Dometic Thermostat cycles furnace only one time

Question

I own a Roadtrek Class B with a Single Zone LCD Dometic Thermostat with that controls the furnace, AC and Heat Pump. In the Furnace mode, the ambient temp causes the furnace to turn on. Temp raises and the furnace turns off. FINE. But next time the temp lowers the furnace will not turn on. I can get the furnace to turn on again but must first turn the thermostat completely off and then select the furnace mode. The furnace will then cycle only one time and then not turn on again.

I can hear a relay click just before the furnace actually starts. No relay click the next time the temp lowers to call for the furnace to turn on.

Can anyone provide suggestion, is it the thermostat, control board, or what?

Thanks for anyone help

beemerphile1 · Answer

I cannot help you but those that can may just want to know the brand and model of furnace.

dougrainer · Answer

The Click you hear is the Upper AC Board, the furnace relay closing sending the 12 volt signal to the furnace. If you do not hear the click the 2nd time, it may be the AC control board. BUT, the ONLY sure way to tell is to access the 2 blue furnace tstat wires at the AC upper control board and see if BOTH wires have 12 volt positive. If NOT, you have a tstat or upper control board problem. VERY rare. I have NEVER seen that happen. IF both blue wires have 12 volts.--  You have a FURNACE problem, not a tstat problem. Usually, when you must cycle a furnace on and off when it won't relight after first cycle, it is a bad Ignition module board on the furnace. Doug

jimbthecamper · Answer

The furnace is a Suburban Model NT-16 SE

jimbthecamper · Answer

Regarding the post from dougrainer, I agree this appears to very rare. Will test the blue wires for 12 volts, just that they are very difficult to access.

jimbthecamper · Answer

Got it. It turns it was the sail switch in the furnace. Once the furnace turns on the sail switch moves to allow the burner to light. When the furnace turns off the sail switch stays in the "Air Moving" position. Give it a little time or knock on the furnace with my hand will nonstick the switch and everything back to normal.

Thanks to all

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Dometic Thermostat cycles furnace only one time

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