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I have a propane wall furnance that is'nt firing, the thermostat is not appearing to trigger the unit. The pilot is fine and the thermostat is a simple unpowered one with mercury that appears to be in working order according to my multimeter.

What may be wrong with the unit? How does one go about further diagnosing the issue? I did have to do some work on the thermopile some time back, is it possible the thermopile may be generating enough voltage to keep the pilot but prevent ignition?

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Try adding a jumper wire to the gas valve to see if that will fire the wall furnace. If it does, the problem is in one of the devices in control system. Jumper out each device until you find the one that has failed. Take down the control's model number and see if it one that is recommended for a thermopile system. All devices used with a thermopile system are made special for that type system. The thermostat used with a thermopile system is not a standard type thermostat; they usually do not use mercury. All the devices have special contacts that have very little resistence since the thermopile has a voltage output of about 3/4 volts. Next, when you jumper the thermostat wires at the gas valve and the unit will not fire, you should have a service tech check to see if the gas valve is opening and allowing gas flow but the flow is not getting to the burners. If this is the case they may need to remove the burner or burners to see if a spider or bug has made a home in the orfice of the burners.

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