Comment by walterbell

Bionaire's elegant $100 commercial window HRV was discontinued, there don't seem to be any alternatives at that price point, https://www.amazon.com/Bionaire-BAP336M-U-EverFresh-Exchange.... Product manual might be useful for DIY: https://ia904509.us.archive.org/29/items/manualzilla-id-7067...

Commercial ERV/HRV units start around $500, https://www.greenbuildingadvisor.com/question/inexpensive-ef...

DIY counter-flow design with coroplast sheets, https://ecorenovator.org/forum/showthread.php?t=891&page=38#...