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Friday, October 23, 2009

No Oven Operation

My Wedgewood oven will not light the main oven burner. The pilot light will light, but when the thermostat calls for heat the main gas valve will not open. I'm just not sure if the pilot flame increases in size or if my pilot flame is just not big enough to heat the thermocouple. It seems very small, but if I wiggle the thermostat, it will briefly get bigger. The way it burns on just the pilot position is definitely not big enough to reach the burner. I have checked the LP pressure and checked the pilot orifice for blockage. These items are good. Any help would be appreciated.
Norm (Kelowna, BC)

Norm, typically after the thermostat calls for heat, here is what happens; assuming the pilot flame is already lit in the oven, when the thermostat is set to any temperature setting the oven pilot should become slightly larger or elongated, (this is also called the extended pilot or high fire), and begins to heat the thermal bulb (it’s not a thermocouple) attached to the safety valve. This thermal bulb, (see photo), when heated by the extended pilot flame, expands a bellows and opens a pathway in the safety valve and allows the LP to flow from the oven thermostat through the safety valve and on to the main burner where it is ignited by the pilot flame. That is why there is a time delay when the oven thermostat is first set to a desired temperature until the main burner actually ignites. Now if there are any blockages in the pilot orifice or its assembly, or if the flame deflection shield is mispositioned, the extended pilot may not fully engulf the thermal bulb of the safety valve. I’d suggest disassembling and cleaning the pilot assembly once again (some spider nests are quite stubborn!) and making sure the thermal bulb is held securely by the small locking screw or against the stop tab. Then run the test as explained below, keeping a close eye on the pilot size. It may be necessary to temporarily remove the main burner flame spreader in order to get a clear view of the pilot assembly. While watching the pilot flame, turn the oven thermostat knob up above 300-degrees F. Immediately, the standing pilot should expand and envelope the thermal bulb portion of the safety valve. If it fails to gain in size, the thermostat is faulty. If it indeed becomes bigger and engulfs the thermal bulb yet the oven main burner fails to ignite, then the safety valve is faulty and needs replacing. The positioning of the thermal bulb in the extended pilot flame mandates it must be fully in the fire of the extended pilot in order to heat the mercury inside.

Also, in some ovens, there is an oven pilot adjustment on the face of the thermostat. If the standing pilot is too small to begin with, the extended pilot might not reach the thermal bulb when the thermostat calls for heat. Remove the thermostat knob and see if there are any adjustment screws visible through the fascia of the front stove panel. You’ll want the standing pilot flame to be large enough to protrude slightly above the height of the pilot tube, but not too large that it heats the thermal bulb in the “OFF” position. If the LP pressure is at 11-inches water column coming into the range and the pilot orifice is clean and adjusted properly, the thermostat and safety valve should work together to get that main burner lit. The good thing is that by running the above tests, you can pinpoint the faulty component before having to tear the range apart.


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