Thanks again. I believe the auxiliary heat is electric and that it must be energized by the system and not the thermosstat. On the mechanical thermostat I'm trying to replace, there is no auxiliary heat setting. It is definitely a heat pump, but it is not located outside; it is actually in my condo unit. The building supplies hot water to the heat pump during the cold season, so it rarely uses the auxiliary heat unless I need heat after they've turned off the hot water supply. The mechanical tstat is the Honeywell round one w/a Q539J subbase. Other than the 4 wires, R/G/W/Y, that were connected to their respective terminals, it has an E/F switch set to "E" and heating cycle rate switch set to 3 cph. I just saw in the installation manual that I can set the cycle rate and I have not tried that yet, although I'm not sure it will make any difference. I have tried setting the 180 function to 1 and the condenser does not power up. I have tried setting the 170 to 4 and get fan and condenser, but it is cold air! I'm running out of options: maybe I'll just stick with the mechanical one.