thermocouple

jaybee

I have a couple of ?'s for the gas guys.
How do u tell if thermocouple is bad in a unit?
Also how do u determine if a gas valve is bad?

Ken_8

Need to know if it's a \"plain jane\"

variety, or a powerpile/thermopile type.

Assuming a plain jane version, the output when properly enveloped by the pilot should be around 30mV. Resistence should be between around .02 ohms. If a 48" long jobbie, the resistence may go to .03 ohms.

A no-load applied powerpile should put out between 600 and 750 mV and read around 2.89 ohms if good.

(Ain't Honeywell Tradeline catalogs grand)?

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jaybee

Where do u get your readings with your meter?

Ken_8

The ohm test is easy,

Simply remove the thermocouple end from the gas valve and put a correctly scaled ohm meter on the "button end" via some sort of alligator clip - and place the other lead on to any part of the copper tube that runs from the tiny insulating gasket/ring that separates the "button/end" from the reste of the "wire/tube" and you should get .02 ohms resistence between the two points.

They make little voltage testers that can be snuck between the engaging button and the valve body, but are a pain and the thermocouple itself so cheap, most of us can tell if the flame is improper and likely "cooked" a thermocouple prenaturely or not and will summari;ly replace the thermocouple anyhow - whether marginal or not, simply because the test will take longer than the replacement. Dialing in the pilot to the correct envelope is critical if you want the replacement to last more than a year!

FYI, it is not uncommon to see thermocouples last 10+ years when the pilot is properly adjusted.

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Nick_13

Thermocouple

That Mv reading is MvDC, and you can also test it w/o unthreading it. Clip one lead to the copper tubing of the thermocouple, and your other lead to a little knob that usually sticks off the gas valve rather close to where the thermocoupl threads in. Your reading here should be 15 MvDC. However, i agree, that normally we just replace them, because of how inexpensive they are.
Talking about long thermocouples, i just replaced a 60" one on an old old pizza oven the other day, talk about fun trying to hunt it down.

Dale

tcpl

I should start out by saying there are quick drop out thermocouples on gas fireplaces that use a thermopile for the main PORTION of the valve and a special thermocouple for the pilot safety, these must not be replaced by a standard type. The special ones have a size reduction on the tip. And some of the new FVIR water heaters have a safety in the tcouple center wire. The interesting question is when to change, honeywell did a survey some time back and found that the average gas furnace was serviced every 5 years and the tcpl life average was 5 years. So, if you don't know when it was changed and have the burners out for cleaning change it. If you changed it last year and there's no problem leave it alone. As to the mv the modern pilots may put out very low mv, a 45 second drop out test on the magnet is a pretty good test. I have "fixed" alot of tcouples by tightening them 1/4 turn past hand tight.

Ken D.

Thermocouple

I prefer to use my DMM on DCMv to check the actual voltage produced. This is especially helpful when there is a call back and the thermocouple has already been replaced. With the actual reading you can't go wrong. Disconnect the thermocouple from the gas valve. put your meter in series with the gas valve and TC. light pilot and hold until reading stabilizes.

Unknown

jaybee, get in touch with

me so I can help you out. give me your postal mailing address and I will send you a procedure for checking thermocouples FREE. As for the gas valve question there are many things that can be wrong with a gas valve. Are you looking for how to check it out electrically? Again I will send you some info to help you out.

Unknown

For everyone who posted to this

question after you replace the thermocouple and try to light the pilot and it still will not hold what do you do?

tim smith

t- couple testing

I can shed some light, us a G bushing adapter in the magnet port of the gas valve. screw the t couple into it. Put one lead on the terminal on the test bushing and the other on the copper t couple lead. I should read about 15 - 25 mv, then turn the pilot off and see where it drops the magnet out in the gas valve. Should be at least 10 mv differencee between where it held and where it dropped out the pilot magnet. Ps use Johnson controls t couples where ever possible, they are much better for millivolt productions. Good luck. Tim.

Jack_19

thermocouples & gas valves

Tim. Could you please send me the same info. that you offered to send to Jaybee? Thank you. J. De Witt 314 red oak rd. Northbrook Il. 60062

Dale

After that

If the pilot flame is good and the new tcpl isn't working I check the open circuit output of the tcpl and then the continuity of the eco. If the eco shows good and it's an older control I loosen and retighen the magnet which sometimes works. The open red/red eco is the usual problem in my area because of lime.