Heat only millivolt non-programmable thermostat stops turning propane stove on after 20 years. Stove turns on/off just fine using manual switch in back of stove. So I assume thermostat bad and replace with another millivolt unit. New unit turns stove on just fine, stove runs fine, reaches temperature, then turns off. Now thermostat will not turn stove on no matter what. Manufacturer says it is bad, I get replacement and it does exactly same thing. Turns stove on once, turns it off, then nothing. Checking thermostats, they are operating fine and closing circuit even after thermostat fails to start stove. Wires are just fine, thermostat about 30' from stove, stove operates as it should with manual switch. Checked all connections and tightened. Shorting the two wires -- usually -- turns stove on but not always. When thermostat won't start stove, shorting wires will not start stove. Stove manual switch works just fine and is closing same circuit in stove?? Obviously something weird, probably has to do with the millivolt generator (pilot light always works fine - no problem) or the thermocouple. Stove dealer scratching head. Anyone have a solution or idea as to what it could be? Maybe millivolt generator putting out too low voltage? I haven't measured that but thermostat turning stove on and off once and then not anymore after that would not make sense. Thanks for any suggestions.
Shorting the two wires -- usually -- turns stove on but not always. When thermostat won't start stove, shorting wires will not start stove.
That might indicate a thermocouple problem. The thermocouple makes the electricity which the millivolt thermostat is turning on and off. That electricity opens the gas valve.
Stove turns on/off just fine using manual switch in back of stove.
Oh, well, then. If that manual switch is directly operating the gas, that still points to thermocouple.
If it electrically shorts the wires (as the thermostat does), then that would seem to exclude the thermocouple. That would point to the wiring in the walls, and 99% of the time, a "wiring in the walls" problem is actually a "loose/corroded/faulty terminal connection".
When the stove does not start but there is a call for heat, check to see if there is voltage on the gas valve terminals. Also, check any controls that are in series in the control circuit ie, the thermostat, limit, and etc. Sometimes older controls will not make good contact due to worn out or corroded contacts. Many years ago,all the contacts used on a millivolt system had gold coated contacts to reduce resistence. Remember, all the controls used on this stove should be rated for use with a thermopile system. A new thermopile generator will produce approximately 750 millivolts (3/4 volts) so any resistence may afect operation. If you decide to replace the wiring in the control circuit replace it with at least 18 gauge wire. Hope this helps.