Thermocouple - question

RayP(MI) · Dec 10, 2015

Having trouble on my propane furnace pilot going out. Pilot seems OK. In the past I have solved this problem by replacing thermocouple. Yet, old one checks out electrically. 445 mv no load, 335 mv with gas solonoid activated. With pilot lit of course.

Any one have a knowledge of thermalcouples going intermittant?

crate78 · Dec 10, 2015

Had the same thing happen on my shop furnace a few weeks ago.

Cleaning the orifices in the pilot cured it. Seems the pilot light was strong enough, but it was too yellow and not hot enough to activate the thermocouple. The pilot orifices were partially clogged with soot.

After we cleaned the orifices in the pilot and have a pure blue pilot flame the furnace works fine.

Steve@Advance · Dec 10, 2015

They can be intermittent. If the pilot flame is too small or not directed at the tip they can go out. Also observe what happens when the blower comes on. If the flame is getting blown around, the heat exchanger could be rusted through. Very dangerous situation!

I have retrofitted several standing pilot furnaces with the Honeywell electric spark intermittent pilot kits. Eliminates the thermocoupler, no more relighting, saves gas.

Jerry/MT · Dec 10, 2015

There"s not much to go wrong with a thermocouple since it just two pieces of dissimilar metal wire spot welded together. They have to be in the correct position relative to the flame to generate the right voltage signal.

led · Dec 10, 2015

The first thing I would do is give a blast of air! About 60lbs.
HTH
Led

rusty6 · Dec 10, 2015

Agree on the air blast to clear the pilot nozzle. I just replaced my thermo couple in the gas heater with no results. Had a really weak pilot flame that would not activate the thermo couple so I opened up the line and blew it out with compressed air. Fired right up after that and hasn't quit yet.

jm. · Dec 10, 2015

Hard to explain and may not be relevant to your problem. At times I have to take a q tip and clean down in the tube where my pilot gets it fuel. there are a couple of small openings down in there and if the air is not coming in the pilot will no burn strong enough to keep the thermocouple contacted.

RandyB(MI) · Dec 10, 2015

I just read all the posts and will ask another Q. I have a 75000 Resnor ceiling hung heater,nat. gas. Only has 2 mos. usage but has sat unused since '93 when it was new. I got it mounted and plumbed and it fired up perfectly. Pilot took a little longer than I thought it should before you could let off button and stay lit. Cleaned out pilot orifices and it burns full and blue and aims correctly. Used it three times for about 15-20 min each time. went to use it tonight and pilot will not stay lit when let off red button. Cleaned thermo couple and made sure not touching anything and still won't stay lit. Worked perfectly 2 days ago and not now. I guess I want to hear from the guy who says they don't go bad. Have had them go bad on water heater before and replaced and worked fine ever since. Are they made in China like our condensers? Maybe what you are saying is that " they shouldn't ever go bad" ? , kinda like an ignition condenser?

RayP(MI) · Dec 10, 2015

Guys, read my post carefully.. PILOT IS FINE. If anything, a little bigger than necessary. Burner house not defective or leaky. Fan nor drafts down chimney not blowing pilot out. Electrical measurements output etc, all appear OK. I Have had this problem more than once over the years. Both when it was using natural gas, and now on propane. Have been all over this furnace like ugly on an ape. Repair is new thermocouple. Only thing I can figure is that the thermocouple goes intermittent!

rrc300 u · Dec 10, 2015

For the price of a thermocouple I would swap it out and have one on hand for next time.Thermocouples are mass produced and I can only imagine the process has taken someshortcuts. Most newer equipment does not have a Thermocouple and the need for them is reduced.

All that aside too big of a flame will hurt a Thermocouple the thing will be red hot and burn out in no time. They will get weaker over time but intermittent is a new one on me.

Bob · Dec 10, 2015

Curious, how are you measuring the voltage with the gas valve connected, is yours an older unit with open connections, or do you have an access adapter?

<img src = "http://s3.supplyhouse.com/images/products/zoom/10-038-3.jpg">

Or, Honeywell sells a tester that is basically part of the innards of a gas valve... you connect the tester with the pilot lit and depress the test button, which will remain latched as long as the pilot is lit, IF the thermocouple is not intermittent.

<img src = "https://images0.plumbersstock.com/product/750/750/371704.jpg">

RWT · Dec 10, 2015

Are you sure the pilot is ok. The thermocouple has to be in the flame not just close.

I also had trouble with a high limit switch going bad.

RT (my 2 cents)

Dr. Walt · Dec 11, 2015

There is a lot of micro-fine dirt in Propane. Turn off the Propane BEFORE it gets to the furnace, then carefully & thoroughly clean out the Regulator, Lines, & Orifices in the Furnace. You may be getting enough Propane to light the Pilot but not enough volume to keep it running properly.

I had a problem similar to yours, in my 40 ft. 5th-wheel Travel Trailer & after checking everything else, it turned out to be an accumulation of dirt in the Propane passageways. After a thorough cleaning, mine has worked like a new furnace ever since.

HTH

Doc

RayP(MI) · Dec 11, 2015

Older unit. Thermocouple connects to control unit via junction block and open screws. Just touch DVM probes to svrews.

Geo-TH,In · Dec 11, 2015

If replacing thermocouple solves the problem then it's simple, thermocouple is the problem. However the problem with standing pilets is down drafts blowing pilot out. Neighbors has an old gas furnace. Every fall she would call me to light her pilot. So if your issue in intermittent, could be the wind.

CJinMI · Dec 11, 2015

Sounds to me you have a ThermoPILE,
not ThermoCOUPLE by your description & meter readings.
Either way, your meter readings are off, low for one, high for the other.
Old school meter;
Untitled URL Link

kevinj54 · Dec 11, 2015

You are correct CJ. It sure sounds like a powerpile to me since he said screw connections.

Going from memory but I think he is looking for 750 open current and no less than 500 valve open.

So dirty pilot or failed powerpile.

However it seems that was a low output powerpile also.

Thermocouple - question

RayP(MI)

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crate78

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Steve@Advance

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Jerry/MT

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led

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rusty6

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jm.

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RandyB(MI)

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RayP(MI)

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rrc300 u

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Bob

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RWT

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Dr. Walt

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RayP(MI)

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Geo-TH,In

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CJinMI

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kevinj54

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