I have two Maple Chasee (Robershaw now)thermostats in my house controlling two separate rooms. Recently one thermostat stopped turning heater on, but still working on cooling. My other thermostat worked fine both on cooling and heating. I bought a new Robershaw 9600 and replaced the old Maple Chase, but new thermostat still could not turn the heater on. It appears that my old thermostat is OK, the problem might be in that white wire or red wire, am I right? And what should I do to fix it?

Usually the thermostat switches power from the red wire from the transformer to the white wire that activates the heat. If this is a zoned hot water system, it may rely on a switch in the zone valve actuator to fire the boiler.