15 ms·

Re-using the heat in bathwater in Britain

172 points by colinprince 4y ago

IanCal 4y ago
A good way of reusing the heat from a shower is to have a heat exchanger to the cold pipe coming in. This has the advantage of also meshing nearly perfectly when you want to heat water.
- kragen 4y ago
  The article mentions this about halfway down, with a diagram.
bitxbitxbitcoin 4y ago
This is so much more reasonable than that …other… Modest Proposal.
- deleted 4y ago
  [deleted]
- yjftsjthsd-h 4y ago
  The other one would have a bigger environmental impact;)
- jitl 4y ago
  What other proposal?
  - Anthony-G 4y ago
    That would be A Modest Proposal For preventing the Children of Poor People From being a Burthen to Their Parents or Country, and For making them Beneficial to the Publick by Jonathan Swift: https://en.wikipedia.org/wiki/A_Modest_Proposal https://en.wikipedia.org/wiki/A_Modest_Proposal
stefan_ 4y ago
As the article points out, only some 9% of domestic energy goes towards heating water for showers and baths. Instead of trying to force yourself to do cold showers, you could save much more just turning down the thermostat.
- Tade0 4y ago
  Exactly. Space heating is the most energy-intensive an expensive item on the list of things you can save on in a detached house - especially that you only really need the heat to be where you are.
  - fomine3 4y ago
    It heavily depends how they use bath, and how cold. In Japan, 25-40% (vary depending on research) of energy at home is used for heating water because people prefer bathing. Heating water is expensive.
    - socialismisok 4y ago
      Japanese houses are more likely to be cooler but have local sources of heat, from what I understand. For example a kotatsu is a warm space. Heating locally is such an interesting idea to me.
      - fomine3 4y ago
        The local heating "idea" is now low valued because moving between cold and hot area isn't good for health (called "heat shock"). It is recommended to make house insulated well and heat all rooms, for new houses. https://japantoday.com/category/national/agency-issues-warning-about-heat-shock-danger-from-hot-baths https://japantoday.com/category/national/agency-issues-warni...
        socialismisok 4y ago
        Fascinating!
sweettea 4y ago
Not discussed: hot water in houses increases humidity. In modern well sealed houses, that can result in unfortunate mold growth.
- yjftsjthsd-h 4y ago
  I've always experienced extremely low humidity in winter months, so I would think the net effect might be a reasonable positive if this saves me needing to run a humidifier. Am I missing some detail? Or is that just unusual?
  - jibbit 4y ago
    You’re in the uk?
    - yjftsjthsd-h 4y ago
      No, but I was given to understand that winter => cold => dry air was driven by the basic physical properties, so I would be very curious if it was different there. Is it different in the UK, and if so, why?
      - mrob 4y ago
        The UK climate has moderate temperatures and a lot of rain. Excess humidity is a common problem here, and excessive dryness is rare.
      - infpeace 4y ago
        It tends to often be humid here, even in winter. Right now, it’s 81% humidity. Our laundry has been taking two days to air dry.
      - stefan_ 4y ago
        Cold air can hold much less water (hence why it's relative humidity). This is good if you take in cold air and heat it to 21°C or so because the air you end up with will have very low humidity, but if the outside is a drizzly 10°C and you are only heating to some 18°C, it can be a problem.
wcoenen 4y ago
I tried this once on a cold winter day: I let the bath cool down to room temperature (which was <20C) before letting it drain, with the bathroom door open. The result was a lot of condensation on the windows in the surrounding rooms. I figured the high humidity was going to cause more damage than it saved money, so I scrapped the idea.
- lucb1e 4y ago
  But did it even do anything in the first place? Because if the temperature wasn't measurably raised, it's not even worth considering the 'how to deal with humidity' question that people are discussing.
- Someone1234 4y ago
  Although higher humidity should make it feel warmer given the same ambient temperature. So if you're increasing the temperature mildly using this method AND increasing humidity, it may compound one another positively. Although, it is a minor efficiency gain either way.
  - cannam 4y ago
    I thought high humidity made it feel colder, when the temperature was relatively low? Hot damp air feels hotter than hot dry air, but I've had the idea that cold damp air feels colder than cold dry air. Is that a misconception?
    - magneticnorth 4y ago
      I agree with you. Water facilitates heat transfer, and the surface of skin is about 95f, so it seems that below that temperature, and assuming you aren't already sweating for the evaporative cooling effect, humid air should move heat out of your body more effectively than dry air.
    - jtc331 4y ago
      I think “dry” and “damp” are two extremes. I feel warmer in 45% rH at 67°F in winter than at 30% rH same temperature, but the air doesn’t feel damp (OTOH 60% would start to approach feeling damp). That being said, I think at normal conditioned air temperature ranges more humidity virtually always feels warmer. I’m not exactly sure where the cutoff is, but I’d guess below 60°F might be a decent guess?
deleted 4y ago
[deleted]
- deleted 4y ago
  [deleted]
convolvatron 4y ago
I reuse my bathwater to start a new load of laundry...that seems a lot simpler
- pshc 4y ago
  How do you get the bath water to the laundry machine?
  - convolvatron 4y ago
    I do my laundry in the bathtub :-)
- throwaway894345 4y ago
  Not if you have a frontload washer. :)
  - ars 4y ago
    Along with the problem of getting the water into the machine, the soap from a bath would damage the machine. Front Loaders require low-sudsing soap, and you have to be very careful on the type of soap, because many types tend to precipitate onto the inside of the machine.
- VBprogrammer 4y ago
  How does the plumbing work for that? I can't imagine it passing plumbing regulations in most countries. Even having a grey water system is tricky under most regulations (the reason for this is the slim chance someone could misconnect a potable system.
maherbeg 4y ago
I was actually looking at heat recuperation systems for bathrooms recently and stumbled upon https://ecodrain.com/en/ https://ecodrain.com/en/ - if we hadn’t already remodeled our bathrooms I’d have loved to have installed this. For now we make do with a heat pump water heater which is very efficient.
- afandian 4y ago
  I wonder how different it would be to use an off the shelf heat exchanger, e.g. this one? https://www.ebay.co.uk/itm/263272702860 https://www.ebay.co.uk/itm/263272702860 You'd probably have to filter the shower water before sending it through, but beyond that I wonder how different the principle is. Maybe being easy to clean is a significant factor in drain showers.
  - shellfishgene 4y ago
    There may also be problems with sanitation rules, only a minimum of water should sit in the pipes, and then ideally hot our cold but not luke warm.
    - afandian 4y ago
      Got a link (or explanation) for that temperature stipulation?
      - shellfishgene 4y ago
        Yes, as tempestn explained this is to prevent bacterial growth, especially Legionella. Otherwise this can be a problem in areas where tapwater is not chlorinated.
      - tempestn 4y ago
        Temperature window for bacterial growth. Above about 120F harmful bacteria commonly found in water won't proliferate, but will survive. As you get hotter from there, they're killed off increasingly rapidly. Above 135 or 140 or so they don't survive long. In cold water they won't die off but also won't multiply quickly. The dangerous range is in the middle.
pshc 4y ago
This is a thing in some newer construction, e.g. in Vancouver BC, the Southeast False Creek Neighbourhood Energy Utility recovers waste heat from a wastewater treatment system to supply heating energy and hot water to mixed-use buildings.
jupp0r 4y ago
"Extra moisture in the air might cause problems with things like mold. Use your judgment. It’s probably fine as long as things don’t stay damp all the time. For many, extra humidity in the air would be welcome." In climates like in the UK, this is a great way to create mold in your bathroom and adjacent rooms if your house is well insulated and does not have central ventilation.
imhoguy 4y ago
Or simply take shorter showers, then this system wouldn't really benefit because before the drainage pipe warms up enough to transfer any heat to cold water, you will be done. Grey water tank systems with recuperation are already known tech, but it requires extra maintenance unfortunately.
fifteenforty 4y ago
I’ve looked into installing a system like this in Australia. It only barely makes financial sense as part of a new build. Maybe in the colder UK climate it would make more, but surely not as a retrofit.
yrgulation 4y ago
Energy poverty can be sorted out by better insulation and investment in cheaper and more sustainable sources such as nuclear, wind and solar. Instead of working around this issue with clever hacks this needs to be a national policy. I think all roofs should be mandated to have solar panels installed, as a start, and all new builds should have proper insulation.
- VBprogrammer 4y ago
  I think a step further is required. I really like the idea of using solar panels as an integral part of the roofing structure. There are people in the UK doing this as integrated solar panels which use a plastic tray underneath. I think with some slight adaptions we could even do better.
  - spockz 4y ago
