Thanks for reading. We have what I've read is a good old thermostat. It is a Honeywell T87F 2873 3 model. The problem is the heat will not kick on when it should. The air conditioning works fine, but the heat only works in Fan mode.

I have tried to reset the furnace by cutting the power and waiting 10 seconds. I have also removed the thermostat cover and checked the wiring, all of which looks tight.

Are there any other suggestions I can do, or would you recommend I replace the thermostat?

The back plate to the thermostat is coded 0039 Q539A 1014 3.

The wiring is as follows: (W) white wire (H) red wire (G) green wire (Y) blue wire (B) no wire attached (O) no wire attached.

Our furnace is a 2004 Rheem model # RBHC 17J14SFA. 208/240 volts. I understand our air conditioning comes from a unit on the roof of the condo. Our Rheem model heater does have a humidifier attached to it.

Any advice greatly appreciated. Thank you so much.