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The way I like to describe HRV/ERVs to people who don't know about them is: "Imagine you could open a window to get fresh air into your house and stale air out
by danans 2y ago
The way I like to describe HRV/ERVs to people who don't know about them is:
"Imagine you could open a window to get fresh air into your house and stale air out, but when you did so, most of the heat/humidity would stay in during the winter, or stay out during the summer, leaving you just the fresh air".
In terms of the effect on environment inside a house, I usually say:
"Imagine it's always a fresh-air spring day inside your house"
This is a great project. One problem with ERV/HRV systems right now is that they are very expensive niche products. While this system doesn't achieve the extremely high heat recovery efficiencies of counter-flow units, the perfect is the enemy of the good, and this seems like it could be orders of magnitude cheaper.
- llm_trw 2y agoDo you have a source explaining how these work? Naively allowing the air columns to thermally mix would result in the average of the inside and outside temp. So how does this do better?
- LukeShu 2y agoCounter-flow heat exchangers. A parallel-flow heat exchanger would result in the average, as you say; but a counter-flow exchanger means that as the formerly-warm air gets progressively cooler, it is exposed to progressively colder air.
- llm_trw 2y agoDuh, thank you for reminding me air flows in a duct.
- gwd 2y agoI've got a counter-flow heat exchanger, but it looks like they're using a different design: > Each OpenERV TW4 module has a very quiet pair of fans, pointed in opposite directions, and a heat exchanger in a 6 inch pipe, that goes through a wall. The hot, polluted air from inside goes out for 30 seconds, and the heat from it is stored in the heat exchanger. > Then, the fan reverses direction, moving clean air from outdoors to the indoors. On it's way in, it picks up that heat from the heat exchanger. This type of heat exchanger is called a regenerative heat exchanger, or less commonly, a regenerator. The kind shown in the video is a recuperative type, not regenerative. Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses. > The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss. https://www.openerv.ca/learn-more https://www.openerv.ca/learn-more
- empiricus 2y agothe intuition: if the 2 colums flow in the same direction, the final temp is the average. but if the 2 columns flow in the oposite directions it is posible to fully exchange the temperature
- IndrekR 2y agoThe direction of flow is reversed every 30s. The cycle is short enough that due to the thermal mass there is thermal gradient within the heat-exchanger. So it effectively works as counter-flow heat exchanger. Same principle (but continuous flow) is used in rotor ERV: https://en.wikipedia.org/wiki/Heat_recovery_ventilation#Thermal_wheel https://en.wikipedia.org/wiki/Heat_recovery_ventilation#Ther... Heat exchanger there is usually an extruded ceramic grid (ERV) or rolled corrugated aluminum (functions closer to HRV than ERV)
- littlestymaar 2y ago> most of the heat/humidity would stay in during the winter Getting rid of humidity in winter is the main reason why you want to bring fresh air in a house though!
- danans 2y agoNot in cold climates where in winter the air outdoors is very dry. Heating systems in such climates often have integrated humidifiers.
- littlestymaar 2y agoThen it just means that you're overventilating! We emit much more steam (through cooking and showering for instance) than we consume oxygen/emit CO2 so controlling the humidity is the main purpose of ventilation, air renewal comes for free as a byproduct of that.
- manmal 2y agoMy system doesn’t over ventilate - we sometimes have too high CO2 in the bed rooms, but humidity is way too low in winters, sometimes below 30%. That means getting sick more often and having irritated airways at times.
- wilted-iris 2y agoI have not observed this to be true. Ventilating enough to keep CO2 low means sub 20% winter humidity in multiple places I’ve lived.
- szundi 2y agoThe less co2 you have the less effect of air exchange does have. Also more co2 in air also decreases this over the decades.
- cwillu 2y agoFor reference, it is currently 84% relative humidity and -20°C outside where I live. This is about the same absolute humidity as 5% relative humidity at 22°C (i.e., inside).
- colordrops 2y agoI can't find anyone to install one in Los Angeles. Is there a particular climate these are suited for?
