One thing I noticed when reading on diy hrvs in the past, is that many porjects are not taking into account air volume in the two ducts, and the amount of nominal flow for the size ducts compared to the ideal amount of air exchange. A 5 inch duct inside a 7 inch duct is more or less the correct size for many homes: about 50 cfm each direction. I do like the fact that there is a ton of dead space in most attics, where you can just drop a long, well insulated pipe hrv and be done with it for a relatively low cost.

I keep thinking window air cons should integrate heat pump and HRV. Such a simple solution for ton of problems.