    We had a few of these projects in the Netherlands. It is nice, until it isn’t. The solar panels are more vulnerable than tiles, they will degrade or need repairs/replacing. The mechanism for sealing it up will leak. The format of the panels and or the inverter was proprietary. Or the connection was and you couldn’t retrofit another inverter. It wasn’t a good experience.
    - VBprogrammer 4y ago
      Sounds like there are a few things being mixed in there. I'm not really that convinced that solar panels are any more subject to damage than they would otherwise be. Maybe you gain a little in terms leak proofing by having a normal roof underneath but it's a lot of material for a marginal gain. Typically roofs these days are constructed with an impermeable plastic sheeting which is waterproof in its own right but requires protection from the elements (particularly the sun). It's impressive that they managed to make a DC electrical connection proprietary though!
- HPsquared 4y ago
  If I was planning national energy infrastructure, I wouldn't plan on dividing the equipment into a million tiny pieces and installing it on roofs, of all places. It seems much simpler to manage a solar farm than 10,000 roofs.
joezydeco 4y ago
These diagrams imply that the incoming water to the heater is heated by drain water from the bath. But if you're filling a bathtub there's no warm water going down the drain. So the exchanger is useless here. And then when you're done and drain the tub the exchanger gets hot but the heater isn't filling any more. This device only really works in a shower situation, it seems.
- bombcar 4y ago
  The only "practical" installation I've seen (besides ones in massive multiplayer showers like you'd see at a college or YMCA) was for a massive house with a huge shower designed for 30+ minute showers. And at that point you probably can "save the environment" more by not building a 10k sq ft mansion. But what do I know?
pmoriarty 4y ago
If you let dirty water fill up your tub while you shower that'll result in a dirty ring around your bathtub which you'll have to clean... with more water (and possibly chemicals)... it'll cost you extra time to clean as well. All together the total costs of the extra cleaning resulting from this may exceed the cost savings from letting the water heat your home.
jitl 4y ago
I read a lot about district heating systems yesterday after seeing an article on NYC's steam system on HN (https://news.ycombinator.com/item?id=33388473 https://news.ycombinator.com/item?id=33388473). District heating systems allow you to capture waste heat at the power plant (called co-generation), and use that to heat & cool (via cooling compressors) many, many buildings at once. The wikipedia article about it is very informative as well: https://en.wikipedia.org/wiki/District_heating https://en.wikipedia.org/wiki/District_heating. Usually district heating systems route heat into your building's pipes using a heat exchanger - so the network's fluid never actually intermingles with your building's fluid lines. These systems also use the same heat exchanger to return excess heat back to the network. District heating systems require a large initial investment, but over the long term are very efficient.
- HPsquared 4y ago
  The mirror image of this is combined heat and power (CHP). That is, instead of burning fuel to heat the home, use the fuel to power a generator of some sort, and using the electricity to power the home. The waste heat (most of the energy from the fuel ends up as waste heat) is then captured to heat the home It's basically free electricity, if you were already heating the home by means of combustion anyway (e.g. oil, natural gas, propane).
- nsteel 4y ago
  The problem with district heat schemes in the UK (at least) is they are becoming mandatory for new large housing developments but are 100% unregulated every step of the way. The builders do the cheapest job possible because the efficiency targets they're supposed to hit are never measured or verified post install. And unlike the rest of domestic energy in the UK, pricing is totally unregulated. The residents can (and are) charged high rates because it's impossible for them to change supplier. Any potential savings are undone by these two things. The energy regulator has so far refused to do its job because they supposedly don't want to stifle growth in this new market. It is a good idea but it's been sabotaged by greed.
  - sokoloff 4y ago
brunick 4y ago
i did hear about some shower drains that reuse the heat like this one: https://www.youtube.com/watch?v=s5bJ61fCA2E https://www.youtube.com/watch?v=s5bJ61fCA2E
ArmandGrillet 4y ago
Tangentially related: has someone found something less energy intensive than taking a bath while bringing the same degree of warmth? I've been looking at heated blankets but they're all made of polyester which is not comfortable due to the moisture being trapped under the blanket. Heated seats in some cars are the closest sense of comfort I've found but that technology doesn't seem to exist for regular seats.
- zubspace 4y ago
  Maybe just one or two hot water bottles? [1] We use them in Winter to warm our feet in bed. But they are very versatile and can also be used to alleviate any kind of tension. I also use one while I code: I put it inside of a small (baby-)sleeping bag wrapped around my feet. Sounds weird, but it's so comfortable! [1] https://www.google.com/search?q=hot+water+bottle&tbm=isch https://www.google.com/search?q=hot+water+bottle&tbm=isch
  - ArmandGrillet 4y ago
    Went for https://www.yuyubottle.eu/ https://www.yuyubottle.eu/ after a bit of googling due to the cotton fabric + the strap, will test it this winter.
  - sixstringtheory 4y ago
    We use this trick on winter backcountry camping trips to good effect. After dinner, melt snow to a boil and fill up all our water bottles and bring them in the sleeping bag. Every little bit helps!
  - gima 4y ago
    Another trick is to put lots of dry rice grains into a sock, tie the sock and warm it up in a microwave. Just be careful, as the rice heats up Really fast in the microwave.
- GavinMcG 4y ago
  https://solar.lowtechmagazine.com/2022/01/the-revenge-of-the-hot-water-bottle.html https://solar.lowtechmagazine.com/2022/01/the-revenge-of-the...
tuatoru 4y ago
Someone (Luu?) coined, a while back, the phrase "cocktail party idea". This is exactly that. Something that sounds like a great idea to people standing around chatting, but in reality is full of fail.
imoreno 4y ago
This seems like a lot of work to save £0.13.
louison11 4y ago
Where we live in Portugal humidity is a big problem, and we actually spend 1kW 24/7 running dehumidifier throughout the house. If we didn’t, mold would grow and we would get severe allergies. So… this seems like a very bad idea even for the UK, considering it’s very rainy there in the winter.
- frankus 4y ago
  A house with plenty of air exchange (either mechanical or because of a leaky air barrier) in a cold climate will tend to suffer from too low of humidity (cold air enters, and as it gets heated to room temperature the relative humidity drops).
lkramer 4y ago
In my experience, there a many lower hanging fruits in British houses to save energy than this. Uninsulated and draughty old houses seems to be the norm here (at least if you're renting). I'm surprised the government is not pursuing this more aggressively.
- mgaunard 4y ago
  Even if you're buying, the most popular houses are ancient Victorian ones which, in spite of their charm, are very badly insulated. They're often listed and require fitted wooden windows, so it's not exactly easy to fix.
  - twic 4y ago
    Vanishingly few houses are actually listed. Some are in conservation areas, which have similar but less strict restrictions on what changes you can make. These are the areas in my borough - almost half the built-up area, but i would guess proportionally less of the homes, because they are lower-density old buildings: https://www.haringey.gov.uk/sites/haringeygovuk/files/haringey_conservation_areas_map-2.pdf https://www.haringey.gov.uk/sites/haringeygovuk/files/haring... You can coax a London-wide map of conservation areas out of this, and see that most of the suburbs aren't covered by them: https://apps.london.gov.uk/planning/ https://apps.london.gov.uk/planning/ It probably is true that the most desirable houses are in those areas, but being in a conservation area helps preserve them and keep them desirable. Anyway, even within those conservation areas, there is plenty you can do to improve insulation. I had internal wall and ceiling insulation fitted, and replaced the window sashes with double-glazed ones.
upsidesinclude 4y ago
Turning your thermostat down by one or two degrees per week and getting acclamation to cold is a better alternative. 5 degrees on the thermostat would be a far better measure than tub heat. Short showers, or 'sea showers', to conserve water and heating would likewise be a far better option than a 50 gallon bath.
- sixstringtheory 4y ago
  And not just short showers, but managing the amount of time the water is actually running. Here’s my strategy for an efficient shower: 1. turn on the water, get in, rinse down, soak a wash cloth 2. turn off water 3. lather hair with shampoo 4. lather wash cloth with soap and scrub body 5. turn on water and rinse down The majority of the time we’re standing in the shower, we don’t actually need the water to be running.
  - pbhjpbhj 4y ago
    It gets really cold without the shower water running. Our indoor temperature in the day is 16-18°C in Winter (UK).
  - switch007 4y ago
    Ufff. I’m not doing that when it’s 4c outside and because I turned the heating off at 9pm my house is 14c. I need the shower to warm me up and stop me turning the heating on early in the morning
    - upsidesinclude 4y ago
