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.
>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 :)
I wish this was as common as having a fridge in the house. The productivity gains from people not being sleepy and tired from shit air would be insane.
> after fine tuning all my heating systems with algos implemented in Home Assistant
I'd be very interested in hearing the details of this.
How is the noise of these systems? And how often/long need they run to give you clean air and humidity extraction. Is there extra noise when it is windy outside? Is it installed inside the windows somehow or do I need to drill through the wall?
> My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model
That is just the HRV, not the design of the system, the ductwork, and the installation. All those add up. In new construction those costs can be shared with the regular HVAC system design, but in a retrofit its far more expensive
Anyone know if there's a good way to control 0-10V dampers? I looked for a solution to control 5 dampers but I didn't find anything, so I started to design my own a couple years ago but never finished the project. I'm having a hard time imagining something so common doesn't have a common solution.
Would you have a link/reference?
> One problem with ERV/HRV systems right now is that they are very expensive niche products.
Most building codes in US/CA mandate them since about 2015, so I'm not sure how niche they are (at least in new construction).
Depending on the (air) volumes involved, ERVs can be had for under CA$ 2000:
* https://gasexperts.ca/product-category/air-exchangers/lifebr...
* 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.
Installation 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.
These 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:
with indoor conditions + 22 °C, 20 % RHOutdoors °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 24,6ERVs 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.
You 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.
Do 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?
The 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#Ther...
Heat exchanger there is usually an extruded ceramic grid (ERV) or rolled corrugated aluminum (functions closer to HRV than ERV)
Counter-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.
the 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
What 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?
CO2 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
CO2 and chemical off gassing from indoor items. Eg: manufacturing chemicals from furniture, natural gas, cooking fumes, etc.
Planning a renovation of my 1947 rowhome in DC, and I’m really looking forward to adding an ERV.
You 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.
I can't find anyone to install one in Los Angeles. Is there a particular climate these are suited for?
Air 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.
> I can't find anyone to install one in Los Angeles.
If you call a random HVAC company, they may not want to deal with "fussy clients" that want something "fancy" like an HRV/ERV. Best to look at folks that perhaps try to adhere to building science more. A quick search for the LA area:
* https://www.jmsacandheating.com/indoor-air-quality/heat-ener...
* https://www.aircomfortexperts.com/additional-products/ervs/
* https://www.azaircond.com/indoor-air-quality/energy-recovery...
* https://www.socalclimatecontrol.com/ervs-and-hrvs-energy-eff...
Or do a dealer search from a manufacturer, e.g.,:
* https://broan-nutone.com/en-us/home/dealer-locator
> Is there a particular climate these are suited for?
Any climate. Modern ones can even handle IECC Zones 6 and 7:
That's surprising. Doesn't CA require them in new construction? All CA HVAC contractors should be familiar with them now.
They 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.
To be fair, the temperature (coldness) of the outdoor air contributes to refreshing the room as well.
I don't think this is meant as a replacement for windows :D
> 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!
Not in cold climates where in winter the air outdoors is very dry. Heating systems in such climates often have integrated humidifiers.
Isn't it the other way around? 50% humidity means that air contains 50% as much moisture as it, at a given temperature. Raising the temperature means that the air can now hold much more moisture.
Bringing in cold air at 50% humidity, then warming it up to room temp makes the humidity fall, leading to dryer air indoors than comfortable.
My house currently is sitting at 35% humidity while being very poorly ventilated (~900ppm CO2). In the summer, it’s around 50% with the same level of ventilation. This generally has been the case everywhere I’ve lived; in the summer, you’re cooling air, which (all else equal) increases the relative humidity of that air. In the winter, you’re heating air, which decreases the relative humidity of that air.
ERV is apparently the term used for HRVs that also exchange humidity and keep it in/out as required, though I'm not sure that makes much logical sense since E is for Energy and H is for Heat, no mention of humidity.
Possibly just different terms used in different countries where the humidity is a bigger problem (very hot and/or very cold outside air).
This current OpenERV product appears to use dessicants for this purpose but might be an optional add-on?
Does very much depend on your location. In the UK a dehumidifier is still often a good idea in the winter because although our humidity drops, its nowhere near enough. Inside can still be high 50's, in older properties it'll never really go below 60%.
Really a lot of our older homes shoud be retrofitted with a MVHR unit to help things as ventelation is awful in most houses. I'm actually quite surprised a lot of landlords in the UK don't do it as theres always a fight between them and their tenants who don't necesserily want to leave a window open all day in the winter to stop mold.
Hey guys, I am the guy behind the OpenERV company, who designed the TW4 and WM12 ERV units.
I'm sorry I don't have a bunch of units ready to ship out, as the site says it's still in beta, I am to be honest kind of taking my time because I have another project, the big quiet fan, which is actually funded a little better, and thus I've been directing most of my time to that. But I do advance this a bit most days. I have a twitter where I tweet my progress : @open_erv, and also I'm on bluesky.
I have shipped a few units to other engineers who have/will test the units so I can share third party confirmation for any skeptics.
To clarify some of the discussion, it is not a counterflow heat exhanger, it is a regenerative type. https://en.wikipedia.org/wiki/Regenerative_heat_exchanger. I prefer this type because they can recover latent heat more effectively than recouperative (such as counterflow) type, and latent heat is 40-50% of the total energy content of the air, seasonal average in Ottawa or Toronto.
I am hoping to get the machine tested by the PassiveHaus institute to show beyond doubt how good the efficiency is with a third party test. I have no doubt, I have tested it myself, though.
These can theoretically handy any temperature differential, but the TW4 and WM12 are currently made of a polymer that I wouldn't trust in an extremely hot climate combined with direct sunlight. For that reason, I am focussed on cold climate scenarios. I am pretty sure it will not frost up even in extremely cold weather like -30. I used it last year in my window and had no problem, and it did get to like -25 at least iirc.
