Hello, all. I have spent the better part of this last week trying to figure out why my Williams natural gas wall heater (cerca 1970's) pilot light went out this last week. There was a cold snap, and I happened to be standing at the thermostat when the heater tried to kick on. There was a brief flash of the burners igniting in a normal fashion, then they turned off. I looked inside and found the pilot off, so I tried to relight the pilot, but it will not stay lit if I release the pilot button. This furnace uses the old 2 wire millivolt system and pilot generator (PG9 type). I tried removing the thermostat wires to see if it was grounding out, but that did not help. I cleaned the pilot and generator. I turned up the pilot flame and I got a millivolt meter, and the generator is putting out about 750mV. Now the pilot stays lit, but the pilot still goes out when I turn on the heat (ie. when I turn up the setting on the thermostat to trigger the burners to ignite, or if I jump the thermostat terminals) and the main burners never light. What's up? Bad gas valve or some kind of short?

 

I suggest having the gas pressure checked. Another possible problem is a partially clogged pilot orifice.

 

Well, you have 750mv, which is right. That says the pilot is putting out enough heat. You know it's a special millivolt thermostat, right? The thermostat hasn't been changed? Find the wires in the furnace where they leave to go to the thermostat. Jump them together [IOW- eliminate the wire run to the stat as a problem but DO NOT jumper across the gas valve terminals] If you get the same results, you probably have a bad gas valve. Any other gas appliances in the house? They working ok?