      Turning off your heat is costing you money. You don't want to cycle all the heat out of the building and then have to warm it back up. It's called heat capacity and the materials themselves require so much heat before you feel warm. TL:DR Turn your heat down, but not off. Save money
- pbhjpbhj 4y ago
  What's a "sea shower". Do you mean when you literally just spray water on yourself, then soap all over, then rinse off?
ytdytvhxgydvhh 4y ago
I appreciate this attitude: > Should you do this? > I render no judgment! > But here’s a thought: It’s OK to use energy for convenience. This is true even in a time of crisis. After all, only a small fraction of the energy we use is necessary to stay alive. We could live dirty and shivering in dark rooms, eating oats and only moving around on foot. But no one does. So the question is how much energy you want to use for how much convenience. Too many articles or conversations about energy usage seem to take an attitude more like, “if you’re not doing the thing we say is the solution then you’re the problem”.
- mrshadowgoose 4y ago
  I couldn't agree more. That type of sentiment is pervasive, and it's absolutely baffling, because we as a society can keep all our excesses and comforts while making the consequential energy expenditures completely green. It seems like people are just incapable of conceiving of solutions that don't involve morally shaming others into being less happy. Or a more cynical take: It seems like a lot of people enjoy "solutions" that stick-it to some other part of the population. "That shower you enjoy uses too much water, you should take shorter and weaker showers." "Shame on you for flying to see your distant relatives, you should take fewer flights." "How dare you use a comfortable car to get around? Unless you're unhappy in a bus with at least 10 other people, you're a bad person." Without even considering other sources of green energy, there is more than enough solar energy reaching the Earth to allow all 8 billion of us to enjoy consumption levels typical of Western nations.
  - intelVISA 4y ago
    I won't be needlessly wasteful but I'm not going to inconvenience myself over a corporation's marketing dept. trying to avail its company's tremendous carbon footprint.
  - hedora 4y ago
    I totally agree. However, if you have the means to spend a bit more on appliances (with payoffs 5-10 years out), then you can easily reduce energy / water (ignoring grass/crops) usage by about 75% vs 10-20 years ago with zero lifestyle impact. Not doing that is needlessly wasteful. In the particular case of showers, get a High Sierra shower head. They have more apparent flow / pressure than a full flow one, but use a small fraction of the water. They cost under $50 and are extremely reliable. (Never had one clog, and never did any maintenance on one, even with extremely hard water.) The article estimates scavenging the bath water heat would reduce total energy consumption by 1.2%. This is much better achieved by lowering the thermostat, or showering less often in the winter. (Or buying a low flow showerhead -- see above!) Note: No affiliation with High Sierra. Happy customer for a decade, across multiple homes. We currently have a 1.5 GPM, but I throttle it down with the trickle valve. The 1.25 GPM is totally fine; the trickle valve might make sense for it too. Note that you can customize colors, etc: https://www.highsierrashowerheads.com/shop/classic/ https://www.highsierrashowerheads.com/shop/classic/
kragen 4y ago
I've done this. It works surprisingly well, and (as Might points out) humidifies the house in a way that radiators and resistance heaters do not. The evaporating water does convert (to use the traditional terms) sensible heat into the latent heat of water vapor, and you might think that this heat should be discounted from the heat you're getting out. Sometimes it should, but not other times, because if the water vapor condenses again inside the house (for example, near the surface of the plaster in cold walls) it will release this heat again as sensible heat. In other cases, it's worse — I've had the displeasure of staying in a cold guest room where the blankets were all slightly damp from such condensation and so failed badly at keeping me warm. In that case, the water vapor had been produced not by evaporating standing water but by burning natural gas. Be careful.
- mistercow 4y ago
  On the latent heat problem, I wonder how the math works out if you put a dehumidifier in the bathroom to counteract the effect of leaving warm water standing around. What's interesting about that is that you'd effectively be using the dehumidifier as a heat pump, indirectly pumping the heat out of the water and into your home, leaving you with cold water in the heat pump's reservoir. You're adding energy, but that energy also ends up as heat, so you'd still certainly end up ahead compared to using electroresistive heating (but not necessarily compared to a traditional heat pump). My guess is that it would be a lot more effective to _directly_ pump the heat out of the waste water, but you can't do that with off the shelf appliances.
sixstringtheory 4y ago
In addition to things like heat exchangers or reducing the amount of time/water in a shower, is there any work being done around optimizing the amount of time a hot water heater keeps the water at high temp? I’ve thought about this for extended absences but this could also bring potential benefits if it just doesn’t run for certain portions of the day or middle of the night.
- bombcar 4y ago
  People have done that (some recent post on HN was about using a RaspberryPi to do just that, including a "get hot enough to remove legionaries' disease"). Likely for most people the tankless heaters would be a better option, or using in-floor water-based heating instead. Something like this: https://www.radiantcompany.com/system/opensystem/ https://www.radiantcompany.com/system/opensystem/
- mvhvv 4y ago
  I'm not sure if things are different outside Australia, but it's probably been 15-20 years since I've lived in a house with a hot-water storage tank. As far as I know most places seem to have instant/continuous systems that heat the water on demand as it passes through the line. Solar-heating definitely makes storage make more sense, but I don't know how the numbers actually work out on that one.
  - frosted-flakes 4y ago
    Or for heat pump water heaters, which have unbeatable efficiency. Still not cost-effective in many areas compared to gas-burning heaters, but they use much less energy.
postultimate 4y ago
I've been doing this, except I have a bath. It finally dawned on me that heating water and leaving it to cool was exactly the way the house is normally heated, so it was costing me nothing to bath as long as I liked. I live there now.
nicd 4y ago
This reminds me of a "re-circulating shower" concept I was looking into recently. By filtering and re-using shower water, it saves both water and heating energy. There is a company in Sweden doing this: https://orbital-systems.com/ https://orbital-systems.com/. There's a bit of an ick factor, but with filtration, it seems this should be more hygienic than bathing? My family remains unconvinced though ;)
- tempestn 4y ago
  Most people are fine with hot tubs aka spas, which are essentially this. (In fact most spa designs don't even filter all the water on each re-circulation, as there are returns both at the bottom and in the skimmer, and only the latter is filtered.) Re-circulating shower sounds like a great idea to me.
  - frosted-flakes 4y ago
    But you don't wash in a hot tub, and you usually shower before entering them.
oliwarner 4y ago
Having read this, I'm strongly considering routing my bath and shower wastes through a poorly insulated hot water cylinder in reverse (new [warm] waste in at the top, old cold waste out the bottom). Hot water will float in there. Cold will be displaced first. It only has to hold the volume of "a shower" and we'd keep that energy within the envelope of the house, without added humidity or state-change heat loss. The only major downside is the "what happens when it clogs" question. To work normally, it would require the discharge to rise to the same height as the top inlet before falling to the drain, but there could be a secondary purge for pressure-flushing it once a month. Rarely does plumbing make so much sense to me. I suspect I'm overlooking something. Answers on a postcard.
- frankus 4y ago
  Something like this could be interesting with active heat transfer, for example using it as the source side of a heat pump water heater. As long as the outgoing water remains above freezing, of course.
tempestn 4y ago
Problem is you're not just adding heat, but humidity. Normally while having a shower you're going to run an exhaust fan in the bathroom. So if you want to conserve heat, the better approach is just to make the shower as short as possible.
- chihuahua 4y ago
  That seems like a more realistic suggestion to me. If you want to take this further, switch to taking Navy showers: get wet, turn off shower, apply soap, turn on shower to rinse off.
  - syockit 4y ago
    What, all my life I thought that was the normal way of showering. So how do you people actually shower? You leave the tap on while soaping?
    - chihuahua 4y ago
      Yes, I believe that is the common way of taking a shower. Turn on the water, do whatever you need for approximately 10 minutes, then turn off the water at the end.
  - mahathu 4y ago
    >get wet, turn off shower, apply soap, turn on shower to rinse off. Isn't that just a regular shower?
- askvictor 4y ago
  Also, cold showers are better for the skin, and good for mental health. Take a bit of getting used to though, particularly in the middle of winter!
divbzero 4y ago