Well this was fun and thanks for the discussion, everyone. People are surprisingly nice and sensible and positive here! I used to have another account but lost the password. Perhaps I'll be a reader here in the future.
Anyway, I've tried to turn the very temporary influx of interest into something positive and lasting by searching for 2 people who can install and document the install of a pair of TW4 energy recovery ventilators, so anyone who wishes to buy thereafter can know what they are in for on that count. There is another guy Alex who will test flow and efficiency, I've already sent him the stuff.
So we get things tested and verified, and I will continue getting a jump on producing units by running the printers and assembling in between when I am doing the more respectably paid work in my life. The kits are on the back burner because even I am still stabilizing the assembly methodology. I even added a new component just recently, a flow straightener that boosts flow by about 10% while allowing noise to be reduced even further.
So stabilize, verify, produce, and then after that, within a couple months, I sell in a more or less ordinary way to anyone who wants them. I'm sorry it's not in time for the cold weather, but we have to remember ERV is about the big picture and long term. Like the rest of a building, it's an investment, and the machines are made for (very) good return and long lifespan.
I will prepare some WM12 units for those who have asked for them. To be clear I only got 12 emails expressing interest, not an absolute flood, but it's encouraging to know some people "get it" at least. I knew there would only ever be a small trickle of relatively wise people from around the world that appreciate good performance and return on investment. I only need to sell a few pairs per month to make it worthwhile, at the eventual $1300 CAD price tag.
I am open to scaling up production with more efficient production methodologies, but I am actually fairly well acquainted with injection molding, machining and other conventional approaches, and they aren't magic. They would help for sure but they wouldn't radically change the price, or the rate of return on investment, and they also take a lot of investment not just for tooling but also re-testing and re-design. I've also changed the design so, so many times after I thought it was done I am highly wary of being locked in.
Good project. I've added links from my MHRV pages which have quite good traction on search.
Small note: the older single-room unit we have with the fan on the outside can ice up and make horrible noises then stall at a few degrees below zero (here in London UK)... B^>
Also: as the creator of a project called OpenTRV, I cannot but help admire your taste in naming! B^> B^>
That's an amazing project! I'm blessed to be in such company. Seriously, I use open source stuff and I prefer to do business with such relatively wise people. If you want a beta unit, email me and I'll put you at the top of the list!
This is neat! I had some issues with ventilation in a foamed house and the only product that’s not a whole home ERV (which, I didn’t have space or ducting) was the Panasonic whispercomfort which actually has some requirements that were hard to meet (minimum duct length) and the overall efficiency isn’t that great. We put in two and have fresh air intake on our HVAC units. Still we’ve taken to running at least one bathroom or laundry fan non-stop.
I’m excited for more competition in this space. Beyond the hardware I’ve found that HVAC installers are way behind the curve on air quality. I hope education and awareness increases in the industry.
>To clarify some of the discussion, it is not a counterflow heat exchanger, it is a regenerative type.
Can someone expand of this?
intuition tells me that a regenerative design can be no better than 50% efficient, and would be worse at recovering latent heat
I think it means that it's like [Lunos](https://www.lunos.de/en/for-heat-recovery). The unit alternates between exhaust and intake every couple of minutes. The air being exhausted heats up a core, which in the next cycle warms the air from the outside. Lunos e2 is advertised to recover 90% of heat and 20–30% of humidity.
The ductless design seems great for smaller units or open spaces. Although for smaller units you want to get building owners to install these. Have you seen interest from them or are you expecting a company to take up this design and sell it to them?
I would consider installing this in my open finished attic even though I already have a whole house ERV. The problem with a whole house ERV, particularly in a multi story house is that it doesn’t necessarily produce a lot of fresh air where you are in the house.
I'm planning to just putter along selling units, making about 50% of my living like that, and if a company comes along and wants to buy the design/company that's good. If not, I get some stuff done, earn a living. This isn't a get rich quick scheme, it's an honest living type stuff.
Fyi the mobile image swipe mechanic on the linked page is inverted. Swiping left takes you to the image on the left, instead of the image on the right and vice versa.
Is there a link to the big quiet fan project?
There is some stuff in the other projects page on the website, that project went really well too, I spend most of my time on that these days, the ERV is a longer term thing, the fan has a finish line but in a way is more epic/important because it's about halting airborne disease transmission. It works extremely well, about 1900 CFM through merv 13 filters (10x 20 by 25 filtrete) at less than 41 dBa.
I'd love to build this. I have access to a 3d printer, use Python, and have some electronics experience. I live in a northern climate and have been eyeing ERV systems for a while. Basically, I'm the perfect target for this.
However, reading the docs, they seem written more to discourage any kind of DIY attempt by saying things A, B or C are difficult, than actually explaining how to do them correctly. I'd love to contribute to the project, but it feels like it's not set-up to foster community contribution.
If I'm mistaken, I'd love to donate some of my time on this!
Unfortunately the recovery core, which is the interesting part of an ERV, is not included in the 3d stl's.
IMO, this feels like a more marketing project than anything open. ERVs are already very simple (a recovery core + blowers/fans). Commercial units last an extremely long time (some with 10 year warranties) and have comprehensive parts availability.
Also a long term window install is a bit janky and is likely to lose out on efficiency due to glass being a poor insulator.
I'm pretty sure stls/regen.stl is the recovery core you're taking about tho I can't open it in a online stl viewer: https://drive.google.com/drive/folders/1py2YwmwBEcvmdw18SKwx...
In the photos it seems to be a bunch of nested single-perimeter cylinders that are joined at a few points to maintain spacing. Easy enough to model, but I agree the documentation is horrible and there's no way to contribute.
Commercial units are not comparable because they're way more expensive despite being so simple
There is the tw4, which is made to be put in a wall, and there is the WM12, which goes in the window. The main focus is the TW4. There are instructions in the manual for making an ERV core. It is not trivial.
