I was not getting any heat from one of the zones for my heating. I tried to replace the thermostat but when I tested the two wires (red and white) only the red wire had power. This house is about 40 years old and never had the thermostat replaced. How do I trace the wire or the source for the white wire? It's really really cold here right now! Thanks

The red should be hot from the 24 volt transformer. The white should run to the zone valve and perhaps the gas valve. Depending on the type of zone valve, the white may connect to the coil of a relay that works the zone valve. You should be able to measure 24 volts between the red and white. If not, try to trace the white to the zone valve or relay. The white should connect to perhaps a blue common connected to the other side of the transformer form the red.

I am asking to have this transferred to Heating and cooling if you need more help.