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Question DetailsAsked on 9/11/2016

I can light the pilot and it lstays lit but goes out after the burner shuts off. Any ideas?

Thermocoupler coupler has been replaced. The intake screen is clean.

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So - pilot stays lit OK when furnace or water heater is not firing, it then fires OK but when the main burner goes off the pilot flame goes out ?


Most likely cause one of the following:


1) thermocouple is not properly placed in the pilot light holder - could be not far enough in the flame (see instructions on package or water heater/furnace instruction manual), or more likely the draft shield (the curved finger-like piece that reaches up around and guards the pilot flame) is not oriented right, so when the burner is off or is firing the thermocouple is getting enough heat to stay lit, but when the burner goes off the continued (for a few seconds) fresh air draft up the flue pulls the pilot flame away from the thermocouple tip enough to let it cool off, thereby shutting off the gas flow to the pilot (and burner). If the pilot can then be relit and keeps going, this is likely the problem - or the next one (2 below).


2) check the cover panel and/or inspection port is properly in place - could be too much draft is getting into firebox and blowing out the pilot when the burner shuts off.


3) if this happened with prior thermocouple and also with the new one, could be the thermopile spring in the gas controller is going bad, so it holds the gas flow open during normal operation but when there is a cool air draft across the thermocouple as the main burner goes out the microvoltage from the thermocouple is not enough to hold it open. Sometimes fixable by putting the thermocouple further into the pilot flame, sometimes requires new gas controller (they are now replaceable only, not repairable, by law). Most plumbers/HVAC contractors these days do not even know how to test the gas control valve for voltage and gas valve spring action.


4) poor thermocouple center button contact in the gas controller can cause this too - and can be confusing, because say the old thermocouple went out from old age, you expect a new one to work right. But might fail to work right (or at all) if it is kinked too much, is not properly in the pilot flame, the contact surface up inside the gads controlled is corroded, or thermocouple tip does not have good contact in the gas control valve.


On the latter - if you remove the thermocouple threaded connection at the gas valve (no gas will come out - this is the thermocouple lead (usually aluminum), NOT the small diameter gas tubing (usually copper) to the pilot flame), straighten the tubing if needed and slide the threaded bushing down on the tubing a few inches, insert the straightened tubing fully up into the recess in the gas valve - there is commonly a shoulder most of the way in, so be sure it is fully seated all the way up in, THEN thread in the bushing while holding the thermocouple tubing snugly in place. It is way too easy to get the tip cockeyed and jammed in the recess, not making good (or sometimes any) contact with the contact point up in the recess if you try to insert the thermocouple tip and the bushing at the same time.


5) rarely, with a few brands, the system uses the outer surface of the thermocouple tubing as part of the circuit, so it the thermocouple is touching any metal along the way it will dissipate the microvoltage and cause intermittent or full failure - but in those cases usually the problem is in keeping the pilot lit, not having it work OK till the burner goes out. Can be a problem with metal contact at the pilot end thermocouple holder (usually has an insulated sleeve around thermocouple if that type), or the tubing touching metal along the way.

Answered 2 years ago by LCD




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