36 ms·

Heat Pumps Work Miracles

251 points by mhb 8y ago

thinkcontext 8y ago
Heat pump water heaters have efficiency of 3x+ vs .9 or so for gas or electric resistance. If you have have an electric resistance water heater get a heat pump model. It costs more upfront but will pay for itself in not too many years, especially considering many places have incentives. In the US natural gas is so cheap the economics are not nearly as good, it takes a long time to pay back the difference but does eventually. With even a modest carbon price obviously this would be different. If you are going to replace your furnace or AC get a quote for replacing with a heat pump instead, either central or multiple mini-split heads. Heat pumps have gotten a lot better in recent years, particularly being able to work in cold weather down to 0F or below, expanding northward the places they are practical.
- heatpumper 8y ago
  I can tell you that my heat pump set up at my condo is absolute trash here in Northern Ohio. Running 24/7 it is unable to get my condo above 62 deg F. It blows cold-ish air under an outside temperature of 40 deg F. Windows are new, doors are weather stripped. I'm having it run it in supplemental mode just to get it above 65 deg F.
  - Robotbeat 8y ago
    There are some very bad heat pumps out there. The difference in usability varies dramatically. Some heat pumps will produce heat even below 0F. Others are crappy at anything under 40F.
  - mr337 8y ago
    Something must be wrong. My heatpump will run you out of the house at 40F outside. Below 20F it puts out so so heat but can maintain. Below 10F electric heat is needed from time to time to maintain. Below 0F heat pump is worthless. Edit: Wanted to add my is air heatpump, not ground source. Also reading other comments looks like heatpumps range from amazing to utter crap.
  - toomuchtodo 8y ago
    1. Have an HVAC tech inspect it (~$100-150). 2. If it's not a coolant or compressor issue, you might need to replace it (either because it is undersized or end of life). Your local government or utility might have energy incentives for replacing it. There are also federal tax credit incentives [1]. [1] https://www.energystar.gov/about/federal_tax_credits/air_source_heat_pumps https://www.energystar.gov/about/federal_tax_credits/air_sou...
  - czechdeveloper 8y ago
    Your heat pump just does not have sufficient capacity. It's not problem of heat pump per se.
  - jandrese 8y ago
    Your heat pump is probably about 20 years old and on its last legs. Newer ones are amazing. We lopped $40 off of our electricity bill each month in the winter and $20 in the summer after we replaced ours. It pays for itself after a few years.
  - dashundchen 8y ago
    From what I understand, cold weather air source heat pumps are relatively new to the market, at least in the US. Previously they were designed more for cooling loads and were essentially space heaters with an COP of 1 in cold weather. Many manufactuers seem to offer warm and cold climate models. For example, pulling a random manufactuerer, Mitsubishi brands their cold climate line "Hyper" and specs out much better efficiency down to -13F temperatures.
  - ethagknight 8y ago
    My HVAC is utter crap ≠ HVACs are crap. This is a great example of a worthless anecdotal comment that distracts from the point. Give us some more detail like model number or install date? Is it a modern unit not coming close to its advertise performance? Is it appropriately sized for your condo?
  - bluGill 8y ago
    A heatpump blows warm air not hot. Most people expect hot air from their vents and so are confused by not getting it. My heat pump in Iowa worked just fine to keep the house comfortable down to 14F (the system automatically switched to backup heat colder than that). Something is wrong with your system if it can't do that. I couldn't begin to guess what though.
- isbjorn16 8y ago
  Speaking specifically of heat pump water heaters - I will absolutely cede that they are more efficient, but holy _crap_ they take forever to recover. My previous water heater was a 70 gallon conventional electric. It didn't have quite enough hot water to get everyone through showering when we had extended family over, but it recovered reasonably quickly so that we only had ~ 30 minute gaps or so in waiting between the person who felt it get cold first and the next in line. Pretty reasonable. Ours was about 20 years old and we also wanted a bunch of other plumbing work done, so we decided we may as well bite the bullet and get a new one. We replaced ours with an 80 gallon hybrid (electric + heatpump). We figured the extra 10 gallons would probably actually completely negate the need to wait between showers for those last people, so we were super stoked. The reality is it exacerbated matters drastically. The refresh rate on our old water heater apparently was almost good enough to keep up with sustained usage between 7 people. The current one runs out of hot water out of 2 and takes well over an hour to even have enough hot water for a single next shower. I tend to take pretty long showers in the morning - like 20 minutes or so. I use a CPAP and clean my mask, I brush my teeth, I brush my invisaligns (which is why I do my teeth in the shower - it's impossible to scrub invisalign braces and not fling infinite toothpaste flotsam everywhere), and I wash myself (which honestly is the fastest part - I keep my hair next to non existent and just do a body wash from head to toe, rinse, and leave. The point is that the 20 minutes I spend in the shower, with a low flow water reducing showerhead, still means my wife gets to get a cold shower an entire hour later. However, from what I was told, there are new regulations regarding electric hot water heaters; namely, you can't get over 60 gallon tanks with a conventional hot water heater. Apparently people are getting around this by buying light industrial ones, which don't have this limitation. I wanted to try to cut down on energy usage AND I was keen on the idea of having a heat pump in my garage, transferring the heat in said garage into the water, thus leaving the garage cooler. It's like a really inefficient air conditioner for my garage, but given how hot that gets in the summer and how little I spend time in my woodworking shop, I wanted to leap at the chance to maybe make it a better environment. All of this is a long winded way of saying it may win on efficiency, and it does have a number of compelling secondary features about it, but it also still suffers from poor refractory performance. If you have one as well - do you notice issues like this? Perhaps the one I got is faulty or just ... not good.
  - 8y ago
- SigmundA 8y ago
  Electric resistance heat is 1.0 or 100% efficient, all electricity put into the heating element is converted to heat. Not sure if your trying to include power plant conversion and transmission losses, but that gets very complicated. Gas units range in efficiency based on how much heat they recover from the exhaust, 80% is typical, 90+ for a condensing heater with two stage heat recovery. The problem with condensing is the exhaust gases fall below condensation point for the water vapor created during combustion requiring a condensate drain complicating the install location and cost significantly. Heat pumps are over unity they use some energy to move more energy from the environment into the water, they are basically solar assisted resistance heaters. The energy cost difference is not as clear when comparing a heat pump tank storage unit to a gas on demand tankless they are both typically more cost effective than tank storage electric or gas. I have a 80% efficient tankless natural gas, it is wonderful having endless hot water at all faucets concurrently with a family of 4 plus occasional guests. I wanted to go condensing 90+ efficient but it was to complex based on its install location.
