Explanation
Years ago I set out to make a low cost really good 3d printable window mount energy recovery ventilator (early prototype, for instance). Then I transitioned to wall embed units because they generally make more sense. They don't cost any more to make, but you get, fundamentally, more value out of them, because they are expected to operate like clockwork 9 months of the year for decades.
Then I realized that window mount units are really in demand, nobody wants to drill holes in their walls, and many people rent etc. and so can't. Also window mount units are a good way to prove out the design of a wall mountable one.
So I got back into window mount, but with the stipulation that it has to be mainly directed towards developing the wall-embed type. The model number of this one is WM12, window-mount 12, for try number 12!
To be clear the way this thing works is the fans reverse direction every 30 seconds or so (actually it's two fans in each side now, so four total in the WM12, pointed in opposite directions, they can each only turn/blow one way), and heat and water vapor is stored and released each time in the porous cylindrical white plastic object in the pipe. It is preferentially released back where it came from, basically. So it works both ways, to prevent intrusion of heat and water vapor, but also to prevent escape of heat and water vapor. The heat exchanger can be full sorption type, it uses silica gel type A to effectively capture and transfer water vapor. I can make them without silica gel too, some people will prefer that to remove water from the basement etc.
You can use the fans in several different ways and there are several accessories which also complicate things. But you don't have to, you can just plug it in and turn it on and it will work fine in energy recovery mode.
Also, importantly, because I'm using mostly the same components as the TW4 wall-embed model, should you buy a house and wish to install it permanently in the wall, you can totally do that.
To cut costs for the window units and accelerate development, I have subtracted some electronic components from the right indoor plate/cover module, which in the context of a window mount unit would be redundant.
The bulk of the value in energy recovery ventilation comes from running the machine a lot, over the long term. Thus, building a TW4 into a building permanently is the most sensible thing. The WM12 is a "gateway". You can buy it and use it as long as you want in the window, and when you get around to it, you install it in the wall.
This is why it is kind of expensive, and also not actually optimized for a window. A window only system is not really a good business proposition, unfortunately, and this is largely because there just isn't that much energy savings that shake out of short term use, and everyone always thinks of window-only units as short term propositions, not an investment, so the ends re retail price vs manufacturing cost don't meet in this context.
This additive manufacturing paradigm can work but only if you are making something really worthwhile.
Specs
The flow rate and efficiency are the same as a pair of TW4 units.
Noise level: 45 dBA at max power. This is remarkably quiet but you can certainly hear it. My house with absolutely nothing turned on in the whole house is 37-40 dBa. My furnace is 43.5 1 meter from a duct when it's turned on. This is higher than the TW4 because there are two modules sided by side, and also noise comes around from the outside in through the foam. The small green pipes on the outdoors help with this. I am experimenting with viscoelastic foam on the indoor cover.
Weight: About 2.5 kg.
Length: 486 mm long. The foam is 450 mm wide.,220 mm high. The pipes are 159 mm in diameter on the outside. The edges of the pipes are 400 mm apart, so that is the smallest window you could use it in, but shimmying it into a window smaller than 460 by 220 would require access on the outdoor side.
Electrical power consumption: 13 watts on max power (measured actual average). Down to nearly zero if you turn it down, of course.
For financial calculations, you can use the spreadsheet on the learn more page, there is a tab for the window mount unit with some suitable values already in it, and you can save a copy and modify the spreadsheet to look at other scenarios. It indicates clearly that time till return on investment is on the scale of a 1-2.5 years, which is extremely good for investments in general and especially anything energy related.
If you look at the energy recovery aspect alone and assume you needed the quiet fans anyway, it is more like 0.7 years payback period.
At this price of $510 usd per complete unit, the ratio of return on investment to capital cost is ~6 times higher than for a centralized ducted unit and about 30 times higher than for a typical decentralized unit. *depending considerably on which unit you compare it to*. The payoff period is affected greatly by your climate and energy costs, of course.
Open Source
It is of course open source, published for non-commercial use, intended primarily to enable maintenance, repair, and others to make replacement parts or add-ons. The files are already in the google drive on the source code page. You can print your own freely if you want, or I can sell you just the parts you need/want, but I haven't invested a lot of time making it easy to clone the device in a diy way.
FAQ
Where is the manual?
Here: https://docs.google.com/document/d/1rLJmJWaWK3KVCfxLsIWu4xua3atvRsI2/edit?usp=sharing&ouid=115633496345407542899&rtpof=true&sd=true
Can you mount it in "portrait mode" i.e. rotated 90 degrees from as shown?
Yes, I have tried this and it works ok. I do shut the window directly on the pipes so that the foam overlaps the window. Then use foam to block off the rest of the window (extra foam will be included in the box for this purpose). Tape the foam around the edges. To make sure it is highly secure in the window, I include a window restrictor, this is a small metal clamp that you can use to prevent the window from opening any further unintentionally. I also cut a 2 by 2 timber to just the right size so that it cannot fall out the window. Instructions will be included in the manual later, with pictures. The pipes sag slightly, but this is harmless and everything is more than strong enough to work well. Small amounts of snow can enter the fans but this has not been a problem during testing. It helps if you can go outside and inspect and adjust things from the outdoor side. If it's too high up for that you can still do it, though. You have to be extremely careful it doesn't go out the window during installation. It's a bit of a hassle and you'd only want to do it once per season, or even just take the heat exchangers out during summer and use it as a window fan (this will be an option in the firmware soon).