I do a version of this when cooking without knowing if it actually has a significant effect: In summer I pour leftover hot water from cooking down the drain right after I finish, while in winter I leave the hot water sitting in the pot so the heat can warm the kitchen.
wutheringh 4y ago
As someone who buys a lot of bathwater on Onlyfans, this is how I break even.
aaron695 4y ago
Put on a hot heater when taking a shower. In theory you'll turn the water heat down which stops a lot of loss, 1 kg air is ~1000 liters, 1 kg water is 1 liter. It'll also flow back into the house. And I get out sooner because it's not scary outside. This person has installed two drain-water heat recovery systems, and measured it https://www.tiktok.com/@johnsavesenergy/video/7046572667121487110 https://www.tiktok.com/@johnsavesenergy/video/70465726671214... This is the proper way forward.
BurningFrog 4y ago
I've noticed that people wildly overestimate the environmental impact of things they personally touch. This, I theorize, is why sub-rounding error things like plastic straws, plastic bags, cups, etc get such disproportionate attention. Bathwater that heats every pore of our naked bodies is probably even more susceptible to this bias.
- Atheros 4y ago
  The plastic straws were for virtue signaling. A bar near me put up signs in the bathroom with pictures of sea turtles advertising that they no longer use plastic straws. Outside the bathroom, the AC units were running (completely on and running; not just the fan) along with household electric space heaters. Outside the building, it was snowing. The plastic bag laws are because municipalities don't want to pay to haul as much garbage and so seek to save themselves money. I don't know what "cups" is but so far, none of it is honest.
- moepstar 4y ago
  You may not be wrong - however, what's your suggestion then? Not doing these things? IMHO, every little bit counts towards a livable future.
  - Maxion 4y ago
    Most people not in the Nordics / Germany / Switzerland would most likely see way more energy savings per year by simply spending the money used to install a waste water energy recovery system to instead just insulate their house.
- fomine3 4y ago
  Plastic straws are too minor and alternative is too crap, so it's outlier. Heating water is somewhat a bigger thing.
simonblack 4y ago
It's very likely be more cost-efficient to just use a standard heat-pump ("air-conditioner") set on the 'heat cycle' instead of intermittently mucking around with bath-size amounts of water.
bemmu 4y ago
In Japan at our place we always do this. After the last person has used the bath water, we let it sit there overnight, and then use the water for the first wash cycles of the washing machine. The less efficient part of this story is that everyone showers before entering, and that does go down the drain.
- deleted 4y ago
  [deleted]
russellbeattie 4y ago
Years ago I was renting a house with a small in-law building in the back which was rented separately. I'd get the electric bill and then split it 75/25 with the guy who lived there. Over time the electric bill kept getting higher and higher, until it was like $500+ a month. I talked to the landlord and the other renter about it and none of us could figure out what was drawing so much electricity. I called the electric company several times, and they'd asked if I had shut off my pool, which I didn't have, or if I had been using the jacuzzi a lot, which I also didn't own. Then they asked about power tools, welders, and all sorts of other things, and when I finally convinced them something was wrong they sent someone out. The electric guy did the obvious thing: He started flipping breakers to see which circuit was drawing so much electricity. I had done that as well, but had no idea what to do from there. He was able to follow it out to the in-law house, which was in complete darkness as it had zero electricity inside - except for a tiny water heater installed out back. It had somehow gotten filled with sediment, so the heating element was basically trying to keep a barrel full of sand warm. (The guy who lived there just assumed the hot water tank was small and took fast showers as a result). The landlord replaced it and suddenly the bill went down to less than $100. Crazy. The point to this story is that there's probably a lot more that could be done before worrying about recouping the heat from your bath water.
tommoor 4y ago
Pretty much everyone in the UK overfills the kettle and boils it 4+ times per day, I'd imagine if we could just get folks to boil the correct amount of water here the energy savings would be far more significant
fsckboy 4y ago
I just want to put in a word for standard microeconomic theory. If our water (or energy) use in aggregate suggests to the govt that our consumption needs to go down, then raise the price of water (or energy) across the board till it does. Individuals can choose which of all your amazing water (and heat) savings ideas they feel would best suit them.
- socialismisok 4y ago
  Doesn't this approach hurt the poor a lot more though? Like, if my mum suddenly got charged more for her heat, she might have to live in a cold house, which would make her deeply uncomfortable or even sick.
- legulere 4y ago
  The problem in reality is that people have wildly different amounts of disposable income and that there are more and less important uses. Rich people use drastically more resources. If you want to curb their resource usage with price you would need to increase prices so much that poor people can’t afford the resource anymore. E.g. you want to stop people watering lawns during drought, so you make water so expensive that rich people cannot afford it anymore, but then poor people cannot afford to wash hands, shower or flush the toilet anymore. One solution is to have the first x units of the resource cheaper, similar to progressive taxation.
baybal2 4y ago
thallium205 4y ago
Why not limit toilets to only flush every 8 hours to allow better heat capture from human waste?
jve 4y ago
> 90% of the heat in that water currently goes down the drain. I think this number is very flawed, at least if we talk about bath and not shower. Kids sometimes play in bath until the water is "cold". Even if they just wash, the resulting water is far cry from what it was when "inputted" into bath. Moreover, all that temp won't be recovered to heat air. Some energy goes to heat bath itself, some goes to heat space under bath and some goes to heat air. So I suspect the impact numbers to be far less benefit. And the result will be slightly warmer bath, but no effect to overall heating. But it is true that water is hungry for energy. My avg energy usage is 550kWh and I have electrical water heater. It takes exactly half of total energy usage per month.
_nalply 4y ago
You shouldn't let warm water stay in a place with air contact. First it makes air humid and the affected rooms prone to mold, and second evaporation takes energy away, and while condensation might give the energy back you will have wet conditions. It's not very sanitary: warm and wet conditions for some time tend to encourage unwanted life. I armchair thought about a scheme: collect the bath and shower water in a tank in the cellar and use a heat exchanger. However the tank will turn disgusting very quickly. Perhaps a better solution is using a large, long pipe with a flat floor with little inclination, such that warm water does not stay for a long time but flows slowly. Cool the floor of the pipe with a heat exchanger. Perhaps have a contraption to regularly have the pipe brushed from inside and flushed with cold water. Instead of a heat exchanger, use cold water, but this has the problem that fresh cold water needs to flow at the same time as the used warm water. It is worthwhile?
- koliber 4y ago
  In the winter, the air is very dry in my house and we use humidifiers. By leaving the bathwater in the bathtub, we could kill two birds with one stone. Everything you say about ventilation is right though. I just don't know if calling this unsanitary is universally true. Maybe in climates where humidity is a problem to begin with, adding more humidity leads to less sanitary conditions.
  - _nalply 4y ago
    I agree, sometimes I do that, too. However the whole October was warm and humid and I never let the water stay in the bathtub. I carefully check for humidity: it should be between 40% and 60%. Sometimes I hang washed clothes in the room to get more humidity.
- mnw21cam 4y ago
  I have been considering a grey water solution that would achieve two things. The bath in my house is upstairs, and I have an additional toilet downstairs, almost directly underneath. So, I could let the bathwater drain into a tank above the toilet cistern, from where it can slowly release its heat, and then use the water to fill the toilet cistern for flushing. That way, I save on heating and on water usage, and the water wouldn't be evaporating into the house causing humidity problems. I did the calculations. A 120l bath has water costing £0.53 and heating (by gas, if we assume the incoming water is about 10C and heating it to 40C, about 4.2kWh) costing about £0.44. We probably get out of the bath and drain it when it's about 35C, so if the house is at 22C then there's about 1.8kWh of energy that would get transferred as heat to the house, about £0.19 worth. So, for a very modest cost of installing a tank and a little bit of plumbing (probably less than £200), we'd be saving £0.53 each day (assuming all that water gets used) plus £0.19 each day that we would have the heating on, probably half the year, which is around £230 per year. It'd pay itself off very quickly. The main concern is that the holding tank doesn't become a bacterial breeding ground, so it may need some chemical discouragement every now and again, but that's hardly difficult.