  - fulafel 8y ago
    In typical mounting of electric radiator elements, some of the heat might be absorbed in the walls and windows that are facing outdoors. Then the temperature gardient in the wall/window insulation is steeper, leaking more heat outside. So yes, all the energy is converted into heat, but some of it may be heating the outdoors.
    - SigmundA 8y ago
      This has nothing to do with the type of heater but rather the placement. When talking about relative efficiency you look at the heater itself and not common factors as they can be eliminated from the equation.
tomohawk 8y ago
Heat pumps based on air do not work economically below 40F. Neighbor has one and his electric bill is out of control in the winter. Drop below freezing and the system will pump heat out of the house to de-ice the external unit, and electric resistance heat will kick in. We have dual fuel, so we burn oil below 40F. Works great. If you can afford it, putting in a heat pump that works off of ground will eventually pay for itself.
- vvanders 8y ago
  How new is the heat-pump? We put one in our house this year and been in the 20-40F range for the last 2 months. So far we haven't seen resistive heating kick in more than when it's above 40F.
- Robotbeat 8y ago
  Doesn't work below 40F? That's just not true unless you're using a very poor, cheap heat pump. Modern, high efficiency air source heat pumps can work well even below 0F.
- willvarfar 8y ago
  Outside temperature definitely affects an air heat-pump's performance, but the ones common in sweden stop being effective at -10c or even -15c. I have a ground-source pump and its brilliant. But plenty of my neighbours are happy enough with air-source pumps.
  - bklyn11201 8y ago
    Recent conversations with net-zero builders indicate that ground-source pumps are being pushed away for most single-family residential projects in favor of the new-generation, highly-effective, cold-climate air-source heat pumps. Mitsubishi and Fujitsu seem to have a large lead in cold-climate air-source heat pumps that are rated effective to -13F (anecdotal reports greenbuildingadvisor.com indicating effectiveness down to -20F). Yet, talking to builders, contractors, friends, and family, you will often still hear old tales about how "heat pumps aren't effective below 40F". Heat pumps come in a wide range of flavors. It's clearly possible to heat your house exclusively using an air-source heat pump in most USA climates.
fastbeef 8y ago
Modern heat pumps are truly a marvel, especially when you get one designed for heating and not just an AC-unit running in reverse. They're very common in Sweden and can easily warm an entire house even at -20 C. Whenever this topic appears on HN I want to get on my soapbox and yell "INSULATION PEOPLE, INSULATION!". Most houses I've seen outside of the Nordics have shit-all insulation and that's depsite it working both ways - house stays cooler in the summer. It's such a small cost to add during construction and it pays off in a year or two, tops. Modern, newly built houses in Sweden, way above the arctic circle, require less than 5000 kWh per year to heat and generate hotwater.
- NeedMoreTea 8y ago
  UK is finally getting there, but we have a very long way to go. Our housing is mostly bloody awful for insulation, draught proofing, and sensible forms of heating. The very worst is anything 60s-90s, particularly if there's a flat roof. We've only been hearing about how well Sweden and other Nordics do insulation, community heating, heat pumps etc for about 40 or 50 years. Various combined heat and power, insulation, passive houses etc would turn up on things like Tomorrow's World (60s-90s technology TV series) fairly often, like it was some sort of breakthrough new discovery. Well it was, to the UK, I guess.
  - pjc50 8y ago
    The UK's deeply dysfunctional planning/housing system contributes to that. Land is expensive, building your own home on purchased land quite rare due to bureaucracy of planning permission.
  - noir_lord 8y ago
    I live in a flat that was built mid 90's and it's a disaster from a heating and cooling point of view. Ventiliation is shit, one side never gets the sun so the outer wall is cold which means condensation is a daily battle, I basically run a big heavy duty dehumidifier from mid-autumn to mid-spring (at least its not a net loss, it dumbs a fair amount of heat as a byproduct so I can run the heaters less). It's rented though so not something I can fix until I buy a property in a few years.
donkeyd 8y ago
Big issue in the Netherlands (and probably more countries) will be noise pollution though. Houses here are built really close to each other and the combined noise could be an issue. Neighbours are already complaining about the noise from my parents heat pump.
- toomuchtodo 8y ago
  Do you have any centralized district heating in NL similar to in Sweden?
  - donkeyd 8y ago
    In some newly developed areas we do, but most of the heating in the Netherlands runs on natural gas, since we have so much of it.
    - Yetanfou 8y ago
      ...or rather since we had so much of it. The Dutch government is, after all, making noises about the country going 'off the gas' to become 'gas-free' around 2050. As to whether this really will happen and what it actually will entail - are they going to tell industry to stop using gas as well? - remains to be seen but heat pumps are being pushed as one of the alternatives to gas. Given that nearly all central heating in the Netherlands is water-borne and most systems are designed for a high working temperature - between 70 and 90 degrees - the switch to a heat pump will need more than just a simple swap of the prime heater. Low-temperature (possibly fan-assisted) convectors can be used to replace high-temperature radiators, these can also be used to cool down the house in summer. More insulation is needed in most Dutch houses, even newly built houses sometimes don't have more than 10 cm of foam insulation in the roof and less in the walls.
  - Freak_NL 8y ago
    I expect this to become more common in cities in the Netherlands for existing neighbourhoods built between the 1920s and 1970s. These older houses (of which there are a lot) are difficult, if not economically infeasible, to convert to heat pump heated houses. Insulation can be done to some extent — I've managed to insulate my 1930s house to a thermal resistance of R⒞ = 4,5 m²K/W in the attic and parts of the level above the ground storey, but this is not realistic for the ground floor — but it is not feasible to reach the R⒞ ≥ 6 m²K/W mandated for newly built houses. The noise is a serious problem too. In terrace housing you simply can't expect everyone to install these things and maintain a sane level of quiet. Better to solve this problem by insulating as much as possible, and to provide heat by collective solutions such as geothermal heat on a neighbourhood scale. The current 'let-the-citizens-solve-this-themselves' approach of our government is making homeowners insecure about what to do though.
torpfactory 8y ago
