I have a reznor xl140-3 fired by natural gas. This problem started a while ago and I found a work around by just manually turning the gas on or off to have heat but I know this is not safe or convenient. The problem is that my thermostat seams to have no bearing on the heater. If electric and gas are on --the heater is on. The fan timing relay is still functioning because when I manually turn the gas down with the electric on it still flushes before shut off. I not see a spark(in the mercury switch) or hear a click which I do on my other units when I turn the theremostat above room temp. This makes me think the relay is jamed "on" in the heater itself. My question is: Does this seem logical as a diagnosis and can I access and replace the hidden relay myself?
ps. I tried disconnecting the thermostat entirely with the wires seperated and the heater still fires up. This also makes me think thermostat is good but the electrical problem is somewhere in the heater.