172 points by colinprince 4y ago

A good way of reusing the heat from a shower is to have a heat exchanger to the cold pipe coming in. This has the advantage of also meshing nearly perfectly when you want to heat water.

kragen 4y ago
The article mentions this about halfway down, with a diagram.

bitxbitxbitcoin 4y ago

This is so much more reasonable than that …other… Modest Proposal.

deleted 4y ago
[deleted]
yjftsjthsd-h 4y ago
The other one would have a bigger environmental impact;)
jitl 4y ago
What other proposal?
- Anthony-G 4y ago
  That would be A Modest Proposal For preventing the Children of Poor People From being a Burthen to Their Parents or Country, and For making them Beneficial to the Publick by Jonathan Swift: https://en.wikipedia.org/wiki/A_Modest_Proposal https://en.wikipedia.org/wiki/A_Modest_Proposal

stefan_ 4y ago

As the article points out, only some 9% of domestic energy goes towards heating water for showers and baths. Instead of trying to force yourself to do cold showers, you could save much more just turning down the thermostat.

Tade0 4y ago
Exactly. Space heating is the most energy-intensive an expensive item on the list of things you can save on in a detached house - especially that you only really need the heat to be where you are.
- fomine3 4y ago
  It heavily depends how they use bath, and how cold. In Japan, 25-40% (vary depending on research) of energy at home is used for heating water because people prefer bathing. Heating water is expensive.
  - socialismisok 4y ago
    Japanese houses are more likely to be cooler but have local sources of heat, from what I understand. For example a kotatsu is a warm space. Heating locally is such an interesting idea to me.
    - fomine3 4y ago
      The local heating "idea" is now low valued because moving between cold and hot area isn't good for health (called "heat shock"). It is recommended to make house insulated well and heat all rooms, for new houses. https://japantoday.com/category/national/agency-issues-warning-about-heat-shock-danger-from-hot-baths https://japantoday.com/category/national/agency-issues-warni...
      - socialismisok 4y ago
        Fascinating!

sweettea 4y ago

Not discussed: hot water in houses increases humidity. In modern well sealed houses, that can result in unfortunate mold growth.

yjftsjthsd-h 4y ago
I've always experienced extremely low humidity in winter months, so I would think the net effect might be a reasonable positive if this saves me needing to run a humidifier. Am I missing some detail? Or is that just unusual?
- jibbit 4y ago
  You’re in the uk?
  - yjftsjthsd-h 4y ago
    No, but I was given to understand that winter => cold => dry air was driven by the basic physical properties, so I would be very curious if it was different there. Is it different in the UK, and if so, why?
    - mrob 4y ago
      The UK climate has moderate temperatures and a lot of rain. Excess humidity is a common problem here, and excessive dryness is rare.
    - infpeace 4y ago
      It tends to often be humid here, even in winter. Right now, it’s 81% humidity. Our laundry has been taking two days to air dry.
    - stefan_ 4y ago
      Cold air can hold much less water (hence why it's relative humidity). This is good if you take in cold air and heat it to 21°C or so because the air you end up with will have very low humidity, but if the outside is a drizzly 10°C and you are only heating to some 18°C, it can be a problem.

wcoenen 4y ago

I tried this once on a cold winter day: I let the bath cool down to room temperature (which was <20C) before letting it drain, with the bathroom door open. The result was a lot of condensation on the windows in the surrounding rooms. I figured the high humidity was going to cause more damage than it saved money, so I scrapped the idea.

lucb1e 4y ago
But did it even do anything in the first place? Because if the temperature wasn't measurably raised, it's not even worth considering the 'how to deal with humidity' question that people are discussing.
Someone1234 4y ago
Although higher humidity should make it feel warmer given the same ambient temperature. So if you're increasing the temperature mildly using this method AND increasing humidity, it may compound one another positively. Although, it is a minor efficiency gain either way.
- cannam 4y ago
  I thought high humidity made it feel colder, when the temperature was relatively low? Hot damp air feels hotter than hot dry air, but I've had the idea that cold damp air feels colder than cold dry air. Is that a misconception?
  - magneticnorth 4y ago
    I agree with you. Water facilitates heat transfer, and the surface of skin is about 95f, so it seems that below that temperature, and assuming you aren't already sweating for the evaporative cooling effect, humid air should move heat out of your body more effectively than dry air.
  - jtc331 4y ago
    I think “dry” and “damp” are two extremes. I feel warmer in 45% rH at 67°F in winter than at 30% rH same temperature, but the air doesn’t feel damp (OTOH 60% would start to approach feeling damp). That being said, I think at normal conditioned air temperature ranges more humidity virtually always feels warmer. I’m not exactly sure where the cutoff is, but I’d guess below 60°F might be a decent guess?

deleted 4y ago

[deleted]

deleted 4y ago
[deleted]

convolvatron 4y ago

I reuse my bathwater to start a new load of laundry...that seems a lot simpler

pshc 4y ago
How do you get the bath water to the laundry machine?
- convolvatron 4y ago
  I do my laundry in the bathtub :-)
throwaway894345 4y ago
Not if you have a frontload washer. :)
- ars 4y ago
  Along with the problem of getting the water into the machine, the soap from a bath would damage the machine. Front Loaders require low-sudsing soap, and you have to be very careful on the type of soap, because many types tend to precipitate onto the inside of the machine.
VBprogrammer 4y ago
How does the plumbing work for that? I can't imagine it passing plumbing regulations in most countries. Even having a grey water system is tricky under most regulations (the reason for this is the slim chance someone could misconnect a potable system.

maherbeg 4y ago

I was actually looking at heat recuperation systems for bathrooms recently and stumbled upon https://ecodrain.com/en/ https://ecodrain.com/en/ - if we hadn’t already remodeled our bathrooms I’d have loved to have installed this. For now we make do with a heat pump water heater which is very efficient.

afandian 4y ago
I wonder how different it would be to use an off the shelf heat exchanger, e.g. this one? https://www.ebay.co.uk/itm/263272702860 https://www.ebay.co.uk/itm/263272702860 You'd probably have to filter the shower water before sending it through, but beyond that I wonder how different the principle is. Maybe being easy to clean is a significant factor in drain showers.
- shellfishgene 4y ago
  There may also be problems with sanitation rules, only a minimum of water should sit in the pipes, and then ideally hot our cold but not luke warm.
  - afandian 4y ago
    Got a link (or explanation) for that temperature stipulation?
    - shellfishgene 4y ago
      Yes, as tempestn explained this is to prevent bacterial growth, especially Legionella. Otherwise this can be a problem in areas where tapwater is not chlorinated.
    - tempestn 4y ago
      Temperature window for bacterial growth. Above about 120F harmful bacteria commonly found in water won't proliferate, but will survive. As you get hotter from there, they're killed off increasingly rapidly. Above 135 or 140 or so they don't survive long. In cold water they won't die off but also won't multiply quickly. The dangerous range is in the middle.

pshc 4y ago

This is a thing in some newer construction, e.g. in Vancouver BC, the Southeast False Creek Neighbourhood Energy Utility recovers waste heat from a wastewater treatment system to supply heating energy and hot water to mixed-use buildings.

jupp0r 4y ago

"Extra moisture in the air might cause problems with things like mold. Use your judgment. It’s probably fine as long as things don’t stay damp all the time. For many, extra humidity in the air would be welcome." In climates like in the UK, this is a great way to create mold in your bathroom and adjacent rooms if your house is well insulated and does not have central ventilation.

imhoguy 4y ago

Or simply take shorter showers, then this system wouldn't really benefit because before the drainage pipe warms up enough to transfer any heat to cold water, you will be done. Grey water tank systems with recuperation are already known tech, but it requires extra maintenance unfortunately.

fifteenforty 4y ago

I’ve looked into installing a system like this in Australia. It only barely makes financial sense as part of a new build. Maybe in the colder UK climate it would make more, but surely not as a retrofit.

yrgulation 4y ago

Energy poverty can be sorted out by better insulation and investment in cheaper and more sustainable sources such as nuclear, wind and solar. Instead of working around this issue with clever hacks this needs to be a national policy. I think all roofs should be mandated to have solar panels installed, as a start, and all new builds should have proper insulation.