It's worth noting that the temperatures at which heat pumps function well is more of a design choice than a fundamental limitation. You can make a heat pump which functions down to -20C with HFC refrigerants, maybe lower with some other types. However, you may choose to optimize 'mild weather' performance by forgoing low temperature performance. Modern heat pumps work pretty darn well across a range of temperatures. What we really need are heat pumps which do not use HFC refrigerants - they are a pretty significant cause of global warming, and there are other options (CO2 being one of the best, in my opinion). https://www.drawdown.org/solutions/materials/refrigerant-management https://www.drawdown.org/solutions/materials/refrigerant-man...
baybal2 8y ago
Question to other engineers here. We were designing a "smart wine chiller" recently. Ran into a problem - small size sealed compressors are very, very, hard to come by, and if you want one that is also quiet, prepare your arm and a leg. We ended up having the compressor custom made by local fridge factory. Engineers there told us that designing such a small piston compressor and hoping it to be reliable and efficient is a futile effort, but they still did their best. Is between piston driven compressors, and peltier coolers, is there just anything on the market? More quiet and robust than small piston compressors, and more efficient than peltier?
- Baeocystin 8y ago
  Sure. Look at fridges built for RVs and the like. The ones that can run off of propane as well as electricity. They use the absorption cycle, so while they have worse efficiency than a compressor-based system, they will easily beat thermoelectrics from a cost/BTU standpoint. Many have no moving parts, so noise and vibration are minimal. https://en.m.wikipedia.org/wiki/Absorption_refrigerator https://en.m.wikipedia.org/wiki/Absorption_refrigerator
- rini17 8y ago
  Stirling engine?
- FiatLuxDave 8y ago
  Have you looked into compressors for mobile applications, such as a Danfoss 35F? I mean, technically its a reciprocating piston compressor, but the smaller ones can be quite quiet. https://assets.danfoss.com/documents/DOC223886433214/DOC223886433214.pdf https://assets.danfoss.com/documents/DOC223886433214/DOC2238...
  - baybal2 8y ago
    Not small enough. The one we custom ordered is like 6x6cm (piston itself is only 9mm,) and they even managed to make it run on DC. The factory and their tech level truly surprised me despite them being complete nonames, who mostly do fridge part kits for local production in 3rd world countries. This reminded me yet another time of just how underappreciated are those no-name factories. They truly are the real unsung heroes of our globalisation era.
deleted 8y ago
[deleted]
cdubzzz 8y ago
The most recent season of This Old House, Jamestown Net-Zero House[0], is worth a watch for folks interested in learning more about home efficiency techniques in general. [0] https://www.thisoldhouse.com/watch/jamestown-net-zero-house https://www.thisoldhouse.com/watch/jamestown-net-zero-house
- SketchySeaBeast 8y ago
  I'll add that to my must watch list - I live in an old (1930's) home that leaks heat like crazy, so I'm interested in possible retrofits, though I'm a bit concerned that it'll be a monstrous task.
  - cdubzzz 8y ago
    Yeah, in this case they spare no expense with the upgrades. It's really cool to see and learn about but probably not realistic for anyone not ready to invest a lot of money. As much I love TOH, I do wish they would take on more "budget" projects or at least talk a little bit about costs. But ATOH kind of fills that gap.
  - antisthenes 8y ago
    Pretty much anything in a pre-1980 house will a nightmare to retrofit. Houses in the 1945 to 1965 era (in the US) especially were mostly built as quickly and as cheaply as possible. The concept of energy efficiency didn't really exist before the 1973 oil crisis as oil use per capita was steadily going up and up until then. Energy Star, for example, was only established in 1992. Energy efficiency is very very recent.
    - SketchySeaBeast 8y ago
      Oh yeah, my home was built strictly to keep the frozen death out, I have to throw a big furnace at it to be livable while it exhausts everything through its impossible to insulate roof.
  - throw0101a 8y ago
    Good channel on building science: * https://www.youtube.com/user/MattRisinger https://www.youtube.com/user/MattRisinger He does renos and new build in Austin, so mostly worries about keep heat out, but air tightness and insulation apply everywhere.
danimal88 8y ago
I build controls for many heatpumps (https://flair.co/pages/mini-splits-and-window-units https://flair.co/pages/mini-splits-and-window-units) and follow the space pretty closely. They are truly magical with respect to pumping heat from the cold outdoors into a warm indoor space. That said, the efficiency gains on the energy consumption side are admirable. From a global warming standpoint however, heat pumps are actually still pretty bad because of the refrigerants.(https://www.drawdown.org/solutions/materials/refrigerant-management https://www.drawdown.org/solutions/materials/refrigerant-man...) This is not an issue with more traditional electric heaters (resistive or otherwise). The real calculus for those concerned with global warming (I understand this was not the topic of the writeup) is the net effect of these systems over their lifetime - aka, Energy Saved and its GHG effect via leveraging a heat pump vs the GHG effect from the release of the refrigerant. Perhaps the real win would be a refrigerant recovery business that is...air tight :)
- fallingfrog 8y ago
  There is a such thing as a solid state heat pump which operates without refrigerants or any moving parts. They are not as efficient as regular heat pumps, though, at least currently. But maybe that could change. https://www.researchgate.net/publication/270646164_Efficiency_of_a_thermoelectric_heat_pump https://www.researchgate.net/publication/270646164_Efficienc... https://www.energy.gov/eere/buildings/downloads/high-efficiency-solid-state-heat-pump-module https://www.energy.gov/eere/buildings/downloads/high-efficie... https://arpa-e.energy.gov/?q=slick-sheet-project/high-efficiency-solid-state-cooling-technologies https://arpa-e.energy.gov/?q=slick-sheet-project/high-effici...
  - vanderZwan 8y ago
    You reminded me of Lonnie Johnson's "Johnson Thermo-Electrochemical Convertor", or JTEC for short. On their webpage they say they are aiming for 85% Carnot efficiency for converting heat to electricity, and it can run in reverse to become a heat pump[0][1]. Sadly, I can find no third-party measurements of how well it performs in practice, so I guess it's still in the R&D phase. I did find a nice talk by Johnson about the project though[2]. [0] http://www.johnsonems.com/about/ http://www.johnsonems.com/about/ [1] http://www.johnsonems.com/benefits/ http://www.johnsonems.com/benefits/ [2] https://www.youtube.com/watch?v=F6FUk2E9-68 https://www.youtube.com/watch?v=F6FUk2E9-68
DoofusOfDeath 8y ago
Slightly OT, but.. I live in a temperate climate. I've sometimes wondered if it would be worthwhile to adjust house construction so that: - In the summer, the kitchen refrigerator's heating coils touch outdoors air instead of indoors air. - In the winter, the refrigerator's cooling is provided, at least partially, by the outside air. - In the summer (only), the kitchen oven vents into the outdoors rather than into the kitchen. - In the winter (only), a heat exchanger recovers some of the heat from our electric clothes dryer.
