Hi, just wondering if anyone can help me here. I have a Honeywell programmable thermostat that was installed by a professional about a year ago. About 3 days ago, the thermostat stopped maintaining a temperature and began dropping. The thermostat is showing HEAT on the display, however, no heat is being produced and the furnace is not even starting up. The only way to get the furnace to start up is to place the system in "FAN" position to run the furnace continuously. However, in "AUTO" mode (the setting I always have it) does not seem to work. I've checked all the wiring and I've even change the thermostat to a new one. Any help would be great! Thanks!

labman

The problem could be in the furnace or the thermostat. You need a meter or test light that works on 24 VAC.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. It may be wired to have the control wires return to the furnace and its controls and then a second wire goes to the A/C unit.

Locate the blue and white wire at the furnace. If you have the 24 VAC at the furnace between the blue and white, the thermostat is working, and the problem is in the furnace. Depending on the furnace, the sequence should be the thermostat calls for heat by switching the power from the red wire to the white wire, any forced draft or enducer comes on, and the gas valve opens. As long as the gas ignites, the blower should come on soon based on either temperature or time. If the gas is coming on, on auto, and shutting off again, likely the control to the blower isn't working, and the gas soon shuts off again. Having the fan on continuos bypasses the fan control, and the gas stays on. Try to identify and check the timer or limit switch that controls the blower.