Our furnace heats up to the set temp But about half the time doesn't start back up when the temp starts to drop. We replaced the thermostat last summer and last winter the furnace did this and the hvac guys checked the drainage. Of course it doesn't do this when the HVAC guy comes so he's got no way of seeing what it's doing. Any suggestions? When I shut it off atty the switch and back on the furnace will kick back on.

Our furnace heats up to the set temp But about half the time doesn't start back up when the temp starts to drop. We replaced the thermostat last summer and last winter the furnace did this and the hvac guys checked the drainage. Of course it doesn't do this when the HVAC guy comes so he's got no way of seeing what it's doing. Any suggestions? When I shut it off atty the switch and back on the furnace will kick back on.

In answer to your NO model number,NO brand Name furnace. It is possible that you have a loose ground wire from the electric panel to the (as you call it) atty switch or from that switch to the connection on the furnace circuit board.Also check where ever the furnace is grounded itself. While you are checking the ground and have the equipment open you might as well check the neutral wire and hot leg (legs) since this furnace could be an electric furnace.

Sorry but with the limited info you offered it is the best I can do. Good luck with your repair.

Sorry! It's a trane 90 plus. No idea how old but the house was built in 93.

The furnace will have a fault code on the control board when it has failed to start. Do not turn the power off to the system or turn your thermostat off(it will reset the control board) go check the control board for for faults. Lower door of the furnace should have a small glass window so you can count the blinks of the lights on the control board.