- MrStonedOne 8y ago
  One idea i've had is to just make a central heatpump system. Why does a fridge need its own compressor that vents heat back to the house even in the summer? Why can't your water heater use that same heat instead? This all started because my brother used to redirect dryer air back into his apartment during the winter, using a saw dust sock to keep the lint contained. I started thinking about how heat could be better managed if everything used a central ac/heat pump. heat generated by the fridge, your computer, the ac, etc could be used by your water heater, your heater heater, your dryer. Add heat recovery systems to your dryer, and your drains, and bam super sustainability.
  - throw0101a 8y ago
    There are AC units that do this: pipe the heat to the hot water tank first. * https://www.rheem.com/innovations/innovation_residential/integrated_systems/ https://www.rheem.com/innovations/innovation_residential/int... * https://techcrunch.com/2011/06/12/air-conditioning-water-heater/ https://techcrunch.com/2011/06/12/air-conditioning-water-hea... Once the water tank reaches its max temperature, then heat is dumped outside. The practical problem is that you need to have the different parts work together, and fridge makers don't don't to hot water makers.
    - henrikschroder 8y ago
      In the apartment I live in now in Hawaii, the utility closet has the AC unit mounted right above the water heater. They are not connected at all. And this is in a building that is about ten years old. I have no words, it's so fucking stupid.
jstapels 8y ago
Maybe I'm missing something, but no matter how efficient a heat pump is, doesn't it have a limit on how much heat it can pump as ∆T increases? Even here in western NY we occasionally have to deal with -10 F (-23 C) weather for a week and so that's a pretty big temperature delta if you want a comfortable 68 F (20 C). Combine that with the fact that gas is still quite a bit cheaper per unit of energy than electricity (in my area) and a heat pump just doesn't make any sense. The ability to have a single unit heat and cool is definitely nice, but without a geothermal-like ambient temperature to work with, I just don't see this working out, even with all the insulation in the world.
- thinkcontext 8y ago
  Elsewhere in this thread fastbeef says "They're very common in Sweden and can easily warm an entire house even at -20 C [-4F]." So, it can work though at -10F you would probably need supplemental heat. Of course in Sweden residential natural gas prices are almost 4 times what they are in the US so its a very different economic decision. Shows what a carbon tax can do. https://www.statista.com/statistics/702735/household-natural-gas-prices-in-selected-countries/ https://www.statista.com/statistics/702735/household-natural...
  - fastbeef 8y ago
    Keep in mind that 1 kW worth of gas gives out 0.9 kW, while 1 kW of electricity entered into a heat pump gives out AT LEAST 3 kW of heat. Also, no one uses gas for heating in Sweden. Very old houses down town use it for cooking only.
- patall 8y ago
  Aren't you supposed (at least when you have a single house) to couple it with a geothermal energy so that your external temperature is always the years median of around 9 C?
  - Yetanfou 8y ago
    Geothermal or some other relatively stable source. If you live near a lake you can drop a water sling on the bottom and use that as a source, it is also possible to bury a sling in a field next to your house at a frost-free depth (2.5 m or deeper in the north of Sweden) and use that. In built-up areas with many houses using deep wells for heat extraction there have been cases of those wells freezing up, this can also happen if the system is under-dimensioned.
agumonkey 8y ago
Talking about fridge, do you guys know if there's a use for heat collector out of the fridge radiator ? Right now it's mostly wasted heat, or IIUC even worse, in closed spaces it forces more energy consumption from the fridge.
- Scoundreller 8y ago
  It’s not a lot of heat in residential use cases. Fridges for enclosed spaces should have a fan to push the air around. The more space you give a radiating fridge, the more efficient it’ll be. I do wonder if a small fan on a radiating fridge would pay for itself if controlled by temperature probes.
deleted 8y ago
[deleted]
lxmorj 8y ago
I had a "geothermal" heat pump put in to my house via Google's Dandelion and it works great so far. It's been -30F at my house but the ground sourcing renders that moot. As far as I can tell their innovation was going vertical so they didn't need to dredge up a huge loop of land. The pump has two channels that go down 300ft each and the fluid runs down and back up each bore hole. It provides heat (working great so far), hot water (but I already had an electric inline heater, so I can't speak to the slow-refill issues), and A/C in the summer. I'm curious to see how well the AC performs!
- WhompingWindows 8y ago
  Is your house entirely electric? That's a dream of mine but I don't know how realistic it is, especially in my state where there aren't good incentives
  - lxmorj 8y ago
    No, we have a propane fireplace that used to heat the whole house (terribly inefficient, the previous owners used it as a summer-only property) and we now run that just on colder evenings to make the house a bit cozier. We also live way out on a dead-end of the grid, so outages are common and sometimes long enough to need backup heat to keep the pipes unfrozen. I also eventually want to swap out my electric stove with propane, though my last experience with induction stoves has me considering them too. Electric conduction stoves just aren't fun to cook on... I'd love to convert the fireplace to run on hydrogen that I create myself via electrolysis one day (and have 0 emission tiki torches on the porch) but I'm not sure how safely I could pull that off! It's a bit of a moot point until I get solar installed anyways, since my power isn't coming from a clean source as it is.
- jonny_eh 8y ago
  I don't see Google mentioned anywhere on their site. What's the relation? https://dandelionenergy.com/ https://dandelionenergy.com/
  - lxmorj 8y ago
kev009 8y ago
This was a really lucid and engaging piece on the mathematics of these systems. I wish more math teachings would be done in long form like this instead of the cargo culted textbooks that are put out by major publishers.
wazoox 8y ago
BTW the whole blog is a goldmine. Rim-filled of evidence of how our energy-hungry civilisation can't last :)
Pulcinella 8y ago
Also heat pumps work a miracle because the fluids they use are NOT perfect, platonic fluids that obey the ideal gas law. https://en.m.wikipedia.org/wiki/Joule–Thomson_effect https://en.m.wikipedia.org/wiki/Joule–Thomson_effect
gdubs 8y ago
We have a heat pump and I love it, but for the longest time it was beyond me how the thing works. When it finally clicked, it blew my mind. The idea that there’s heat to be found, even on a cold day, is amazing. It’s actually what made me want to understand thermodynamics more deeply.
elmolino89 8y ago
The most commonly used heat pumps utilize the external air as a source. Which is quite silly on a sunny but chilly day. Even 2m^2 black & isolated panel exposed to the sun will be tens of degrees warmer. Just putting the heat exchanger in some micro glass house with a valve would increase the efficiency.