I expected a community open source project from the title, but reading the docs led me to the same conclusion: The website is about convincing you to buy one while discouraging you from attempting to build one.
It looks like a fun project. I don’t want to discount what has been designed and built. It is confusing to start reading about the project and discover that it’s more of a business than a community project while simultaneously being unavailable for purchase. The person who built it commented on HN that they’re focused on a 3rd different fan project right now, which brings the future of this project into question.
It would be great if a community effort could fork this project and work on making it easier to DIY so the community could push it forward.
EDIT: After exploring the files I’m not sure I’d even call this open source. I either can’t find some key files or they’re deliberately excluded. True open source projects would also include the CAD source, not only .STLs so others could adapt and modify the source. I think the open angle on this project is more marketing than substance.
The step files are also there, which is the best common denominator for CAD files. Again, it's open source for the purpose of maintenance and repair, not cloning, and frankly earlier on I did make it more community oriented and nobody ever contributed even a little bit, so I just gave up on that idea.
The most likely scenario for longer term is that people may submit minor patches or suggestions, which I roll into the hardware or firmware. In reality, hardware is not like software. You can't make changes easily. Some wizards may take it upon themselves to spruce up the firmware with fancy features and release something, which anyone is free to do. There would then be multiple compatible versions of the firmware, one which I curate for reliability with minimal features, and others which others can provide. Same as for 3d printer firmware.
The firmware is Micropython, which is extremely easy to understand and modify.It is not open source (per OSI definition), as it is under CC BY-NC-SA.
just saw this video https://www.youtube.com/watch?v=U-hVUczzlL4 and you get a very smart solution for a fair price, that does coordination etc.. and it's even esp32 based should the need arise. see https://www.bpcventilation.com/bsk-zephyr-single-room-heat-r...
I also have this DIY bookmarked: https://www.youtube.com/watch?v=wJB3dyHDa-8
I'm confused how the heat retention could be around 80-90% without expending a ton of energy.
Naively, if on average the same amount of air goes in and out, I'd expect the temperature of the heat exchanger (on average in space and time, eventually) to be the average of the outside and inside temperatures. If the outside is hotter, the air coming in would be cooler than the outside air (which is a win), but it couldn't be cooler than the average of the temperatures. So, it would still not be anywhere as cool as the inside air, which doesn't sound like 90% heat retention.
Is the heat exchanger attached to a heater or a cooler? The linked video, https://www.youtube.com/watch?v=CDCu0IbEn8Q , would suggest not, as it talks about saving the energy needed for cooling or heating. Is there another clever trick?
What you describe is kind of like a theoretical heat exchanger that only averages temperature at a single point.
You can improve this by exchanging heat across a continuous length along opposing flows. Imagine two parallel pipes thermally bonded where fluids flow in opposite directions. Each point still averages the temperatures, but the average temperature varies across the length and approaches the interior temperature on the interior side and the exterior temperature on the exterior side.
Yeah, I think this makes sense. If you connect many of my heat exchangers in series, the temperature gradient increases; only the middle one will work at the average of the inside and outside temperature (the example of 3 in a row makes sense to me). At the limit, it becomes what you described.
Thanks!
There isn't a uniform temperature across the entire exchanger. There's a smooth gradient extending from one end to the other. If the outside is hotter, then the inbound air gradually cools as it gives up heat to the outbound air which is gradually warming.
I find the idea of reversing the air flow direction every 30s simpler to understand than two counter-flowing pipe side by side.
Imagine a pipe filled with 3 metallic grid sections (such that the air temperature in the section will equalize with the metal temperature) separated by plastic grids (such that the heat isn't conducted through the metal), and you push air alternatively from one hot side at 20°C to a cold side at 0°C for 30s and in the other direction for 30s.
For symmetry reason, the pipe will passively (we don't count the energy required to move the air) have a gradient of temperature from the hot side to the cold side. The first section will be ~15°C, the second ~ 10°C, the third ~5°C. (Each section temperature is the temporal average of the temperature of the air flowing from previous sections : so because air switch direction, it means it's the average of left and right sections.)
From the point of view of the house, you only lose energy from the first section of the pipe which will be more like 15°C rather than 0°C.
Counter-flow heat exchangers can be very efficient, without a heater or cooler attached. That said, I don't think I've seen a commercial ERV claim to be more than 80% efficient, so I'm skeptical of the 90% measurement.
(I've seen ERVs with heaters attached; but for the purpose of avoiding frost buildup when it's below freezing outside.)
Commercial HRVs often use rotating disks, not counterflow heat exchangers. Disks are freeze-proof, but they need to be powered.
In my experience 80% plus is quite common in the models sold for colder environments. E.g. Mitsubishi electric Lossnay advertises 86%.
> I'm confused how the heat retention could be around 80-90% without expending a ton of energy.
Imagine two air streams counter-flowing. They "swap places" within your heat exchanger, so you can (theoretically) get 100% heat recovery.
This principle is used by animals to minimize the heat waste, by counter-flowing warm and cold blood: https://en.wikipedia.org/wiki/Rete_mirabile
Co-current flow (both flows moving in the same direction) work the way you described.
Countercurrent-flow heat exchangers (where the two channels of the fluid/gas) move in opposite directions on both sides of the heat-transfer mechanism maintain a heat flow gradient over the entire length of the heat exchanger. This can result in an almost complete transfer of heat from one current to another.
High efficiency HRV/ERVs use counter-current flow heat exchangers.
I'm not 100% sure about this. But with ERV (opposed to HRVs), iirc, also the moist of the air is transferred to the incoming air. The moist contains a lot of the energy.
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I so need this, and I so need it to function with Home Assistant. I would love to ventilate based on values of my Aranet 4 (a bluetooth CO2 sensor). Also, would be nice if it coordinates with multiple units, ie what this brand does: [0]
EDIT: It does, if you click on "learn more", you'll learn more: "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." ...Perfect!
