Pilot light still won't stay on after changing Thermocouple

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Old 10-31-11, 01:52 PM
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Pilot light still won't stay on after changing Thermocouple

Hi guys,

3 days ago I tried to turn on the pilot light and it won't stay on. So I googled and figured it's probably the thermocouple. Yesterday I changed the thermocouple to a new one I got from homedepot, however, same result. So it's probably not the thermocouple. That leaves the gas control valve (Honeywell standing pilot, forgot the exact model #).

Now I also got a multi-meter to check the actual power to the control valve itself, hoping that the valve doesn't have to be replace... The incoming wire seems to have a stable voltage. One wire (black) then goes through the thermal cutoff and comes back, and at this point, the voltage is stable as well. Then these 2 wires are connected to two final plug wires that go into the control valve. Now when I check the voltage on those 2 end plugs, it jumps around between +20mV and -20mV constantly. Is this normal? Is the problem with the electrical input rather than the control valve?

I'm hoping I dont have to call a contractor to change the control valve, but if that is the case, what is your suggestion for a new control valve? one with intermittent pilot or some other model? And what would be the average parts + labor for a job like this?

Thanks, I will appreciate any help I can get.

(new home owner... bad headaches... thank god I found this forum).
 
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Old 10-31-11, 04:02 PM
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Most of the time the reason a pilot wont stay lit is that the pilot orifice and pilot burner are dirty and need to be cleaned.

Clean those first before doing anything else. It's routine maintenance that needs to be done in any case.

You need to remove the pilot burner, take the pilot tubing off and remove the pilot orifice. Sometimes you can clean the pilot orifice with the wire from a wire brush.

Use a wire brush to clean the pilot burner.

The pilot burner should look like a small blowtorch with the flame engulfing 3/8" of the tip of the thermocouple.
 
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Old 11-01-11, 08:36 AM
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Thanks for the quick reply, SeattlePilot.

The pilot light is strong, blue flame and engulfs a good third of the thermalcouple. I cleaned it anyway and the flame looks about the same. Still no luck.

What should my next step be?
 
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Old 11-01-11, 10:28 AM
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The thermocouple safety system is independent of the power supply to the gas valve. The thermocouple generates a small voltage used to power a magnet the holds open a gas valve allowing gas to flow through the valve. When you push the control to get gas flowing to the pilot, you are pushing that valve into contact with the electromagnet. As long as the thermocouple is hot enough to generate the needed millivolts, the valve is held open.

If the pilot goes out, the millivolts aren't generated and the gas valve snaps shut. So no outside power is needed for this system.

I recommend against DIYers replacing the gas valve. That's something that should be done by a competent repairman. Unfortunately, it sounds like you have a gas valve with a bad magnet, which happens occasionally.

A competent repairman should use a special fitting to verify this by using a special fitting to monitor the thermocouple millivolts and to verify that the magnet wont hold when the proper milivolts are applied. That confirms the need to replace the gas valve.
 
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