I'm the OpenERV guy. It's been a long road and it's not very open source any more I'll admit, however I did publish a bit Wikifactory article with all the cad

I'm the OpenERV guy. It's been a long road and it's not very open source any more I'll admit, however I did publish a bit Wikifactory article with all the cad etc. for a quite efficient window mount unit. I now have the know how to boost the flow through that thing by 2.5x or so so it could actually be a viable product/machine. However it's not a good business proposition. There is a whole class of energy recovery ventilators called decentralized, ductless or sometimes push-pull which are quite amenable to DIY. All you need is a good reversible fan and the regenerator and a pipe. I have shared elsewhere how to 3d print a regenerator. I use a 0.3 mm nozzle and it takes about 24 hours to print a regenerator that can do about 45 cfm at about 80 percent sensible efficiency. Basically just take a cylinder in Cura of the desired diameter, and use lines infill, no top layer, no bottom layer. That's basically it. Make the lines as thin as you can and the line spacing as small as you can while getting the desired airflow. About 2.7 mm on center spacing was about right for me and a typical fan with 6.5 mm stall pressure. The operating point was about 3.5 mm h2o and 45 cfm at maximal fan power. These devices do capture some water vapor during cold weather because the water vapor condenses on the heat exchanger then re-evaporates. I have tried sprinkling zeolite and silica gel in there to get good latent heat recover as well and it seems to work pretty well. You don't need much, only about 5 grams of silica gel, more for zeolite 3A molecular sieve. I have built two window mount units. One for myself, the mega sized one, which there are pics of on my twitter (@open_erv) and also one for my friend Alex, whose landlord wouldn't let him use it, so I sold it for the cost of the parts. It took forever to sell that thing even at the cost of the parts, so clearly window mount units are not a great business proposition. Also for other reasons, mostly people don't think of them as long term propositions so they aren't willing to amortize the cost of manufacturing a good machine over long periods. There is one called the purifresh, for windows too. The ductless models for houses include the lunos e2 for about $1800 CAD and only 25 cfm, and the blauberg Vento (there is another related one called Twinfresh), however I don't believe their noise or efficiency claims, and I have investigated. There are many cheap chinese knockoffs and some of them aren't too bad Holtop makes one. But mostly they are very poor. Ultimately I would like to make a hackaday or something that explains how to do all this, with a raspberry pi pico etc. Also an anemometer based on thermistors and the hot wire technique is useful to regulate flow, any extra flow in either direction hurts efficiency.