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dimjim 03-21-2011 10:53 PM

Thermocouple and Gas Valve
Last week I hired a professional HVAC person to repair my 20 y/o Coleman furnace when the pilot light went out and I could not make it stay lit. He diagnosed the problem as a failure of the thermocouple and replaced it. The pilot light was successfully lighted and the burner came on and then the blower. We cycled through the start up and shut down a couple times and all was great.
When I was putting the panels back on the furnace I brushed the copper tubing of the thermocouple and the furnace immediately quit.
We lit the pilot again and the furnace started right up again until he moved the thermocouple tubing again and the furnace again shut down immediately.
He rerouted the copper tubing of the thermocouple to an out of the way place and fired up the pilot light and the furnace. All was working well.
He then told me that the gas valve must be getting weak to be so sensitive to the thermocouple tubing being moved and said it should probably be replaced.
My Question ~ Does it sound right that moving the copper thermocouple tubing would be an indication that the gas valve is about to fail?

The gas valve in question is a White Rogers 36C67.
Your thoughts.....?

hvactech126 03-22-2011 05:12 AM

test the gas valve and thermocouple using a tester. Check the milivolt output of the t-couple. If good, then blow out the pilot and see how long the gas valve pilot stays open and what milivolt it closes at.

dimjim 03-22-2011 10:09 AM

Thanks hvactech126,
I don't know how to do that or even use my multichecker. The HVAC tech sure didn't check it - just gave me his "opinion" and suggested that I have him install the $358 gas valve ($155 at HomeDepot).
I guess what I was looking for was another "opinion".
thanks again

kenmac 03-22-2011 10:48 AM

If there is a safety tied into the thermocouple circuit. That safety could be bad using up to many m.v's

how 03-22-2011 11:21 AM

Doing the test that hvactest126 suggested would of told him the strength of the thermocoupler and the pilot electromagnets in the gas valve. It is a basic, minimum test for any hvac tech with a pilot/thermocoupler/valve problem and takes a minute or so to do.
It could be other factors but by your description of it going out with a simple touch to the coupler does point to a faulty coupler or gas valve but if he skipped the most basic of tests, maybe he also didn't tighten the thermocoupler connection into the valve securely enough.

rightit 03-22-2011 11:28 AM

I agree with How. The fact that simply brushing against the thermocouple closes the valve is indicative of a bad connection. The 1st thing I would check is the tightness of the thermocouple connection to the gas valve. Gently grasp the copper tubing and gently 'wiggle' it to see if it's 'bottomed out' in the valve. If it's tight, there may be debris in the bottom of the 'socket'.


heatycooly 03-22-2011 02:08 PM

You can also over tighten thermocouple into valve

kenmac 03-22-2011 02:28 PM

He may not installed it correctly in the pilot flame. I've seen some, where a tech just stuck the t.couple in the pilot assy. without securing it or, stuck it in the flame too far

kswaby10 03-22-2011 03:24 PM

Makes sense kenmac.

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dimjim 03-22-2011 05:56 PM

hmmm, guess I should have made one more statement...
The furnace has operated perfectly for the past 4 days.
I'm just trying to decide whether to replace the gas valve.

Per suggestion above, I did move the copper tube of the T/C near where it goes into the gas valve and it is tight and as I moved it, the pilot light did not go out.
The "tech" may not have tightened it correctly the first time and then when he unscrewed the T/C from the gas valve to reroute the copper tube, he got it tight the second time.
Thanks for the comments but I'm thinking I best not mess with this and try to find me a more honest HVAC guy to check the gas valve and recommend replace or don't replace the gas valve.

how 03-22-2011 10:05 PM

If its been working for four days, wasn't mentioned as a safety concern, was only going to be changed for a pilot problem that may not be happening anymore... why not wait and see if you still need to change it.

There is one test you can try without equipement.
If the pilot flame is fully enveloping the tip of your new thermocouple, you can get a sense of if the gas valve electromagnets are actually weak by blowing out the pilot and timing how long it takes for the safety solenoid in the gas valve to lock out. You will hear a click from the valve when that happens. If it takes 20 seconds or less you are probably going to continue to have pilot outages. If it takes 30 to 90 seconds then you might not need to do anything.
Of course if you blow out the pilot and then can't get it to stay on after relighting it...nobody in your family is going to be very happy.

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