For anyone consider in this mod, it works perfectly, no need for an extra switch like the hunter thermostat upgrade. I used a white Rodgers 1F85 model.
Basically it goes like this...
Stage 1 heat - heat strip terminal on AC unit
Stage 2 heat - gas furnace
Fan output - low fan on AC unit
Stage 1 cool (compressor output from thermo) - cool input on AC unit
Stage 2 cool - high fan terminal on AC unit
12v+ to thermostat Rc /Rh
12v- to blue terminal on AC unit
I added a switch which allows high fan only, or a simple terminal swap I can force low fan only.
It works like a charm! Fully programmable, with fast or slow second stage. Runs on low fan / electric heat most of the time. Translations to using both gas and electric if the setppoint is off by more than a degree (adjustable spread). Engages both heat units or high AC temporarily to bring temperature to setpoint. This thermostat also has full 5+1+1 programmability with auto heat cool changeover. Cadillac......
Cheers!