Currently I have two holes in my wall for ventilation, when it is windy it's too much (feel the wind blowing inside), when some people visit and there is no wind, boom, >3000 ppm CO2 in 20 minutes.
I just really hope it is very quiet, although it says ~37 dBa (which is quite a lot imho), I replaced my bathroom ventilator recently, it produces 25 db! [1]). The previous one [2] produced 52 dB (cheapest around), that was pretty annoying, you'd hear it in the bedrooms above the room it was used in. Maybe 37 dB it isn't so bad, especially since you can wind it down and mostly need it when it's busy/noisy (many people) anyway.
Btw, don't buy a CO2 sensor, pretty soon you're a ventilation nerd, or as my wife would call it, a ventilation curmudgeon.
[0] https://blaubergventilatoren.de/en/series/vento-expert-a50-1...
[1] https://www.filterfabriek.nl/ventilatoren/badkamerventilator...
[2] https://www.hornbach.nl/p/rotheigner-toilet-badkamerventilat...
> 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." ...Perfect!
Absolutely.
> A room is not heated by increasing its internal energy but by decreasing its entropy due to the fact that during heating, the volume and pressure remain constant and air is expelled.
https://pubs.aip.org/aapt/ajp/article-abstract/79/1/74/10418...
The point about balancing airflow is crucial, but I think underappreciated by non-professionals. Thermodynamics is highly non-intuitive in places, and the enclosed climate-controlled spaces we love to inhabit are certainly included in that.
Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
Oh man I had this discussion with my wife yesterday, we have a small electric heater in a room where a pipe burst and I still need to fix that (no heating means instant fungus problems). It keeps its fan rotating always, that way it determines the input temp for its thermostat more accurately. But wife insists it is sometimes blowing cold air and thus very very bad... I explain what a thermostat is (bimetals and all) and that she experiences "coldness" because a layer of warm air is blown from her skin, it's not blowing cold air... she doesn't follow... I even measure the energy usage and the thing only uses 20 W or so when just blowing, not heating. Even when just blowing it's moving cold moist air from the walls so overall good. It's difficult dealing with her like this.
I'll pay someone to tell me how to deal with someone like this and maintain a positive atmosphere. The thing is, I also do it for things that really are probably not worth discussing... I should pick my battles better, is there ever a good time for some mansplaining? Or should I say... Nerdsplaining?
> Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
On that note, I'm curious if hybrid heating/cooling solutions will ever take off. Other than this OpenERV product I mean, which I guess technically counts!
Low Tech Magazine mentioned some experiments in their article on compressed air energy storage (CAES). Instead of trying to make that form of energy storage an adiabatic process, the idea is to use the heat produced/required in the compression/decompression steps in the household to improve the energy efficiency (e.g. use heat produced during compression to heat water; do the decompression in a space that should be cold anyway like a basement used for food storage).
[0] https://solar.lowtechmagazine.com/2018/05/ditch-the-batterie...
Check out Komfovent units if you want a ready solution. My setup is Komfovent HRV (over MODBUS TCP), NIBE heatpump (over MODBUS UDP + esphome-nibe), Vaillant gas boiler (over eBUS-WiFi) + a bunch of AirGradients scattered around the house. Nothing has access to the internet, everything is glued together with HA. Works surprisingly well :)
37dba is practically nothing. Like a very soft whisper from a couple meters away. Remember the scale is logarithmic, 37 is almost 2 orders of magnitude below 52.
Thank you, this is correct. Using my class 2 sound meter, if I stand in my house in nowhereland cornwall with power shut off to the whole house, it's 38 dBa. 37 dBa is audible in that environment but nearly inaudible in a normal environment where your computer cooling fan is making 45 dBa at 1 meter, etc. 42 dBa is pretty quiet too, my furnace makes 43 dBa at 1 meter from the duct when it turns on. And that 42 dBa is a full 60 cfm, full blast. That's more than twice the airflow of competing units like the blauberg vento. You don't turn it up that high when you are sleeping.
Why not just buy an ERV? It's available, comparable in costs, and in 5-10 years, there will still be parts, unlike this project where the author hasn't actually shared the most crucial component, the exchanger.
I was reading up on counter-flow heat exchangers a few weeks ago after I'd just installed a MVHR system and realised that the actual heat exchanger components themselves were, counter-intuitively, a fraction of the price of the whole unit.
I was surprised when I saw they're mostly made of thin plastic and don't depend on thermal capacity at all (unlike, say, HX espresso machines). The way they work is quite simple:
It's just a bunch of thin parallel channels where warm and cold air flow in opposite directions, separated by thin plastic walls. Because the flows are counter to each other, there's always a temperature difference driving heat transfer across the dividing walls, even as the warm air gradually cools and the cold air gradually warms.c w o a │ │ l r └─────────────────────────────────────────────────────┘ d m air at 50 deg ────────────────────────► air now 5 deg o i u d ───┬──┬──┬──── heat exchanging surface─────┬──┬──┬───── t o ── ▼ ▼ ▼ ─────────────────────────────── ▼ ▼ ▼ ──── d o o r air now 45 deg ◄─────────────────────────air at 0 deg o r a ┌─────────────────────────────────────────────────────┐ i │ │ a r i rThe lightweight plastic walls are advantageous here - while plastic isn't particularly conductive, the walls are so thin that heat transfers readily. It's how these heat exchangers can achieve 80-90% efficiency without needing any expensive materials or thermal mass. The warm exhaust air leaves only slightly warmer than the incoming cold air, having transferred most of its heat to the incoming stream.
Clever design.
Thank you for the illustration! I was sitting here, wondering that the whole system sounds paradoxical but seeing it drawn down with the arrows really helped grasp how this works!
Not sure what you mean by the heat exchanger core being a fraction of the price, but I've seen replacement cores cost around 1/3 of the total unit.
