With the recent storm in the Northeast, we installed a 5000 watt backup generator to run a few circuits and an oil boiler. The furnace has 4 zones and one of the thermostats doesn't function because it draws power from an A/C air handler instead of the furnace. None of the proposeed solutions are great: 1. Add a second thermostat, one for the A/C and one for the heat. Works, but at some point, someone in the house will inadvertantly run both systems at the same time. 2. Install a larger transfer switch to power the AHU, which in turn will power the thermostat. Could cause problems in the summer when the AHU is running, but the condenser is not. 3. Install some type of relay to a second thermostat so when the AHU loses power, the control is passed to the second thermostat that will then control the heat. Could cause an extra load on the AHU although a small enough relay should be OK. The A/C thermostat is a Carrier Integrity, and the heat is just 3 wire zone.