Thermocouple testable?

[PaPaPeng]

My gas furnace has a thermocouple problem. Stronmg winds blew out the pilot light. I can relight the pilot light, but no heat. Given that the thermocouple worked without trouble for some 20 years it has more than paid for itself. This was the only replacement in a 30 year old furnace. The trouble is that the replacement thermocouple I bought from Canadian Tire doesn't fit. The barrel at the base is undersized and even the adaptor ring doesn't bring it up to size. That's the only model (other than the copper tube conductor length) they have. The trip to a appliance parts store is quite a drive. To cut a long story short is there a way to test it with a multimeter to see if the original thermocouple is still good? I had already cleaned it and the circuit is good enough to keep the pilot light on but not enough to fire up the furnace.

I don't think it is a thermostat problem since it is one of those old style mercury switch types that doesn't have any fancy electronic circuitry to go bust. I do have an electronic one that I had disconnected long ago. I can never remember how to program it. Its still on the wall and used as a thermometer.

This thermocouple was in the plumbing (hot water tank) section and is the only thermocouple available. I would think its the same one used for the furnace as all is does is to sense the pilot light ON so that the gas can be fed to fuel the flame.

[Tony Hwang]

Hi, Thermocouple's function is to provide small amount of gas for pilot light. Main gas valve is actuated when thermostat sends 24V signal(W termnal from the 'stat) What makes you think thermocouple is your problem? 30 year old furnace? Isn't it time to replace it? Do you see

24V AC on the gas valve terminals when heat i called for?

[jim]

Suggest you get some one to look at it or phone someone to find out about part as most are the same size and are hence marked universel on package. You sound to have more wrong than what you think suggest a pro or at least some one with more experience take a look for you it is gas after all. Strongly look at a new furnace if you can.

[PaPaPeng]

Just because you are ignorant and insecure doesn't mean that everyone else is. If you don't have anything useful to suggest the than calling a tradesman I suggest you keep your non advice to yourself.

Tony Hwang did remind me of a simple voltage test. Its there at 28 Vac. Thanks Tony. I don't do this often enough to have missed a simple test llike that. But it was midnight, the light was poor and I was tired. My next step is to take out the current thermocouple and get a match. Its a $8 item and gets me out of the house. If it doesn't fix the problem I leave it there anyway as a new one saves me a future failure. Then I'll troubleshoot something else. The kind of repairs require only a module change, no fancy parts repairs. That's how things are done these days be it a module or a whole PCB for even the most complex equipment. I have more more than 20 years of fixing oilfield and industrial equipment.

Calling in one of your fellow tradesmen will easily set me back more than $100 plus the aggravation of hearing him cry death and destruction if I don't get him to replace the whole furnace immediately. I do my own maintenance and my furnace looks as new as the day it was installed. I had the heat exchanger tubes inspected by the gas company two years ago and there is a lot of life left in them. I asked and he showed me what to look for. When there comes a day for them to be replaced I'll do it myself and I can do a better job than many a tradesman.

[DonC]

Just a shot in the dark -- but it saved me years ago when I returned from Florida late at night in the dead of winter to find my gas furnace had shut down.

If your water heater was installed at the same time as your furnace -- assuming it was also gas fired -- it may well have the same thermocouple. Mine did so I pirated it over to the furnace and headed off to bed. I bought a new one the next day. But you situation may well be different.

Best of luck!

[PaPaPeng]

Actually that swap idea did occur to me. But its not that cold yet (58 deg F in the house) where I have a problem - frozen house plants. I can tolerate cold quite well. Anyway its off to the parts store for now.

[Meat Plow]

Thermocouple is ok if it holds the pilot valve open. Need to verify that there is a call for heat via the thermostat and the low voltage supply is available. Check for 18 - 24 volts AC at the gas valve. If so then the vavle is stuck or the coil is open. If not then check your low voltage transformer and wiring. BTW the thermocouple should put out maybe a tenth of a volt or so.

[PaPaPeng]

Went to two more places and they both have only the new "slim" thermocouples. I'll be darned if I am going to drive across town in the hope of finding an antique at probably double the price. So went home. Cut out a rectangle from a food can. Drilled the appropriate sized hole for the replacement thermocouple. Clipped that over the existing holder and secured the clip with a small screw. Works like a charm.

Should have thought of this originally but I am not that enthusiastic about duct tape and bailing wire stuff anymore. Its a lot easier to just buy the right stuff and replace.

To the Jeremiah's. The majority of house equipment problems are simple problems requiring simple fixes. Use a bit of common sense to get them running again. You don't have to go to get a university degree to learn how to design and manufacture the darn things in order to fix them. And when things go wrong don't tear apart the house to find the problem. I have fixed everything from fine wind up watches to that hernia box of a TV set, the electric organ, my car, the lawn mower, plus much more. The fun is in figuring out how things work as you go along and fix them. The worse that can happen is to toss them out.

[Moe Jones]

You are right and I total agree with you. He may have a defective gas valve and if he does did the valve just wear out or is there a short in the system, that took the valve out?

[Meat Plow]

Well he posted again and said he modified the way the new slim thermocouple mounted and now all is well. I suppose if the gas valve was stuck shut that all the vibration from drilling etc.. could have unstuck it. It's been years and I don't recall if I've run into a situation where a thermocouple could have just enough output to hold the pilot open but not enough for the safety on the main valve or even if if the older valves were designed in a way that this could be possible. I have my doubts that this is the end of his problems. As far as a short taking out the gas valve on a 20 year old system, that's not likely to happen as a short would remove the voltage path to the gas valve and possible take something else out instead.

[Don Young]

Some of the old gas valves supplied gas to the pilot independent of the thermocouple. If the pilot went out or the thermocouple went bad, the pilot gas was not cut off. Some systems even had a small separate valve which supplied gas to the pilot, totally independent of the automatic main gas valve.

Don Young

[Meat Plow]

Seems very hazardous to allow gas to the pilot if there isn't a flame to burn it doesn't it? I would hope this wasn't the case in valves manufactured within the past 50 years or so during which heating migrated from coal to gas.

[Don Young]

Yes, there was a hazard which is why that type is no longer used. I think the idea was that such a small amount of gas was not likely to be a problem as long as the venting was good. I do not know when they went out of use but it may have been when LPG became common since LPG is heaver than air and does not dissipate as well. Cookstove pilots used to be that way too and I do not know if any are still.

Don Young