They should be made of high thermal conductivity material like resin or ceramic.
I installed a central ERV in my home, a Zehnder ComfoAir Q. Installing it was quite involved, but not hard. Definitely within reach of anyone with basic DIY skills.
The hardest part is finding a good spot for the ventilation unit, which is about the size of a large, old CRT TV. You have to run ducting from there to multiple rooms, but their ducting system is easy to install.
Mine draws about 20W/hour at its typical setting, and it greatly improves comfort. Keeps some of the humidity out when it's humid out, keeps some moisture in when it's dry out. Fresh air year round. Keeps mosquitos out. Keeps some dust/particulates out.
Worth the effort, even in my small house.
A decentralized unit would be a lot easier to install, but I imagine it's less efficient, less suitable for larger dwellings, and probably louder. YMMV.
Cool project! Around that topic I can also recommend a channel of this engineer (switch to auto-translate): https://www.youtube.com/watch?v=Cv0s6TgwbJg
Paraphrased:
- push-pull ventilation is easy to install and comparatively cheap
- it's prone to hygiene issues like blowing dirt out of the filters back into the air and providing a moist environment for microorganisms in some operational conditions
- it's prone to windy conditions
- the numbers stated by commercial vendors seem to have no basis in reality, there seems to be no vendor providing data based on the relevant testing standard for these systems. OPenERV states they want to get it tested by Passivehause institute but also say no lab data measured yet.
Might be just my counter-factual gut-feeling, maybe a mechanical window opener based on EspHome for short pulsed passive ventilation intervals is actually more efficient, easier to implement and need less maintenance? Not aware of any comparisons though and last time I checked I could only find some finicky 3d printed actors that might not survive a guest opening the window.
The TW4 is light years ahead. Higher flow, better efficiency, much quieter, wind compensation, Internet of things functionality. It's not just yet another machine of the same kind. The heat exchanger is very different, the whole design and construction is quite different.
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I use similar decentralized (single-room) ventilation units here in Northern Germany: https://www.bayernluft.de/de/frame.cgi?page=start.
I am super happy with them. We now always have air that feels fresh and warm in winter, and the humidity has dropped significantly.
There are two types of such ERV devices:
1. Those with only one air channel that switches directions periodically. They use a heat storage element in the airflow. OpenERV belongs to this group.
2. Those with two separate air channels for intake and exhaust at the same time. The air does not mix but passes through a heat exchanger. Bayernlüfter works like this.
The only thing I don't like about Bayernlüfter is that it is not open source. It is controlled by a Raspberry Pi (or a similar clone), and I don't have access to it.
In the Netherlands these systems are fairly common in new houses. Mostly because the law mandates a certain level of energy efficiency of new houses. There are other ways of obtaining this required efficiency level, but an ERV unit is pretty cost effective.
I've personally been looking at installing such a system [1]. However since houses in the Netherlands are almost all made out of concrete installing such a system in an existing house is pretty hard.
[1] https://www.duco.eu/uk-ie/products/mechanical-ventilation/ve...
Same in Denmark, we pretty much had to install one when building our house, to make up for energy loss from the large window area we wanted. We didn't have it properly calibrated at first, but once that was (professionally) done it has worked perfectly and kept a pleasant indoor-climate ever since.
The old-and-trusted brand here is https://www.genvex.com/en (ours was supplied by Ecovent though https://ecovent.dk/?lang=en )
Similar story here in Germany. New energy standards require a ventilation concept. Some people choose to rely on daily ventilation to save some money, but most people nowadays opt for an ERV. At least here in Germany, for some reason there are quite a lot of people who are super against the idea of having an ERV. Personally, I wouldn't want to miss is for having fresh air alone, not having to deal with pollen is an added bonus
My basic understanding is that the thermal energy also costs a lot more over there than it does in north america, like 4x as much.
I'm confused if this is true open source hardware and software or not?
I don't think it's FOSS at the moment. I actually would build two or three of these units on my own and could provide some feedback along the way. My definition of open source means that I should be able to do so.
Unfortunately, the gDrive files are not providing enough information for me to build one of these in a DIY manner. I didn't find enough information on the hardware side, no BOM, no hardware documentation. I think that, if the author would like to actually boost DIY adoption, it'd be worth having a step by step assembly guide. At the same time, when reading the page, I had a feeling like it's more supposed to be a way of advertising a future commercial product, not really focusing on the FOSS/DIY side.
The software is provided, but from my experience with such projects, it's maybe half of the minimum information needed to build a full fledged device.
I like the project and would love to build it in the near future though.
Under "source code", there's a link to a GDrive with a ton of design files and documentation, as well as source code.
These are licensed CC BY-NC-SA 4.0, so depending on your personal definitions, they may or may not be "open source" (IMO they're open source but not FOSS but I've seen others equate open source with FOSS).
As a community we need to do some thinking on how open source may sensibly be applied to hardware. Unfortunately Prusa, who used to be a real champion, has departed from the assumed True Path, and they have discussed their reasons, which are largely valid. That said their design at a more fundamental level has also departed from a maintainable, simple and elegant design.
The purpose of the source code is to enable maintenance, not cloning, I say that on the website. That is this context, there are many others. It improves the economics because the machine lasts longer and there is no planned obsolescence. People are welcome to make their own units from the source if they have the skill, but although it would be fun, I don't really have time to make it easy. Some day there may be a kit which is very economical but it will still take a whole day of work to assemble, probably.
Me too, when reading "open source" I was expecting some design docs or the like. Aside from the general confusion of the website, I haven't been able to find some of the most important information. For example, there's no diagram or immediate explanation of the general working principle and airflow path. The heat exchanger itself is published only as-is for those designs, while the author writes that he uses a custom python script tuned for the design size and his 3d printer to generate it.