- jeroenhd 2y agoAir exchange HRVs are quite common here in the Netherlands. Not exactly ubiquitous, but common enough that you can find them in most recently built apartments at the very least. If you're going to have mechanical ventilation installed, you may as well save a buck in the long run on the heat loss. There's only so much temperature gradient these setups can handle economically, and it's quite possible that the hot LA summers combined with the cool AC air are too much for such an installation not to leak energy at an unacceptable rate. Then again, just like with ACs that also serve as heat pumps, it could just be a matter of not enough people (or professionals) knowing about these installations to make it viable to build a business around them.
- TheSpiceIsLife 2y agoHVAC folk ought to know about ERVs. Type in HRV system in to your preferred search provider and hit the shopping tab. Here in Australia they range from about 1500 Antipodean Dineros for a single room through-wall mounted systems, and around 5500 upward for a centralised unit. Plus installation costs, but HVAC install is one of my paid activities, so I mostly don’t pay that part. NB heat pump is a more accurate term for refrigeration type air conditioners, as in cooling mode they’re ‘pumping’ the heat out of the inside environment and rejecting it outside. But have you heard about brown?
- driverdan 2y agoThat's surprising. Doesn't CA require them in new construction? All CA HVAC contractors should be familiar with them now.
- bluGill 2y agoThey have been required by code in all new houses in Minnesota for about 25 years. I'm sure CA requires them too. Though in older houses they are a waste of money as your house already leaks much air. These are a good thing if you have a well sealed house, but older houses universally are not sealed that well and so they won't give you anything.
- zerr 2y agoTo be fair, the temperature (coldness) of the outdoor air contributes to refreshing the room as well.
- d--b 2y agoI don't think this is meant as a replacement for windows :D
- freetonik 2y agoIn Finland, many people avoid opening windows altogether, and only rely on various devices for mechanical movement of air. I find it quite alarming actually, as many homes have stale air and subpar ventilation systems, always justified by "never let heat escape the house". The idea of opening windows to refresh the air in the house is basically alien to many Finns, while it's a normal thing to do in other countries; in German, there's a term Stoßlüften.
- BlobberSnobber 2y agoStoßlüften isn’t merely a term, it’s a German religion
- pzduniak 2y ago>that they are very expensive niche products My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model that I can directly control over MODBUS / 0-10V signal (even turning it into a "dumb" unit). Most of the expenses were running the ducts. YMMV It's just awesome. Every single room has fresh-smelling air and after fine tuning all my heating systems with algos implemented in Home Assistant - I'm getting ~60-100ppm over outdoor CO2, perfectly clean air, temperature within 1C of the set value, on-demand humidity extraction after showers etc. All it needs to be properly overengineered now is a bunch of dampers and per-room CO2/humidity feedback :)
- yrcyrc 2y agoWould you have a link/reference?
- tinco 2y agoAnyone know if there's a good way to control 0-10V dampers? I looked for a solution to control 5 dampers but I didn't find anything, so I started to design my own a couple years ago but never finished the project. I'm having a hard time imagining something so common doesn't have a common solution.
- pzduniak 2y agoIf you're OK with large controllers, the cheap Chinese RS485 stuff seems to work perfectly fine. For "digital inputs" I started out with Polish $150 devices, eventually ended up using the cheapest AliExpress listings for some expansions and so far they've been working exactly the same, with the only difference being the quality of the docs. I'd expect the 0-10V modules to be exactly the same.
- batushka5 2y agoThese units are ubiquitus in northernish Europe, as any new/renovated building needs them to reach A/A++ energy effiency. Brands like Komfovent, SystemAir, offering 200 m³/h ducted units for 2000 Euros, with efficiency like: Outdoors °C -23 -15 -10 -5 0 25 30 35 After unit, °C 12,9 14,5 15,5 16,5 17,5 22,6 23,6 24,6 with indoor conditions + 22 °C, 20 % RH
- jonstewart 2y agoPlanning a renovation of my 1947 rowhome in DC, and I’m really looking forward to adding an ERV.