VBprogrammer 4y ago
I think a step further is required. I really like the idea of using solar panels as an integral part of the roofing structure. There are people in the UK doing this as integrated solar panels which use a plastic tray underneath. I think with some slight adaptions we could even do better.
- spockz 4y ago
  We had a few of these projects in the Netherlands. It is nice, until it isn’t. The solar panels are more vulnerable than tiles, they will degrade or need repairs/replacing. The mechanism for sealing it up will leak. The format of the panels and or the inverter was proprietary. Or the connection was and you couldn’t retrofit another inverter. It wasn’t a good experience.
  - VBprogrammer 4y ago
    Sounds like there are a few things being mixed in there. I'm not really that convinced that solar panels are any more subject to damage than they would otherwise be. Maybe you gain a little in terms leak proofing by having a normal roof underneath but it's a lot of material for a marginal gain. Typically roofs these days are constructed with an impermeable plastic sheeting which is waterproof in its own right but requires protection from the elements (particularly the sun). It's impressive that they managed to make a DC electrical connection proprietary though!
HPsquared 4y ago
If I was planning national energy infrastructure, I wouldn't plan on dividing the equipment into a million tiny pieces and installing it on roofs, of all places. It seems much simpler to manage a solar farm than 10,000 roofs.

joezydeco 4y ago

These diagrams imply that the incoming water to the heater is heated by drain water from the bath. But if you're filling a bathtub there's no warm water going down the drain. So the exchanger is useless here. And then when you're done and drain the tub the exchanger gets hot but the heater isn't filling any more. This device only really works in a shower situation, it seems.

bombcar 4y ago
The only "practical" installation I've seen (besides ones in massive multiplayer showers like you'd see at a college or YMCA) was for a massive house with a huge shower designed for 30+ minute showers. And at that point you probably can "save the environment" more by not building a 10k sq ft mansion. But what do I know?

pmoriarty 4y ago

If you let dirty water fill up your tub while you shower that'll result in a dirty ring around your bathtub which you'll have to clean... with more water (and possibly chemicals)... it'll cost you extra time to clean as well. All together the total costs of the extra cleaning resulting from this may exceed the cost savings from letting the water heat your home.

jitl 4y ago

I read a lot about district heating systems yesterday after seeing an article on NYC's steam system on HN (https://news.ycombinator.com/item?id=33388473 https://news.ycombinator.com/item?id=33388473). District heating systems allow you to capture waste heat at the power plant (called co-generation), and use that to heat & cool (via cooling compressors) many, many buildings at once. The wikipedia article about it is very informative as well: https://en.wikipedia.org/wiki/District_heating https://en.wikipedia.org/wiki/District_heating. Usually district heating systems route heat into your building's pipes using a heat exchanger - so the network's fluid never actually intermingles with your building's fluid lines. These systems also use the same heat exchanger to return excess heat back to the network. District heating systems require a large initial investment, but over the long term are very efficient.

HPsquared 4y ago
The mirror image of this is combined heat and power (CHP). That is, instead of burning fuel to heat the home, use the fuel to power a generator of some sort, and using the electricity to power the home. The waste heat (most of the energy from the fuel ends up as waste heat) is then captured to heat the home It's basically free electricity, if you were already heating the home by means of combustion anyway (e.g. oil, natural gas, propane).
nsteel 4y ago
The problem with district heat schemes in the UK (at least) is they are becoming mandatory for new large housing developments but are 100% unregulated every step of the way. The builders do the cheapest job possible because the efficiency targets they're supposed to hit are never measured or verified post install. And unlike the rest of domestic energy in the UK, pricing is totally unregulated. The residents can (and are) charged high rates because it's impossible for them to change supplier. Any potential savings are undone by these two things. The energy regulator has so far refused to do its job because they supposedly don't want to stifle growth in this new market. It is a good idea but it's been sabotaged by greed.
- sokoloff 4y ago

brunick 4y ago

i did hear about some shower drains that reuse the heat like this one: https://www.youtube.com/watch?v=s5bJ61fCA2E https://www.youtube.com/watch?v=s5bJ61fCA2E

ArmandGrillet 4y ago

Tangentially related: has someone found something less energy intensive than taking a bath while bringing the same degree of warmth? I've been looking at heated blankets but they're all made of polyester which is not comfortable due to the moisture being trapped under the blanket. Heated seats in some cars are the closest sense of comfort I've found but that technology doesn't seem to exist for regular seats.

zubspace 4y ago
Maybe just one or two hot water bottles? [1] We use them in Winter to warm our feet in bed. But they are very versatile and can also be used to alleviate any kind of tension. I also use one while I code: I put it inside of a small (baby-)sleeping bag wrapped around my feet. Sounds weird, but it's so comfortable! [1] https://www.google.com/search?q=hot+water+bottle&tbm=isch https://www.google.com/search?q=hot+water+bottle&tbm=isch
- ArmandGrillet 4y ago
  Went for https://www.yuyubottle.eu/ https://www.yuyubottle.eu/ after a bit of googling due to the cotton fabric + the strap, will test it this winter.
- sixstringtheory 4y ago
  We use this trick on winter backcountry camping trips to good effect. After dinner, melt snow to a boil and fill up all our water bottles and bring them in the sleeping bag. Every little bit helps!
- gima 4y ago
  Another trick is to put lots of dry rice grains into a sock, tie the sock and warm it up in a microwave. Just be careful, as the rice heats up Really fast in the microwave.
GavinMcG 4y ago
https://solar.lowtechmagazine.com/2022/01/the-revenge-of-the-hot-water-bottle.html https://solar.lowtechmagazine.com/2022/01/the-revenge-of-the...

tuatoru 4y ago

Someone (Luu?) coined, a while back, the phrase "cocktail party idea". This is exactly that. Something that sounds like a great idea to people standing around chatting, but in reality is full of fail.

imoreno 4y ago

This seems like a lot of work to save £0.13.

louison11 4y ago

Where we live in Portugal humidity is a big problem, and we actually spend 1kW 24/7 running dehumidifier throughout the house. If we didn’t, mold would grow and we would get severe allergies. So… this seems like a very bad idea even for the UK, considering it’s very rainy there in the winter.

frankus 4y ago
A house with plenty of air exchange (either mechanical or because of a leaky air barrier) in a cold climate will tend to suffer from too low of humidity (cold air enters, and as it gets heated to room temperature the relative humidity drops).

lkramer 4y ago

In my experience, there a many lower hanging fruits in British houses to save energy than this. Uninsulated and draughty old houses seems to be the norm here (at least if you're renting). I'm surprised the government is not pursuing this more aggressively.

mgaunard 4y ago
Even if you're buying, the most popular houses are ancient Victorian ones which, in spite of their charm, are very badly insulated. They're often listed and require fitted wooden windows, so it's not exactly easy to fix.
- twic 4y ago
  Vanishingly few houses are actually listed. Some are in conservation areas, which have similar but less strict restrictions on what changes you can make. These are the areas in my borough - almost half the built-up area, but i would guess proportionally less of the homes, because they are lower-density old buildings: https://www.haringey.gov.uk/sites/haringeygovuk/files/haringey_conservation_areas_map-2.pdf https://www.haringey.gov.uk/sites/haringeygovuk/files/haring... You can coax a London-wide map of conservation areas out of this, and see that most of the suburbs aren't covered by them: https://apps.london.gov.uk/planning/ https://apps.london.gov.uk/planning/ It probably is true that the most desirable houses are in those areas, but being in a conservation area helps preserve them and keep them desirable. Anyway, even within those conservation areas, there is plenty you can do to improve insulation. I had internal wall and ceiling insulation fitted, and replaced the window sashes with double-glazed ones.

upsidesinclude 4y ago

Turning your thermostat down by one or two degrees per week and getting acclamation to cold is a better alternative. 5 degrees on the thermostat would be a far better measure than tub heat. Short showers, or 'sea showers', to conserve water and heating would likewise be a far better option than a 50 gallon bath.

