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!

That's very kind, but I have all the (SR)MHRV units that I can reasonably fit!

FWIW the email address on your site page is bouncing for me.

Please do add my email (in my profile here) to a low-volume mailing/updates list if you have one.

And if my limited experience of bringing an open hardware project to market might be of help, let me know!

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.

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.

yes i get that, but say you have an indoor temp of 30c and an outdoor temp of 0c. the average of this heat exchanger is going to be 15c. so on average youre only cooling the exhaust down to 15, and heating up the intake to 15c.

a counter flow heat exchanger can get the temperature higher than the average because 30c exhaust is meeting partially warmed intake, and 0c intake is meeting partially cooled exhaust.

Unless theres a phase change???

Though the average temp of the overall core may be 15c, there may be a thermal gradient along the length, so maybe the inside end averages 28c and the outside end averages 2c, or something like that.

Perhaps.

I was trying to reason it out, you have the thermal mass of the heat exchanger material, that you aren't going to be able to tune to every temp differential.

at the start of the cycle, it may be very efficient, but at the end of the cycle where the heat exchanger temperature is going to approximate the temperature of the air, efficiency is going to drop, and that would be happening along the length of the heat exchanger, so by the end of the cycle the inlet would be basically doing nothing, and the exhaust quickly getting to that point (in the best case).

I was starting to confuse myself at this point, so the constant temp model seemed easier to reason about.

Although, having written all this the model would basically be a constantly shortening counter flow heat exchanger? with a thermal mass not necessarily tuned to the temp diff.

So its still going to be worse than a counterflow heat exchanger, at least theoretically.

Perhaps https://news.ycombinator.com/item?id=42428537 or its parent will make sense to you: connecting 3 or more perfect constant-temperature regenerative heat exchangers in a series would make the ones at the ends work at a higher/lower temperature (of the constant-temperature heat sink for those exchangers) than the one in the middle, increasing efficiency of the overall system.

I'm not proposing this as a practical design, but it convinces me that 50% efficiency is not the limit.

Maybe? but doesn't that just become a poor mans counter flow heat exchanger?

It's closer to a counter-flow heat exchanger, but you could still have air only go in one direction at a time. Say your indoor temperature is 20 degrees and the 3 heat exchangers are at 25, 30, and 35 degrees, and outside is 40 degrees (I'm thinking in Celsius, though 40 is a bit extreme). You blow air out until they cool to 20, 25, and 30 degrees. Then you blow air the other way until they heat back up to 25, 30, and 35 degrees, with the air coming in being somewhere in the 20-25 degree range instead of 40 degrees that the outside air is at.

This assumes that the heat exchanger has just enough thermal capacity so that raising/lowering its temperature by 5 degrees would get the air to the same temperature. In fact, it might be easier to imagine if the air doesn't blow continuously. Each chamber could fill up with air and wait for the heat exchanger and the air to get to the same temperature, before moving the air to the next chamber.

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.

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.