When i saw this I immediately thought of studying it and reuse some of its designs for my custom use case, which does not appear to be currently possible.
At first glance it appears to be "open source" in the sense that you can buy it, but if and when something breaks you can print/reorder it easily.
Correct me if i'm wrong
No, it's not. Files are available under the CC BY-NC-SA licence which, because it does not allow commercial usage, is not open source.
Looks interesting. Especially since it seems to be much cheaper that the closed solutions...
Have looked in this kind of systems, for my parents. The use case was basically, not about energy efficiency, but rather noise protection - to be able to sleep with a closed windows. I think so far I always had two issues (in that usecase).
- First the device by itself - produces a bit noise like 42db might be too much for some people if you want to sleep. Especially some of the devices are using one ventilator, which switches directions and won't produce homogeneous noise.
- Second 60 CFM is fine, but if you want to have the feeling of an open window - it should be much more and most devices can't deliver that. Also the heat exchange thing is kind of cool in the winter for sure. In the summer, you often have the case that in the evening you house is much warmer than the air outside - so you would like to turn the heat exchange off in the winter.
PS: Actually, maybe looking for a complete different use case. But I think what would be very cool, would be some idea to make at least one room 100% quite (with fresh air ) in a cheap way. Guess this would be a huge life changer for a lot of people, who suffer from noise pollution.
I think that summer/winter distinction is really important. In the UK where most houses don't have air conditioning, you really don't want heat recovery for 1/3 of the year. On really hot days you might want to use heat recovery during the peak few hours, but otherwise you are trying to cool the house down with colder outside air.
I would love some sort of intelligent house ventilation system which could do all that. Heat recovery when it makes sense, normal ventilation when it doesn't. All automated based on dT and relative humidities.
I think with this model you can do this as long as you install them in a synced pair (which I think is the baseline assumption).
Normal (heat recovery mode) you have them reverse flow every 60 seconds or so to swap heat.
In cooling mode you just run then continually. One is bringing in fresh air and the other is removing stale air.
The heat store in the intake will soon cool to the outside temp and the heat store in the output is irrelevant (apart from maybe slowing the air flow and creating noise).
If you manually control the system you could combine with a few open windows to create cross breezes even on still evenings.
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Here OpenERV use a push-pull ventilation design where air direction is reversed every 30s. This allows energy recuperation and dispense connecting the inlet and the outlet to each other, as each ventilation port alternate role simultaneously.
The alternative design is a counter-flow heat exchanger. Using 3d printing and gyroids it seems possible to build quite compact ones. (metal 3d printed heat exchanger for helicopter https://www.youtube.com/watch?v=1qifd3yn9S0 )
3d-printing a counter-flowing heat-exchanger seems interesting but maybe there are some molding issues that need to be taken care of (maybe HEPA filters on the inside in/outlet are sufficient).
The main advantage of the heat-exchanger solution is that you won't need specific electronic control and can reuse the standard fans for controlled ventilation, but there is more thermally isolated piping required (and the pipes are quite big (~10cm diameter) because they need to move a lot of air even if the fans are weak).
The push-pull system is harder to DIY because most of the off-the shelf fans can't be reversed easily (and 3d printed fans are noisy and inefficient).
I'm not understanding if it also filters the outdoor air coming in.
I live in a place where supposedly the air is of good quality, and yet, when I open the windows, the all place gets a thin film of black dust on all surfaces - most probably due to the particle emissions and tire degradation dust from vehicles from the highway nearby.
The solution I've found is to open the windows every day for about half an hour and then put an air purifier to work.
it appears the air filter is an optional extra, but incompatible with the storm vent.
The filter is a bit messed up right now, you can put one on the TW4 but not the WM12.
So, this doesn't use a traditional heat exchanger. Instead it appears to "pulse" air inwards then outwards through a series of fins.
It effectively heats the fins slightly in one direction, then cools them again slightly in the opposite direction.
Same with wetting and drying for recovering moisture.
I am surprised that the fins appear to be plastic - one would imagine that steel fins would have far better thermal capacity
I am looking at buying one of these and have had a look at similar (but not open-source) products available on Alibaba, e.g. https://www.alibaba.com/product-detail/Fresh-Air-Ventilation...
Prices there start at $150 + shipping.
Has anyone tested these?
I've installed ERV about a year ago. It's great, but kinda overbuilt. The core is some very thin plastic film (some are aluminum). The body around it is 2mm metal. The entire device is 2-3mm metal. There are 2 motors inside that likely need good support, but not reason for this device to cost $1k and weight 30 kg.
There's some comments here suggesting that the incoming and outgoing air pass each other in a heat exchanger. But this is a different model that passes he two streams in turn through a heat sink:
> 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.
I have to admit I am slightly more dubious about this type as they are new to me, though I did see a YouTube video about a commercial one recently and they seem to be a hot new thing.
Possibly this is something sensible that only becomes practical with software and wireless communication? Rather than running ducts to a central location.
Though then fitting two side by side in a window seems odd. Why not use the traditional type in that case?
I have two equivalent regenerative commercial units (HRV, no vapour handling) fitted at opposite sides of a mostly open plan ground floor. They use a heavy ceramic core, and sync for opposite or coordinated flow (optional). They go up to 60m3/h (~35CFM) which is extractor fan level for me, 60CFM (~100m3/h) is quite a step up. They were under €200 a unit about 18 months ago.
They are rated 90% recovery at low speed. Today it's 11C 75%RH outside, 18C 65%RH inside, at low speed (15m3/h rated at 1.2W) there's barely a difference: 17.8-17.9C air intake temperature. They keep the air noticeably fresh, drier and also keep the CO2 down (<600ppm right now). I'm running them below the "recommended" 50% air-change per hour (ACH about 35%), and boost when needed.
There's a recuperative ducted type in the attic for the first floor, when I checked last month it was 4C outside, 18C at the outlet vent, and 17C at the inlet vents. That runs at 50% ACH.