251 points by mhb 8y ago

Heat pump water heaters have efficiency of 3x+ vs .9 or so for gas or electric resistance. If you have have an electric resistance water heater get a heat pump model. It costs more upfront but will pay for itself in not too many years, especially considering many places have incentives. In the US natural gas is so cheap the economics are not nearly as good, it takes a long time to pay back the difference but does eventually. With even a modest carbon price obviously this would be different. If you are going to replace your furnace or AC get a quote for replacing with a heat pump instead, either central or multiple mini-split heads. Heat pumps have gotten a lot better in recent years, particularly being able to work in cold weather down to 0F or below, expanding northward the places they are practical.

heatpumper 8y ago
I can tell you that my heat pump set up at my condo is absolute trash here in Northern Ohio. Running 24/7 it is unable to get my condo above 62 deg F. It blows cold-ish air under an outside temperature of 40 deg F. Windows are new, doors are weather stripped. I'm having it run it in supplemental mode just to get it above 65 deg F.
- Robotbeat 8y ago
  There are some very bad heat pumps out there. The difference in usability varies dramatically. Some heat pumps will produce heat even below 0F. Others are crappy at anything under 40F.
- mr337 8y ago
  Something must be wrong. My heatpump will run you out of the house at 40F outside. Below 20F it puts out so so heat but can maintain. Below 10F electric heat is needed from time to time to maintain. Below 0F heat pump is worthless. Edit: Wanted to add my is air heatpump, not ground source. Also reading other comments looks like heatpumps range from amazing to utter crap.
- toomuchtodo 8y ago
  1. Have an HVAC tech inspect it (~$100-150). 2. If it's not a coolant or compressor issue, you might need to replace it (either because it is undersized or end of life). Your local government or utility might have energy incentives for replacing it. There are also federal tax credit incentives [1]. [1] https://www.energystar.gov/about/federal_tax_credits/air_source_heat_pumps https://www.energystar.gov/about/federal_tax_credits/air_sou...
- czechdeveloper 8y ago
  Your heat pump just does not have sufficient capacity. It's not problem of heat pump per se.
- jandrese 8y ago
  Your heat pump is probably about 20 years old and on its last legs. Newer ones are amazing. We lopped $40 off of our electricity bill each month in the winter and $20 in the summer after we replaced ours. It pays for itself after a few years.
- dashundchen 8y ago
  From what I understand, cold weather air source heat pumps are relatively new to the market, at least in the US. Previously they were designed more for cooling loads and were essentially space heaters with an COP of 1 in cold weather. Many manufactuers seem to offer warm and cold climate models. For example, pulling a random manufactuerer, Mitsubishi brands their cold climate line "Hyper" and specs out much better efficiency down to -13F temperatures.
- ethagknight 8y ago
  My HVAC is utter crap ≠ HVACs are crap. This is a great example of a worthless anecdotal comment that distracts from the point. Give us some more detail like model number or install date? Is it a modern unit not coming close to its advertise performance? Is it appropriately sized for your condo?
- bluGill 8y ago
  A heatpump blows warm air not hot. Most people expect hot air from their vents and so are confused by not getting it. My heat pump in Iowa worked just fine to keep the house comfortable down to 14F (the system automatically switched to backup heat colder than that). Something is wrong with your system if it can't do that. I couldn't begin to guess what though.
isbjorn16 8y ago
Speaking specifically of heat pump water heaters - I will absolutely cede that they are more efficient, but holy _crap_ they take forever to recover. My previous water heater was a 70 gallon conventional electric. It didn't have quite enough hot water to get everyone through showering when we had extended family over, but it recovered reasonably quickly so that we only had ~ 30 minute gaps or so in waiting between the person who felt it get cold first and the next in line. Pretty reasonable. Ours was about 20 years old and we also wanted a bunch of other plumbing work done, so we decided we may as well bite the bullet and get a new one. We replaced ours with an 80 gallon hybrid (electric + heatpump). We figured the extra 10 gallons would probably actually completely negate the need to wait between showers for those last people, so we were super stoked. The reality is it exacerbated matters drastically. The refresh rate on our old water heater apparently was almost good enough to keep up with sustained usage between 7 people. The current one runs out of hot water out of 2 and takes well over an hour to even have enough hot water for a single next shower. I tend to take pretty long showers in the morning - like 20 minutes or so. I use a CPAP and clean my mask, I brush my teeth, I brush my invisaligns (which is why I do my teeth in the shower - it's impossible to scrub invisalign braces and not fling infinite toothpaste flotsam everywhere), and I wash myself (which honestly is the fastest part - I keep my hair next to non existent and just do a body wash from head to toe, rinse, and leave. The point is that the 20 minutes I spend in the shower, with a low flow water reducing showerhead, still means my wife gets to get a cold shower an entire hour later. However, from what I was told, there are new regulations regarding electric hot water heaters; namely, you can't get over 60 gallon tanks with a conventional hot water heater. Apparently people are getting around this by buying light industrial ones, which don't have this limitation. I wanted to try to cut down on energy usage AND I was keen on the idea of having a heat pump in my garage, transferring the heat in said garage into the water, thus leaving the garage cooler. It's like a really inefficient air conditioner for my garage, but given how hot that gets in the summer and how little I spend time in my woodworking shop, I wanted to leap at the chance to maybe make it a better environment. All of this is a long winded way of saying it may win on efficiency, and it does have a number of compelling secondary features about it, but it also still suffers from poor refractory performance. If you have one as well - do you notice issues like this? Perhaps the one I got is faulty or just ... not good.
- 8y ago
SigmundA 8y ago
Electric resistance heat is 1.0 or 100% efficient, all electricity put into the heating element is converted to heat. Not sure if your trying to include power plant conversion and transmission losses, but that gets very complicated. Gas units range in efficiency based on how much heat they recover from the exhaust, 80% is typical, 90+ for a condensing heater with two stage heat recovery. The problem with condensing is the exhaust gases fall below condensation point for the water vapor created during combustion requiring a condensate drain complicating the install location and cost significantly. Heat pumps are over unity they use some energy to move more energy from the environment into the water, they are basically solar assisted resistance heaters. The energy cost difference is not as clear when comparing a heat pump tank storage unit to a gas on demand tankless they are both typically more cost effective than tank storage electric or gas. I have a 80% efficient tankless natural gas, it is wonderful having endless hot water at all faucets concurrently with a family of 4 plus occasional guests. I wanted to go condensing 90+ efficient but it was to complex based on its install location.
- fulafel 8y ago
  In typical mounting of electric radiator elements, some of the heat might be absorbed in the walls and windows that are facing outdoors. Then the temperature gardient in the wall/window insulation is steeper, leaking more heat outside. So yes, all the energy is converted into heat, but some of it may be heating the outdoors.
  - SigmundA 8y ago
    This has nothing to do with the type of heater but rather the placement. When talking about relative efficiency you look at the heater itself and not common factors as they can be eliminated from the equation.

