daestrom wrote:

2.2=60C0.075(wi-wo), so C = 37 cfm, with a heat loss of about
37(70-31.8) = 1420 Btu/h, which lowers the heater system efficiency
to 93%, compared to an HHV-based 100%.

70 degF air at RH 50% is about 25.5 Btu/lbm and 31.8 degF air at RH 100%
is about 11.7 Btu/lbm. So the heat loss from 37 cfm is
37*60*.075*(25.5-11.7)=2298 Btu/hr, more like 88.5% efficiency.

Ah yes. I was just thinking about the sensible loss. Then again,
we might build that Coroplast condensing air-air heat exchanger.

As long as you stay above freezing to avoid frost I suppose...

Maybe we need 2 fans, with a Thermocube and a relay to turn off the outdoor
fan if the outgoing airflow is less than 35 F. With equal capacity fans and
outgoing condensation, freezing seems unlikely. One $9 sheet of Coroplast
and a tube of 3M 4693 H adhesive could make a 2' wide x 6" high x 8" deep
box that sits in a window with about 64 ft^2 of heat transfer surface and
Cmin = 50 cfm and NTU = AU/Cmin = 64x1.5/50 = 1.9 and E = 1-e^-1.9 = 0.85.

Nick