- bluGill 2y agoYou probably cannot do a renovation the tightens up your house enough to matter. Of course you have not specified what you are doing, it is certainly possible to do that, but it is a major effort that makes the house unlivable for a couple months and costs a lot of money. If you don't do that level of renovation your house will have enough leaks that a ERV will not make any difference in air quality (and even that level doesn't always make the house airtight enough to need an ERV). Making a house airtight is very hard - worth doing because of the energy savings, but not easy. If you are doing that level of renovation is is probably better to just tear down the house and rebuild. The costs will be similar and there are a lot of other things people demand of a new house layout that cannot be retrofitted in the old shell. Often the law will not allow this and so you are forced to renovate just to keep some now illegal feature that is worth keeping, but otherwise a tear down would be better.
- BizarroLand 2y agoAnyone can do a renovation that "tightens up your house enough to matter". Use a qualified professional. Get multiple inputs. It will cost money, and more money as you approach perfection, but it is doable.
- bluGill 2y agoNot really as the structrure of most houses leaks. It can be done but you are doing a lot of work that is easy to skip
- BizarroLand 2y agoThat is true, and I guess technically you don't want a house with no leaks, only with leaks that you have complete control over. Spray foam insulation can seal essentially any structure, and it doesn't require much more than a small hole drilled into the wall between the beams which is easily spackled over. It can also be put on as insulation in attics and crawlspaces. Combine that with a full ducting audit if any of your ducting pierces the envelope, a full intrusions audit for power boxes and the like, and new windows/house sheathing/ proper roofing, you can get very close to as good as a new built. Most of these I would not suggest doing as a DIY, therefore I still say it is expensive, and even moreso if you have lathe and plaster instead of more modern drywall or encounter any of a myriad of issues likely to be uncovered when doing these things to a very old house.
- throw0101b 2y ago> One problem with ERV/HRV systems right now is that they are very expensive niche products. Most building codes in US/CA mandate them since about 2015, so I'm not sure how niche they are (at least in new construction). Depending on the (air) volumes involved, ERVs can be had for under CA$ 2000: * https://gasexperts.ca/product-category/air-exchangers/lifebreath/ https://gasexperts.ca/product-category/air-exchangers/lifebr... * https://bphsales.ca/collections/high-quality-erv-air-exchangers https://bphsales.ca/collections/high-quality-erv-air-exchang... HRVs for less, but it's probably worth the extra few hundred for better humidity management.
- danans 2y agoInstallation is usually the most expensive part, and can easily send the total price into 5 digits of $, especially in a retrofit, depending on the market.
- turtlebits 2y agoERVs are not expensive, 1500 USD will get you a decent whole house unit. The installation is the expensive part, which this project doesn't change.
- kccqzy 2y agoWhat makes outdoor air "fresh" compared to indoor air? You said that the temperature and humidity of indoor air are preserved. So is it just CO2 concentration? Would installing a chemical CO2 scrubber have an effect similar to an ERV system then?
- runjake 2y agoCO2 and chemical off gassing from indoor items. Eg: manufacturing chemicals from furniture, natural gas, cooking fumes, etc.
- NavinF 2y agoCO2 ppm is the main number that research studies look at, but it's also a proxy for the "freshness" of the air. All the air quality metrics are correlated. Eg a lot of cheap CO2 meters measure something else like TVOC and convert it to eCO2 using a lookup table. CO2 scrubbing would be better than nothing, but it's really expensive and won't improve other metrics like TVOC
- turtlebits 2y agoYou have it backwards. Counter flow units have lower efficiency than these "regenerative" type ERVs. The downside of this is that the high efficiency is limited to small spaces (based on the mass of your core), where counter flow units are great for entire homes. One point often overlooked with counter flow units, is that you can place exhaust ducts in spaces that you want to purposefully remove air, like bathrooms and kitchens, while providing fresh air to places with little air movement, like closets, basements. Regenerative core ERVs do little for fresh air circulation.