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500 millivolts is good to fired that heater, the minimum is 250. disconnect the wires connect to the gas valve be very carefulland see if the pilot stay on, if stay on get a piece the solid copper wire and jump the gas valve where the wires from the safety switches were connected and see if the heater fired. but before that check everi wire and safety switches for resistem ,also you water pressure switch,but also for any loose connection .
This symptom indicates something wrong with the millivolt system. Start by checking the wires from the thermopile to the gas valve, insuring they are all tight and in place. Check the hi limit thermostat, if you have an ohm meter it should read .1 or .2 across it. With you meter set to dc millivolts, and flame present on the pilot, read across the thermocouple wires (red and white) to see the millivolts generated, with the thermostat off and the gas valve in pilot position you should get approx 600 millivolt plus minus 50. The pilot will have to be on for a minute or so to get maximum millivolts. If the millivolts are correct, hi-limit closed, and all wiring is connected, you likely have a faulty gas valve. If the hi limit reads more than .1 or .2 replace the limit, if you are not getting the millivolts from the thermopile, replace it.
First check and see if the dial on the gas valve is on the ON position. Next turn the thermostat up all the way.if it does not come on then, this sounds crazy, but, tap the gas valve twice lightly with the back end of a screw driver, sometimes a spec of dirt can clog the gas valve and this releases it. if it still doesnt come on, you need a Volt Ohms meter or multi meter, set the meter to millivolts, you want to connect the 2 leads from the meter to the terminals on the gas valve that the 2 wires from the pilot generator, located next to the main burner underneath tank. turn the meter on and the millivolts should read a minimum of 400 milivolts, if lower, replace pilot generator. If still doesnt work, replace gas valve.
Check if there is 500 to 750 millivolts across the thermopile wires; thermopile generates 500 to 750 millivolts if properly heated to energize pilot and main valve solenoids of the gas valve for pilot and main burner operation. If the voltage across the thermopile is good and all wiring connections are ok, I think you have to replace the gas valve. Regards.
Has the control knob been turned from the PILOT position to the ON position? If that has been done, check the wiring from the thermostat to the Gas Control Valve in the heater. Make sure the wires are connected in both places (T-Stat & Control Valve). If all of that appears to be correct, there's a good chance that the Thermopile (I think Williams calls it a Pilot Generator) has failed. This part generates millivolts of electricity to open and close the gas valve when the Thermostat calls for heat. But, don't confuse the Thermopile with the Thermocouple. Your unit has both. And since the pilot is burning, that tells me it's not a problem with the Thermocouple.
Anyway, the only way to test the thermopile is with a millivolt multimeter. You will have to consult your Owners Manual for the proper millivolt output for your heater.
Hope this helps head you in right direction in solving this problem.
The safety feature for your fireplace is probably a millivolt pilot generator. It serves 2 functions. It holds open the pilot safety magnet AND operates the solenoid on the millivolt gas valve. The pilot generator is weak. It will hold open the safety but not work the operator selenoid on the gas valve. Replace it. And party on!
It sounds like the gas valve is dropping out. Make sure the system is venting the exhaust(no restrictions). Then make sure all your wiring connections in the unit are tight. In a millivolt system if all wiring connections aren't tight and clean your circuit will drop in millivolts and the gas valve will drop out. The gas valve requires a certain millivolt to stay open. I hope I was of some assistance!! Good luck!!
the pilot light is operated off a millivolt generator. that is the small tube directly in the pilot flame. its purpose is to generate a small dc current which is connected to the pilot valve inside the main valve. setting the control to pilot opens just the pilot gas supply. when operating normally the millivolt generator keeps a small electromagnet pulled in that keeps the pilot gas supply open. if it don't work the pilot gas supply valve closes. replace the thermocouple.