tomohawk 8y ago

Heat pumps based on air do not work economically below 40F. Neighbor has one and his electric bill is out of control in the winter. Drop below freezing and the system will pump heat out of the house to de-ice the external unit, and electric resistance heat will kick in. We have dual fuel, so we burn oil below 40F. Works great. If you can afford it, putting in a heat pump that works off of ground will eventually pay for itself.

vvanders 8y ago
How new is the heat-pump? We put one in our house this year and been in the 20-40F range for the last 2 months. So far we haven't seen resistive heating kick in more than when it's above 40F.
Robotbeat 8y ago
Doesn't work below 40F? That's just not true unless you're using a very poor, cheap heat pump. Modern, high efficiency air source heat pumps can work well even below 0F.
willvarfar 8y ago
Outside temperature definitely affects an air heat-pump's performance, but the ones common in sweden stop being effective at -10c or even -15c. I have a ground-source pump and its brilliant. But plenty of my neighbours are happy enough with air-source pumps.
- bklyn11201 8y ago
  Recent conversations with net-zero builders indicate that ground-source pumps are being pushed away for most single-family residential projects in favor of the new-generation, highly-effective, cold-climate air-source heat pumps. Mitsubishi and Fujitsu seem to have a large lead in cold-climate air-source heat pumps that are rated effective to -13F (anecdotal reports greenbuildingadvisor.com indicating effectiveness down to -20F). Yet, talking to builders, contractors, friends, and family, you will often still hear old tales about how "heat pumps aren't effective below 40F". Heat pumps come in a wide range of flavors. It's clearly possible to heat your house exclusively using an air-source heat pump in most USA climates.

fastbeef 8y ago

Modern heat pumps are truly a marvel, especially when you get one designed for heating and not just an AC-unit running in reverse. They're very common in Sweden and can easily warm an entire house even at -20 C. Whenever this topic appears on HN I want to get on my soapbox and yell "INSULATION PEOPLE, INSULATION!". Most houses I've seen outside of the Nordics have shit-all insulation and that's depsite it working both ways - house stays cooler in the summer. It's such a small cost to add during construction and it pays off in a year or two, tops. Modern, newly built houses in Sweden, way above the arctic circle, require less than 5000 kWh per year to heat and generate hotwater.

NeedMoreTea 8y ago
UK is finally getting there, but we have a very long way to go. Our housing is mostly bloody awful for insulation, draught proofing, and sensible forms of heating. The very worst is anything 60s-90s, particularly if there's a flat roof. We've only been hearing about how well Sweden and other Nordics do insulation, community heating, heat pumps etc for about 40 or 50 years. Various combined heat and power, insulation, passive houses etc would turn up on things like Tomorrow's World (60s-90s technology TV series) fairly often, like it was some sort of breakthrough new discovery. Well it was, to the UK, I guess.
- pjc50 8y ago
  The UK's deeply dysfunctional planning/housing system contributes to that. Land is expensive, building your own home on purchased land quite rare due to bureaucracy of planning permission.
- noir_lord 8y ago
  I live in a flat that was built mid 90's and it's a disaster from a heating and cooling point of view. Ventiliation is shit, one side never gets the sun so the outer wall is cold which means condensation is a daily battle, I basically run a big heavy duty dehumidifier from mid-autumn to mid-spring (at least its not a net loss, it dumbs a fair amount of heat as a byproduct so I can run the heaters less). It's rented though so not something I can fix until I buy a property in a few years.

donkeyd 8y ago

Big issue in the Netherlands (and probably more countries) will be noise pollution though. Houses here are built really close to each other and the combined noise could be an issue. Neighbours are already complaining about the noise from my parents heat pump.

toomuchtodo 8y ago
Do you have any centralized district heating in NL similar to in Sweden?
- donkeyd 8y ago
  In some newly developed areas we do, but most of the heating in the Netherlands runs on natural gas, since we have so much of it.
  - Yetanfou 8y ago
    ...or rather since we had so much of it. The Dutch government is, after all, making noises about the country going 'off the gas' to become 'gas-free' around 2050. As to whether this really will happen and what it actually will entail - are they going to tell industry to stop using gas as well? - remains to be seen but heat pumps are being pushed as one of the alternatives to gas. Given that nearly all central heating in the Netherlands is water-borne and most systems are designed for a high working temperature - between 70 and 90 degrees - the switch to a heat pump will need more than just a simple swap of the prime heater. Low-temperature (possibly fan-assisted) convectors can be used to replace high-temperature radiators, these can also be used to cool down the house in summer. More insulation is needed in most Dutch houses, even newly built houses sometimes don't have more than 10 cm of foam insulation in the roof and less in the walls.
- Freak_NL 8y ago
  I expect this to become more common in cities in the Netherlands for existing neighbourhoods built between the 1920s and 1970s. These older houses (of which there are a lot) are difficult, if not economically infeasible, to convert to heat pump heated houses. Insulation can be done to some extent — I've managed to insulate my 1930s house to a thermal resistance of R⒞ = 4,5 m²K/W in the attic and parts of the level above the ground storey, but this is not realistic for the ground floor — but it is not feasible to reach the R⒞ ≥ 6 m²K/W mandated for newly built houses. The noise is a serious problem too. In terrace housing you simply can't expect everyone to install these things and maintain a sane level of quiet. Better to solve this problem by insulating as much as possible, and to provide heat by collective solutions such as geothermal heat on a neighbourhood scale. The current 'let-the-citizens-solve-this-themselves' approach of our government is making homeowners insecure about what to do though.

torpfactory 8y ago

It's worth noting that the temperatures at which heat pumps function well is more of a design choice than a fundamental limitation. You can make a heat pump which functions down to -20C with HFC refrigerants, maybe lower with some other types. However, you may choose to optimize 'mild weather' performance by forgoing low temperature performance. Modern heat pumps work pretty darn well across a range of temperatures. What we really need are heat pumps which do not use HFC refrigerants - they are a pretty significant cause of global warming, and there are other options (CO2 being one of the best, in my opinion). https://www.drawdown.org/solutions/materials/refrigerant-management https://www.drawdown.org/solutions/materials/refrigerant-man...

baybal2 8y ago

Question to other engineers here. We were designing a "smart wine chiller" recently. Ran into a problem - small size sealed compressors are very, very, hard to come by, and if you want one that is also quiet, prepare your arm and a leg. We ended up having the compressor custom made by local fridge factory. Engineers there told us that designing such a small piston compressor and hoping it to be reliable and efficient is a futile effort, but they still did their best. Is between piston driven compressors, and peltier coolers, is there just anything on the market? More quiet and robust than small piston compressors, and more efficient than peltier?