sixstringtheory 4y ago
And not just short showers, but managing the amount of time the water is actually running. Here’s my strategy for an efficient shower: 1. turn on the water, get in, rinse down, soak a wash cloth 2. turn off water 3. lather hair with shampoo 4. lather wash cloth with soap and scrub body 5. turn on water and rinse down The majority of the time we’re standing in the shower, we don’t actually need the water to be running.
- pbhjpbhj 4y ago
  It gets really cold without the shower water running. Our indoor temperature in the day is 16-18°C in Winter (UK).
- switch007 4y ago
  Ufff. I’m not doing that when it’s 4c outside and because I turned the heating off at 9pm my house is 14c. I need the shower to warm me up and stop me turning the heating on early in the morning
  - upsidesinclude 4y ago
    Turning off your heat is costing you money. You don't want to cycle all the heat out of the building and then have to warm it back up. It's called heat capacity and the materials themselves require so much heat before you feel warm. TL:DR Turn your heat down, but not off. Save money
pbhjpbhj 4y ago
What's a "sea shower". Do you mean when you literally just spray water on yourself, then soap all over, then rinse off?

ytdytvhxgydvhh 4y ago

I appreciate this attitude: > Should you do this? > I render no judgment! > But here’s a thought: It’s OK to use energy for convenience. This is true even in a time of crisis. After all, only a small fraction of the energy we use is necessary to stay alive. We could live dirty and shivering in dark rooms, eating oats and only moving around on foot. But no one does. So the question is how much energy you want to use for how much convenience. Too many articles or conversations about energy usage seem to take an attitude more like, “if you’re not doing the thing we say is the solution then you’re the problem”.

mrshadowgoose 4y ago
I couldn't agree more. That type of sentiment is pervasive, and it's absolutely baffling, because we as a society can keep all our excesses and comforts while making the consequential energy expenditures completely green. It seems like people are just incapable of conceiving of solutions that don't involve morally shaming others into being less happy. Or a more cynical take: It seems like a lot of people enjoy "solutions" that stick-it to some other part of the population. "That shower you enjoy uses too much water, you should take shorter and weaker showers." "Shame on you for flying to see your distant relatives, you should take fewer flights." "How dare you use a comfortable car to get around? Unless you're unhappy in a bus with at least 10 other people, you're a bad person." Without even considering other sources of green energy, there is more than enough solar energy reaching the Earth to allow all 8 billion of us to enjoy consumption levels typical of Western nations.
- intelVISA 4y ago
  I won't be needlessly wasteful but I'm not going to inconvenience myself over a corporation's marketing dept. trying to avail its company's tremendous carbon footprint.
- hedora 4y ago
  I totally agree. However, if you have the means to spend a bit more on appliances (with payoffs 5-10 years out), then you can easily reduce energy / water (ignoring grass/crops) usage by about 75% vs 10-20 years ago with zero lifestyle impact. Not doing that is needlessly wasteful. In the particular case of showers, get a High Sierra shower head. They have more apparent flow / pressure than a full flow one, but use a small fraction of the water. They cost under $50 and are extremely reliable. (Never had one clog, and never did any maintenance on one, even with extremely hard water.) The article estimates scavenging the bath water heat would reduce total energy consumption by 1.2%. This is much better achieved by lowering the thermostat, or showering less often in the winter. (Or buying a low flow showerhead -- see above!) Note: No affiliation with High Sierra. Happy customer for a decade, across multiple homes. We currently have a 1.5 GPM, but I throttle it down with the trickle valve. The 1.25 GPM is totally fine; the trickle valve might make sense for it too. Note that you can customize colors, etc: https://www.highsierrashowerheads.com/shop/classic/ https://www.highsierrashowerheads.com/shop/classic/

kragen 4y ago

I've done this. It works surprisingly well, and (as Might points out) humidifies the house in a way that radiators and resistance heaters do not. The evaporating water does convert (to use the traditional terms) sensible heat into the latent heat of water vapor, and you might think that this heat should be discounted from the heat you're getting out. Sometimes it should, but not other times, because if the water vapor condenses again inside the house (for example, near the surface of the plaster in cold walls) it will release this heat again as sensible heat. In other cases, it's worse — I've had the displeasure of staying in a cold guest room where the blankets were all slightly damp from such condensation and so failed badly at keeping me warm. In that case, the water vapor had been produced not by evaporating standing water but by burning natural gas. Be careful.

mistercow 4y ago
On the latent heat problem, I wonder how the math works out if you put a dehumidifier in the bathroom to counteract the effect of leaving warm water standing around. What's interesting about that is that you'd effectively be using the dehumidifier as a heat pump, indirectly pumping the heat out of the water and into your home, leaving you with cold water in the heat pump's reservoir. You're adding energy, but that energy also ends up as heat, so you'd still certainly end up ahead compared to using electroresistive heating (but not necessarily compared to a traditional heat pump). My guess is that it would be a lot more effective to _directly_ pump the heat out of the waste water, but you can't do that with off the shelf appliances.

sixstringtheory 4y ago

In addition to things like heat exchangers or reducing the amount of time/water in a shower, is there any work being done around optimizing the amount of time a hot water heater keeps the water at high temp? I’ve thought about this for extended absences but this could also bring potential benefits if it just doesn’t run for certain portions of the day or middle of the night.

bombcar 4y ago
People have done that (some recent post on HN was about using a RaspberryPi to do just that, including a "get hot enough to remove legionaries' disease"). Likely for most people the tankless heaters would be a better option, or using in-floor water-based heating instead. Something like this: https://www.radiantcompany.com/system/opensystem/ https://www.radiantcompany.com/system/opensystem/
mvhvv 4y ago
I'm not sure if things are different outside Australia, but it's probably been 15-20 years since I've lived in a house with a hot-water storage tank. As far as I know most places seem to have instant/continuous systems that heat the water on demand as it passes through the line. Solar-heating definitely makes storage make more sense, but I don't know how the numbers actually work out on that one.
- frosted-flakes 4y ago
  Or for heat pump water heaters, which have unbeatable efficiency. Still not cost-effective in many areas compared to gas-burning heaters, but they use much less energy.

postultimate 4y ago

I've been doing this, except I have a bath. It finally dawned on me that heating water and leaving it to cool was exactly the way the house is normally heated, so it was costing me nothing to bath as long as I liked. I live there now.

nicd 4y ago

This reminds me of a "re-circulating shower" concept I was looking into recently. By filtering and re-using shower water, it saves both water and heating energy. There is a company in Sweden doing this: https://orbital-systems.com/ https://orbital-systems.com/. There's a bit of an ick factor, but with filtration, it seems this should be more hygienic than bathing? My family remains unconvinced though ;)

tempestn 4y ago
Most people are fine with hot tubs aka spas, which are essentially this. (In fact most spa designs don't even filter all the water on each re-circulation, as there are returns both at the bottom and in the skimmer, and only the latter is filtered.) Re-circulating shower sounds like a great idea to me.
- frosted-flakes 4y ago
  But you don't wash in a hot tub, and you usually shower before entering them.

oliwarner 4y ago

Having read this, I'm strongly considering routing my bath and shower wastes through a poorly insulated hot water cylinder in reverse (new [warm] waste in at the top, old cold waste out the bottom). Hot water will float in there. Cold will be displaced first. It only has to hold the volume of "a shower" and we'd keep that energy within the envelope of the house, without added humidity or state-change heat loss. The only major downside is the "what happens when it clogs" question. To work normally, it would require the discharge to rise to the same height as the top inlet before falling to the drain, but there could be a secondary purge for pressure-flushing it once a month. Rarely does plumbing make so much sense to me. I suspect I'm overlooking something. Answers on a postcard.

frankus 4y ago
Something like this could be interesting with active heat transfer, for example using it as the source side of a heat pump water heater. As long as the outgoing water remains above freezing, of course.

tempestn 4y ago

Problem is you're not just adding heat, but humidity. Normally while having a shower you're going to run an exhaust fan in the bathroom. So if you want to conserve heat, the better approach is just to make the shower as short as possible.

chihuahua 4y ago
That seems like a more realistic suggestion to me. If you want to take this further, switch to taking Navy showers: get wet, turn off shower, apply soap, turn on shower to rinse off.
- syockit 4y ago
  What, all my life I thought that was the normal way of showering. So how do you people actually shower? You leave the tap on while soaping?
  - chihuahua 4y ago
    Yes, I believe that is the common way of taking a shower. Turn on the water, do whatever you need for approximately 10 minutes, then turn off the water at the end.
- mahathu 4y ago
  >get wet, turn off shower, apply soap, turn on shower to rinse off. Isn't that just a regular shower?
askvictor 4y ago
Also, cold showers are better for the skin, and good for mental health. Take a bit of getting used to though, particularly in the middle of winter!

divbzero 4y ago

I do a version of this when cooking without knowing if it actually has a significant effect: In summer I pour leftover hot water from cooking down the drain right after I finish, while in winter I leave the hot water sitting in the pot so the heat can warm the kitchen.

wutheringh 4y ago

As someone who buys a lot of bathwater on Onlyfans, this is how I break even.

aaron695 4y ago

Put on a hot heater when taking a shower. In theory you'll turn the water heat down which stops a lot of loss, 1 kg air is ~1000 liters, 1 kg water is 1 liter. It'll also flow back into the house. And I get out sooner because it's not scary outside. This person has installed two drain-water heat recovery systems, and measured it https://www.tiktok.com/@johnsavesenergy/video/7046572667121487110 https://www.tiktok.com/@johnsavesenergy/video/70465726671214... This is the proper way forward.