The reasoning for the paired up window model isn't obvious, maybe a simple increase in capacity. The website is quite clear you need a push/pull pair to be efficient, and an immediately adjacent such pair is not going to work so well.
I have traced the air with a smoke generator, there is a youtube video of me doing this on my youtube channel. There is not significant re-inhalation (short circuiting) of the air. The reason for the window mount is that a lot of people rent or otherwise cannot punch holes in their walls. However the reason the window mount is nearly the same as the TW4 (through wall) type, is that I cannot invest the time and money to re-design a whole new machine for a window. The primary value is in the TW4, but a lot of people wanted window mount ones so I helped some people out by deving a quick window adapter and then they helped me out by testing some of the components/the fundamentals of the system in the real world. Unfortunately the window mount units nobody is willing to really pay the cost of manufacture for, so they have to use grade B parts etc. and they have to serve the purpose of testing the TW4 or they aren't worth making.
What happens if power fails or one unit stops working? Without valves, you could get unwanted air exchange through a non-operating unit. The commenters discuss various aspects of the design in detail, but surprisingly no one brings up this potential failure mode.
Is there any reason the source is available as a Google Drive link and not on Github (or whatever alternative, Gitlab, etc)? Having been burned out by open-source hardware project, checking how healthy the git looks is a good indicator.
The file limit sizes on github are a problem, there is some workaround but I haven't gotten around to dealing with it yet.
Try Oxen.ai
Try git lfs
What happens to the humidity in the outflow air when it cools down and condense?
It condenses on the heat exchanger, then it evaporates when the airflow reverses direction. That's if you don't have sorbent. If you have sorbent, it gets grabbed out of the air before it can condense.
That is very nice, and I really appreciate that the design files are open source.
Great work.
I cannot help to wonder what brings the total cost to $600, the price of a modern, powerful computer.
And yes, I am familiar with the economy of scale :)
Looks to me like a low cost version of the same could be designed with 2 CPU fans ($1), and a large 3D print ($1 - for the DIY version) or injection moulding for a commercial version ($0.30). Do time-sync between the units with grid frequency sampling (free), and have the whole thing controlled by a 2 cent microcontroller and a pair of triacs.
The whole thing, designed and made in China could probably come to a BOM under $4, and retail in the USA for $12.
While I agree with your points, $0.30 for injection moulding would need quite the scale, and I have doubts about whether two CPU fans would have enough power to flow enough air even in the absence of a HEPA filter.
You get similar ones from alibaba from around $175 : https://www.alibaba.com/product-detail/Fresh-Air-Ventilation...
Look up the price of a blauberg Vento or a Lunos e2. They are ~$1800 CAD, and they get a fraction of the airflow. Computers have a large ecosystem behind them and have been in development for a very long time. This is still at the start of the deployment curve.
This has not answered my question. I can also come up with many examples of things that are expensive.
What does really inflate the BOM ?
And this is in no way to discount your effort or your results, I'm genuinely curious.
Great project. Hrv and erv should be a lot more common. Is there a diagram of the airflow in that unit? It looks like the intakes and exhaust on both sides might be very close to each other.
Less "open" than the name suggests, because the design is for non commercial use only, yet this is very much the kind of product that needs to be sold to see widespread adoption.
Mass market product would have a different design, intended for manufacturing at scale. This design is intended for DIY, mass producing with DIY-intended design is too expensive (see Ergodox as an example).
How does it avoid short-circuiting when the supply and exhaust vents are so close together?
There's lots of text on that web-site, but details of the actual design of the thing are pretty scant.
This is really cool! I try to keep CO2 at reasonable levels at home, and that occasionally results in running the heater with the windows open. My friends/family do not understand why I care or would waste energy like that, and seem to think it is a mental illness or something.
The worst scenario is during high wildfire smoke events... trying to keep the house sealed enough to keep the smoke out often requires taping door seams, etc. and the CO2 skyrockets.
I guess increasing atmospheric CO² by maximizing your fossil fuel usage is one way to equalize the interior and exterior CO² of your house...
It's actually pretty minimal- I live in a mild climate, and even doing this sometimes I probably spend under $50/year on home heating and leave the windows cracked most of the time.
My home was most likely 'retrofit' in the '90s to make it airtight for efficiency, and even with the windows cracked is too airtight to be healthy, and still more so than it was probably designed for in the ~1950s.
I’m interested in this, but I think I just realized that what I want it to do is impossible.
We have a fireplace, which is not efficient at all, in part because it sucks cold air in from the outside. I was thinking it would be great it we could use an ERV to condition the air that gets brought in.
However, as far as I can tell, the moment you exhaust air from your house in any way except through the ERV itself, the ERV cannot help you with the replacement air that comes in.
Is that correct?
That is half correct. The TW4 units operate in synchronized pairs. When one is exhausting (egressing), the other is ingressing (bringing air in). This always happens. They even have quite nice pressure sensors that precisely regulate the pressure the fans exert, controlling flow precisely and preventing the house from becoming pressurized at any time, which as you say would imply heat loss.
It would be cool if someone could build a chimney ERV: extract heat from the dirty air produced by fire, and inject it into fresh air pulled into the living space. So I guess basically it would function like a regular ERV, but with a fire in the exhaust path. (Probably not feasible with a wood fire, but maybe with gas?)
Maybe this thing works that way? https://www.modlar.com/brands/nu-air-ventilation-systems-inc...
If you can use the exhaust heat to temper the incoming outside air, that's all an ERV does. Or, get a direct vent fireplace (draws in outdoor air)
What would be the advantage compared to commercial products like: https://les.mitsubishielectric.it/en/products/ventilation_37...
This product cost around 500$ and also has heat exchange. A friend of mine has installed it and is very happy with it.