Baeocystin 8y ago
Sure. Look at fridges built for RVs and the like. The ones that can run off of propane as well as electricity. They use the absorption cycle, so while they have worse efficiency than a compressor-based system, they will easily beat thermoelectrics from a cost/BTU standpoint. Many have no moving parts, so noise and vibration are minimal. https://en.m.wikipedia.org/wiki/Absorption_refrigerator https://en.m.wikipedia.org/wiki/Absorption_refrigerator
rini17 8y ago
Stirling engine?
FiatLuxDave 8y ago
Have you looked into compressors for mobile applications, such as a Danfoss 35F? I mean, technically its a reciprocating piston compressor, but the smaller ones can be quite quiet. https://assets.danfoss.com/documents/DOC223886433214/DOC223886433214.pdf https://assets.danfoss.com/documents/DOC223886433214/DOC2238...
- baybal2 8y ago
  Not small enough. The one we custom ordered is like 6x6cm (piston itself is only 9mm,) and they even managed to make it run on DC. The factory and their tech level truly surprised me despite them being complete nonames, who mostly do fridge part kits for local production in 3rd world countries. This reminded me yet another time of just how underappreciated are those no-name factories. They truly are the real unsung heroes of our globalisation era.

deleted 8y ago

[deleted]

cdubzzz 8y ago

The most recent season of This Old House, Jamestown Net-Zero House[0], is worth a watch for folks interested in learning more about home efficiency techniques in general. [0] https://www.thisoldhouse.com/watch/jamestown-net-zero-house https://www.thisoldhouse.com/watch/jamestown-net-zero-house

SketchySeaBeast 8y ago
I'll add that to my must watch list - I live in an old (1930's) home that leaks heat like crazy, so I'm interested in possible retrofits, though I'm a bit concerned that it'll be a monstrous task.
- cdubzzz 8y ago
  Yeah, in this case they spare no expense with the upgrades. It's really cool to see and learn about but probably not realistic for anyone not ready to invest a lot of money. As much I love TOH, I do wish they would take on more "budget" projects or at least talk a little bit about costs. But ATOH kind of fills that gap.
- antisthenes 8y ago
  Pretty much anything in a pre-1980 house will a nightmare to retrofit. Houses in the 1945 to 1965 era (in the US) especially were mostly built as quickly and as cheaply as possible. The concept of energy efficiency didn't really exist before the 1973 oil crisis as oil use per capita was steadily going up and up until then. Energy Star, for example, was only established in 1992. Energy efficiency is very very recent.
  - SketchySeaBeast 8y ago
    Oh yeah, my home was built strictly to keep the frozen death out, I have to throw a big furnace at it to be livable while it exhausts everything through its impossible to insulate roof.
- throw0101a 8y ago
  Good channel on building science: * https://www.youtube.com/user/MattRisinger https://www.youtube.com/user/MattRisinger He does renos and new build in Austin, so mostly worries about keep heat out, but air tightness and insulation apply everywhere.

danimal88 8y ago

I build controls for many heatpumps (https://flair.co/pages/mini-splits-and-window-units https://flair.co/pages/mini-splits-and-window-units) and follow the space pretty closely. They are truly magical with respect to pumping heat from the cold outdoors into a warm indoor space. That said, the efficiency gains on the energy consumption side are admirable. From a global warming standpoint however, heat pumps are actually still pretty bad because of the refrigerants.(https://www.drawdown.org/solutions/materials/refrigerant-management https://www.drawdown.org/solutions/materials/refrigerant-man...) This is not an issue with more traditional electric heaters (resistive or otherwise). The real calculus for those concerned with global warming (I understand this was not the topic of the writeup) is the net effect of these systems over their lifetime - aka, Energy Saved and its GHG effect via leveraging a heat pump vs the GHG effect from the release of the refrigerant. Perhaps the real win would be a refrigerant recovery business that is...air tight :)

fallingfrog 8y ago
There is a such thing as a solid state heat pump which operates without refrigerants or any moving parts. They are not as efficient as regular heat pumps, though, at least currently. But maybe that could change. https://www.researchgate.net/publication/270646164_Efficiency_of_a_thermoelectric_heat_pump https://www.researchgate.net/publication/270646164_Efficienc... https://www.energy.gov/eere/buildings/downloads/high-efficiency-solid-state-heat-pump-module https://www.energy.gov/eere/buildings/downloads/high-efficie... https://arpa-e.energy.gov/?q=slick-sheet-project/high-efficiency-solid-state-cooling-technologies https://arpa-e.energy.gov/?q=slick-sheet-project/high-effici...
- vanderZwan 8y ago
  You reminded me of Lonnie Johnson's "Johnson Thermo-Electrochemical Convertor", or JTEC for short. On their webpage they say they are aiming for 85% Carnot efficiency for converting heat to electricity, and it can run in reverse to become a heat pump[0][1]. Sadly, I can find no third-party measurements of how well it performs in practice, so I guess it's still in the R&D phase. I did find a nice talk by Johnson about the project though[2]. [0] http://www.johnsonems.com/about/ http://www.johnsonems.com/about/ [1] http://www.johnsonems.com/benefits/ http://www.johnsonems.com/benefits/ [2] https://www.youtube.com/watch?v=F6FUk2E9-68 https://www.youtube.com/watch?v=F6FUk2E9-68

DoofusOfDeath 8y ago

Slightly OT, but.. I live in a temperate climate. I've sometimes wondered if it would be worthwhile to adjust house construction so that: - In the summer, the kitchen refrigerator's heating coils touch outdoors air instead of indoors air. - In the winter, the refrigerator's cooling is provided, at least partially, by the outside air. - In the summer (only), the kitchen oven vents into the outdoors rather than into the kitchen. - In the winter (only), a heat exchanger recovers some of the heat from our electric clothes dryer.