BurningFrog 4y ago

I've noticed that people wildly overestimate the environmental impact of things they personally touch. This, I theorize, is why sub-rounding error things like plastic straws, plastic bags, cups, etc get such disproportionate attention. Bathwater that heats every pore of our naked bodies is probably even more susceptible to this bias.

Atheros 4y ago
The plastic straws were for virtue signaling. A bar near me put up signs in the bathroom with pictures of sea turtles advertising that they no longer use plastic straws. Outside the bathroom, the AC units were running (completely on and running; not just the fan) along with household electric space heaters. Outside the building, it was snowing. The plastic bag laws are because municipalities don't want to pay to haul as much garbage and so seek to save themselves money. I don't know what "cups" is but so far, none of it is honest.
moepstar 4y ago
You may not be wrong - however, what's your suggestion then? Not doing these things? IMHO, every little bit counts towards a livable future.
- Maxion 4y ago
  Most people not in the Nordics / Germany / Switzerland would most likely see way more energy savings per year by simply spending the money used to install a waste water energy recovery system to instead just insulate their house.
fomine3 4y ago
Plastic straws are too minor and alternative is too crap, so it's outlier. Heating water is somewhat a bigger thing.

simonblack 4y ago

It's very likely be more cost-efficient to just use a standard heat-pump ("air-conditioner") set on the 'heat cycle' instead of intermittently mucking around with bath-size amounts of water.

bemmu 4y ago

In Japan at our place we always do this. After the last person has used the bath water, we let it sit there overnight, and then use the water for the first wash cycles of the washing machine. The less efficient part of this story is that everyone showers before entering, and that does go down the drain.

deleted 4y ago
[deleted]

russellbeattie 4y ago

Years ago I was renting a house with a small in-law building in the back which was rented separately. I'd get the electric bill and then split it 75/25 with the guy who lived there. Over time the electric bill kept getting higher and higher, until it was like $500+ a month. I talked to the landlord and the other renter about it and none of us could figure out what was drawing so much electricity. I called the electric company several times, and they'd asked if I had shut off my pool, which I didn't have, or if I had been using the jacuzzi a lot, which I also didn't own. Then they asked about power tools, welders, and all sorts of other things, and when I finally convinced them something was wrong they sent someone out. The electric guy did the obvious thing: He started flipping breakers to see which circuit was drawing so much electricity. I had done that as well, but had no idea what to do from there. He was able to follow it out to the in-law house, which was in complete darkness as it had zero electricity inside - except for a tiny water heater installed out back. It had somehow gotten filled with sediment, so the heating element was basically trying to keep a barrel full of sand warm. (The guy who lived there just assumed the hot water tank was small and took fast showers as a result). The landlord replaced it and suddenly the bill went down to less than $100. Crazy. The point to this story is that there's probably a lot more that could be done before worrying about recouping the heat from your bath water.

tommoor 4y ago

Pretty much everyone in the UK overfills the kettle and boils it 4+ times per day, I'd imagine if we could just get folks to boil the correct amount of water here the energy savings would be far more significant

fsckboy 4y ago

I just want to put in a word for standard microeconomic theory. If our water (or energy) use in aggregate suggests to the govt that our consumption needs to go down, then raise the price of water (or energy) across the board till it does. Individuals can choose which of all your amazing water (and heat) savings ideas they feel would best suit them.

socialismisok 4y ago
Doesn't this approach hurt the poor a lot more though? Like, if my mum suddenly got charged more for her heat, she might have to live in a cold house, which would make her deeply uncomfortable or even sick.
legulere 4y ago
The problem in reality is that people have wildly different amounts of disposable income and that there are more and less important uses. Rich people use drastically more resources. If you want to curb their resource usage with price you would need to increase prices so much that poor people can’t afford the resource anymore. E.g. you want to stop people watering lawns during drought, so you make water so expensive that rich people cannot afford it anymore, but then poor people cannot afford to wash hands, shower or flush the toilet anymore. One solution is to have the first x units of the resource cheaper, similar to progressive taxation.

baybal2 4y ago

thallium205 4y ago

Why not limit toilets to only flush every 8 hours to allow better heat capture from human waste?

jve 4y ago

> 90% of the heat in that water currently goes down the drain. I think this number is very flawed, at least if we talk about bath and not shower. Kids sometimes play in bath until the water is "cold". Even if they just wash, the resulting water is far cry from what it was when "inputted" into bath. Moreover, all that temp won't be recovered to heat air. Some energy goes to heat bath itself, some goes to heat space under bath and some goes to heat air. So I suspect the impact numbers to be far less benefit. And the result will be slightly warmer bath, but no effect to overall heating. But it is true that water is hungry for energy. My avg energy usage is 550kWh and I have electrical water heater. It takes exactly half of total energy usage per month.

_nalply 4y ago

You shouldn't let warm water stay in a place with air contact. First it makes air humid and the affected rooms prone to mold, and second evaporation takes energy away, and while condensation might give the energy back you will have wet conditions. It's not very sanitary: warm and wet conditions for some time tend to encourage unwanted life. I armchair thought about a scheme: collect the bath and shower water in a tank in the cellar and use a heat exchanger. However the tank will turn disgusting very quickly. Perhaps a better solution is using a large, long pipe with a flat floor with little inclination, such that warm water does not stay for a long time but flows slowly. Cool the floor of the pipe with a heat exchanger. Perhaps have a contraption to regularly have the pipe brushed from inside and flushed with cold water. Instead of a heat exchanger, use cold water, but this has the problem that fresh cold water needs to flow at the same time as the used warm water. It is worthwhile?

koliber 4y ago
In the winter, the air is very dry in my house and we use humidifiers. By leaving the bathwater in the bathtub, we could kill two birds with one stone. Everything you say about ventilation is right though. I just don't know if calling this unsanitary is universally true. Maybe in climates where humidity is a problem to begin with, adding more humidity leads to less sanitary conditions.
- _nalply 4y ago
  I agree, sometimes I do that, too. However the whole October was warm and humid and I never let the water stay in the bathtub. I carefully check for humidity: it should be between 40% and 60%. Sometimes I hang washed clothes in the room to get more humidity.
mnw21cam 4y ago
I have been considering a grey water solution that would achieve two things. The bath in my house is upstairs, and I have an additional toilet downstairs, almost directly underneath. So, I could let the bathwater drain into a tank above the toilet cistern, from where it can slowly release its heat, and then use the water to fill the toilet cistern for flushing. That way, I save on heating and on water usage, and the water wouldn't be evaporating into the house causing humidity problems. I did the calculations. A 120l bath has water costing £0.53 and heating (by gas, if we assume the incoming water is about 10C and heating it to 40C, about 4.2kWh) costing about £0.44. We probably get out of the bath and drain it when it's about 35C, so if the house is at 22C then there's about 1.8kWh of energy that would get transferred as heat to the house, about £0.19 worth. So, for a very modest cost of installing a tank and a little bit of plumbing (probably less than £200), we'd be saving £0.53 each day (assuming all that water gets used) plus £0.19 each day that we would have the heating on, probably half the year, which is around £230 per year. It'd pay itself off very quickly. The main concern is that the holding tank doesn't become a bacterial breeding ground, so it may need some chemical discouragement every now and again, but that's hardly difficult.