My home is heated by an older model Empire wall heater. The heater did not start up last week and I could not figure out the problem so I called a repair man. He found a wire in the heater that had become disconnected and hooked it back up, after which he was able to start the heater by lighting the pilot and turning the gas to "On." However, the thermostat would not turn the heater off so he said I would need to replace the thermostat. I have installed a new thermostat (a simple 750 millivolt from Ace Hardware) but the heater does not repond to the thermostat. I can get the burner to light up by putting the gas to "On" but to turn it off I have to turn the gas down to "pilot" again. The heater burns even when I disconnect one of the two wires at the thermostat which makes me think there is another factor at work, maybe something else electrical between the thermostat and the heater. Any ideas? Thank you!

Gas valves can stick open, it's rare but I've seen it. Has this furnace ever been flooded by water?

The heater has never been flooded. Is there a way to check out the idea that the gas valve may be stuck? Thanks for the reply.

If you beat on the side of the gas valve with a hammer handle, or a screwdriver handle it may unstick a stuck valve.

Thanks for the suggestion but beating on the gas valve had no effect. I wonder if there is some type of electrical problem. There are three wires leading to the gas valve, lined up vertically above one another. The top and bottom ones are wires coming from the thermostat. The middle wire leads to the thermocoupler and it connects with another wire to the thermocoupler which connects to the bottom wire from the thermostat. If I disconnect the thermostat wire on the top connection the burner is not affected and keeps running. If I loosen either the middle or lower wires the burner and pilot immediately turn off. Does that mean anything?

Power flows from usually a red wire at the transformer to the thermostat which switches it to a white wire to the gas valve. To complete the circuit, the other side of the gas valve coil must be connected to the other side of the transformer. It sounds to me like the thermostat is constantly calling for heat through the one wire. It could be bad, or the wires to it could be shorted.

The 3 terminals must be 2 different firing rates plus a common back to the transformer.

The wall heater is working fine but when it reaches the set temperature, the heater turns off so as with the pilot light. What could be the problem?

Thermocouple