It's better compared to a blauberg vento or lunos e2. That product probably gets poor efficiency, I have not checked the technical sheet but if they say 80% that means at the minimal flow levels, which are only 10 cfm or something. The TW4 gets >85% sensible and comparable latent efficiency, at 60 cfm. It also has twice the maximal flow of that device. It's got many other features as well, and is more durable. Ultimately, it's about return on investment. You have to make a spreadsheet and see which one is best, given the actual tested values for efficiency flow, maintenance cost, etc. If that's not possible, it's a shot in the dark.
I was just recently researching these units..
This model is available in Europe for about 900 EUR: https://www.international.zehnder-systems.com/en/comfortable...
Is it safe? I think I heard that home made heat exchangers cause listeria or something like that. Something about condensation.
IINM, listeria is usually spread by contaminated food, not air.
Probably meant Legionnaires. Melbourne recently had an outbreak when outdoor cafes used cooling towers (essentially mist sprayers) with some sus water - https://www.health.vic.gov.au/health-alerts/outbreak-of-legi...
Feature request: Possibility to have more air intake than going out. This creates a positive pressure in your building, e.g., so that you don't get air from your garage into your living space.
This looks great. How well does the WM12 circulate the air, with the intake and exhaust being so close?
This guy on YouTube built a very clever DIY HRV, but got rid of it and went with a commercial ERV: https://www.youtube.com/watch?v=LiptsaKmq80
This looks wonderful, but it operates on the assumption that outdoor air is something you'd actually want to breathe where you live :)
Unfortunately, due to cars and dirty heating systems around where I live, outdoor air tends to be not great.
Nice idea, but why do they use Google Drive for sharing their code?
Poor man's CDN?
Wait, is the air outdoors generally cleaner than the air indoors? Certainly not true in the Salt Lake valley for the 4 months of the year we get persistent inversion.
The CO2 levels outside are definitely lower than inside. Often by 4x. Fixing that without freezing is the main use case for an ERV
I have a full system in my house. The unit is in the attic space. It's great although it makes a lot of noise, especially in the bedroom below the unit
This sounds great! The site left me a bit confused however. Is it open in respect to software/firmware? Or also the hardware? Can I just build my own with stock components? Something was mentioned about a DIY kit... The WM12 is basically two TW4 modules ... Um, TW4? As an ignoramus I need some introduction please...
Apparently it isn't open source at all, firmware is CC BY-NC-SA 4.0. It appears that the author does not know what open source means.
Could you post a higher quality photo of the heat exchanger? Is it 3d printed plastic?
How did you get the fans to run backwards? They look like standard PC fans
Some of these instructions have fiber glass or similar for insulation which appears to be used for this. It's not something anyone would want next to their ventilation system or inside it.
This isn't something to even consider without some expert reviews. The projects are also work in progress and overall incomplete with many details missing.
Be careful when you do anything involving ERV and HRV. It's very easy to cause serious damage to the property you live in, harm yourself and others in an irreversible way, or even both.
This looks really cool! I started to build something like this using thin square aluminum tubing but I never finished the project.
This is so cool!
This is amazing but needs much clearer docs rbh
Why can't I just glue a fan to an air filter?
You could do this as an air purifier, but it will primarily just remove particulates and other HEPA-y things. It won't actually bring fresh outside air in.
You could glue a fan to an air filter and then position the thing in a window, to bring in outside air that then gets scrubbed by the filter. But now you're bringing in cold air in the winter, or hot air in the summer.
An ERV brings in fresh air and mostly solves the problem of having cold air rushing in on a winter day, or hot air rushing in on a summer day.
Or open windows, but outside's cold in many places this time of the year. Energy recovery ventilators run stale outgoing air through a heatsink to pre-heat incoming fresh air to save heating costs. Sounds BS but it's a well established and widely used thing.
That won't decrease CO2 or VOC levels.
Air filters remove particulates down to some size, varying based on the filter. They do not scrub CO2 or CO, nor do they (generally) remove other things like VOCs - unless they’re really, really expensive filters. Opening a window exchanges all that junk to be roughly equal to the baseline of your environment, which for most people is at least lower CO2 levels.
> clean outdoor air
Is the air outdoor or clean? It can't be both!
Marketing bullshit aside, this looks great!
Fairly standard language for describing what an ERV does.
Low-CO2 outdoor air vs high-CO2 indoor air, if you prefer. Important for how air-tight modern energy-efficient construction is.
That kind of depends on where you live. And this can be combined with air filtration.
People breathe, so you need outdoor air to replenish the oxygen and get rid of the carbon dioxide. That's "fresh" air.
Unfortunately, outdoor air has particulate and ozone pollution. Filtering it gives you "clean" air.
In winter and summer, you also heat or cool the indoor air for comfort. If you just pump in outside air, you effectively also pump out the indoor air. This wastes the energy that had gone into heating or cooling it.
These systems save that energy by transfering heat between the air that's getting pumped in and the air that's getting pumped out.
> Fresh outdoor air
But this is actually treating indoor air, that's very confusing.
Fresh outdoor air is easy to find: just go outside!
You're also only going to get clean outdoor air in rural places where there is no/minimal farming and construction.
What’s so good about fresh air? Like I don’t want stinky stuffy air but as someone with central HVAC I had no issues with my indoor air. Are we trying to get outdoor smells? Or is it something else?
High CO2 levels impair cognition and stale air accumulates pathogens, not just smells. The V in your HVAC stands for Ventilation, so you're already getting fresh air, that's probably why you have no complaints. If you live in an air tight apartment with no forced circulation where CO2 levels spike super fast requiring ventilation several times a day, it's a different story.
In the winter it's cold outside and opening the window cools down the room -> no ventilation most of the time.
In the summer it's not a problem for me, I leave my windows partially open all the time but in the winter especially when working from home this would be quite neat. Also, I live in a small town in germany so the air quality here is comparatively good to many of the city folks here.
Possibly could help with radon poisoning