MrStonedOne 8y ago
One idea i've had is to just make a central heatpump system. Why does a fridge need its own compressor that vents heat back to the house even in the summer? Why can't your water heater use that same heat instead? This all started because my brother used to redirect dryer air back into his apartment during the winter, using a saw dust sock to keep the lint contained. I started thinking about how heat could be better managed if everything used a central ac/heat pump. heat generated by the fridge, your computer, the ac, etc could be used by your water heater, your heater heater, your dryer. Add heat recovery systems to your dryer, and your drains, and bam super sustainability.
- throw0101a 8y ago
  There are AC units that do this: pipe the heat to the hot water tank first. * https://www.rheem.com/innovations/innovation_residential/integrated_systems/ https://www.rheem.com/innovations/innovation_residential/int... * https://techcrunch.com/2011/06/12/air-conditioning-water-heater/ https://techcrunch.com/2011/06/12/air-conditioning-water-hea... Once the water tank reaches its max temperature, then heat is dumped outside. The practical problem is that you need to have the different parts work together, and fridge makers don't don't to hot water makers.
  - henrikschroder 8y ago
    In the apartment I live in now in Hawaii, the utility closet has the AC unit mounted right above the water heater. They are not connected at all. And this is in a building that is about ten years old. I have no words, it's so fucking stupid.

jstapels 8y ago

Maybe I'm missing something, but no matter how efficient a heat pump is, doesn't it have a limit on how much heat it can pump as ∆T increases? Even here in western NY we occasionally have to deal with -10 F (-23 C) weather for a week and so that's a pretty big temperature delta if you want a comfortable 68 F (20 C). Combine that with the fact that gas is still quite a bit cheaper per unit of energy than electricity (in my area) and a heat pump just doesn't make any sense. The ability to have a single unit heat and cool is definitely nice, but without a geothermal-like ambient temperature to work with, I just don't see this working out, even with all the insulation in the world.

thinkcontext 8y ago
Elsewhere in this thread fastbeef says "They're very common in Sweden and can easily warm an entire house even at -20 C [-4F]." So, it can work though at -10F you would probably need supplemental heat. Of course in Sweden residential natural gas prices are almost 4 times what they are in the US so its a very different economic decision. Shows what a carbon tax can do. https://www.statista.com/statistics/702735/household-natural-gas-prices-in-selected-countries/ https://www.statista.com/statistics/702735/household-natural...
- fastbeef 8y ago
  Keep in mind that 1 kW worth of gas gives out 0.9 kW, while 1 kW of electricity entered into a heat pump gives out AT LEAST 3 kW of heat. Also, no one uses gas for heating in Sweden. Very old houses down town use it for cooking only.
patall 8y ago
Aren't you supposed (at least when you have a single house) to couple it with a geothermal energy so that your external temperature is always the years median of around 9 C?
- Yetanfou 8y ago
  Geothermal or some other relatively stable source. If you live near a lake you can drop a water sling on the bottom and use that as a source, it is also possible to bury a sling in a field next to your house at a frost-free depth (2.5 m or deeper in the north of Sweden) and use that. In built-up areas with many houses using deep wells for heat extraction there have been cases of those wells freezing up, this can also happen if the system is under-dimensioned.

agumonkey 8y ago

Talking about fridge, do you guys know if there's a use for heat collector out of the fridge radiator ? Right now it's mostly wasted heat, or IIUC even worse, in closed spaces it forces more energy consumption from the fridge.

Scoundreller 8y ago
It’s not a lot of heat in residential use cases. Fridges for enclosed spaces should have a fan to push the air around. The more space you give a radiating fridge, the more efficient it’ll be. I do wonder if a small fan on a radiating fridge would pay for itself if controlled by temperature probes.

deleted 8y ago

[deleted]

lxmorj 8y ago

I had a "geothermal" heat pump put in to my house via Google's Dandelion and it works great so far. It's been -30F at my house but the ground sourcing renders that moot. As far as I can tell their innovation was going vertical so they didn't need to dredge up a huge loop of land. The pump has two channels that go down 300ft each and the fluid runs down and back up each bore hole. It provides heat (working great so far), hot water (but I already had an electric inline heater, so I can't speak to the slow-refill issues), and A/C in the summer. I'm curious to see how well the AC performs!

WhompingWindows 8y ago
Is your house entirely electric? That's a dream of mine but I don't know how realistic it is, especially in my state where there aren't good incentives
- lxmorj 8y ago
  No, we have a propane fireplace that used to heat the whole house (terribly inefficient, the previous owners used it as a summer-only property) and we now run that just on colder evenings to make the house a bit cozier. We also live way out on a dead-end of the grid, so outages are common and sometimes long enough to need backup heat to keep the pipes unfrozen. I also eventually want to swap out my electric stove with propane, though my last experience with induction stoves has me considering them too. Electric conduction stoves just aren't fun to cook on... I'd love to convert the fireplace to run on hydrogen that I create myself via electrolysis one day (and have 0 emission tiki torches on the porch) but I'm not sure how safely I could pull that off! It's a bit of a moot point until I get solar installed anyways, since my power isn't coming from a clean source as it is.
jonny_eh 8y ago
I don't see Google mentioned anywhere on their site. What's the relation? https://dandelionenergy.com/ https://dandelionenergy.com/
- lxmorj 8y ago

kev009 8y ago

This was a really lucid and engaging piece on the mathematics of these systems. I wish more math teachings would be done in long form like this instead of the cargo culted textbooks that are put out by major publishers.

wazoox 8y ago

BTW the whole blog is a goldmine. Rim-filled of evidence of how our energy-hungry civilisation can't last :)

Pulcinella 8y ago

Also heat pumps work a miracle because the fluids they use are NOT perfect, platonic fluids that obey the ideal gas law. https://en.m.wikipedia.org/wiki/Joule–Thomson_effect https://en.m.wikipedia.org/wiki/Joule–Thomson_effect

gdubs 8y ago

We have a heat pump and I love it, but for the longest time it was beyond me how the thing works. When it finally clicked, it blew my mind. The idea that there’s heat to be found, even on a cold day, is amazing. It’s actually what made me want to understand thermodynamics more deeply.

elmolino89 8y ago

The most commonly used heat pumps utilize the external air as a source. Which is quite silly on a sunny but chilly day. Even 2m^2 black & isolated panel exposed to the sun will be tens of degrees warmer. Just putting the heat exchanger in some micro glass house with a valve would increase the efficiency.