[mackstann]

I've been thinking more about design.

I originally wanted to make a counter-flow HX, for maximum efficiency, but it seems like sealing up the alternating channels at the ends would be really finicky and prone to leaking. A square cross-flow HX seems much easier, but efficiency isn't as good. So what I'm thinking about doing is simply making many small cross-flow HX's and putting them in series. It would get close to counter-flow efficiency, but is simpler to make. The hexagonal ones seem like a pain to make and still just a compromise between counter-flow and cross-flow. The counter-flow HX is just so elegant and simple, I find myself maybe irrationally biased towards it.

It doesn't get that cold here in Portland, so I'm thinking I could just have a thermistor in the outside air inlet and shut down at the freezing point and wait until it warms back up before turning back on. The house leaks more during deep cold anyway, due to the stack effect. The most useful time for the HRV is during temperate times when there is no stack effect. Also, I could use a microcontroller to make the HRV run at higher CFM for a while when coming out of a below-freezing shutdown period. So during the winter it'd run at higher speed when above freezing, and then not at all when below freezing, while during warm times it would just run constantly at a low/moderate speed.

I'm a little concerned about condensate draining. Creating a sloped pan in the bottom of the enclosure seems like it might be tricky, and how do you keep it from leaking air? A trap will only work during times that create enough condensation to keep the trap full. During spring and fall, the trap might dry out, and then what? Air would be leaking out the condensate tube. Maybe it's a small enough leak (though a small long tube with lots of airflow resistance) to not care.