Furnace pilot won't stay lit. Thermocouple good

[Hettinga]

1984 Bigfoot 17.
Original Hydroflame furnace with piezo.
Last year I managed to get this old furnace running after pulling it out and cleaning it out. Everything was packed with dead flies. I had bought it from someone who had it sitting on a farm for 30 years.
At that time I replaced the thermocouple. Fan runs fine. After a good cleaning it fired up.
Now, at the end of the summer, early fall, I tried to fire it up, to no avail. The pilot will light but will not stay lit when I release the button. I have held it multiple times for up to 5 minutes. Nothing. I put a volt meter on the thermocouple and I was getting 30mv, that is supposed to be good. But still stuck. I can't believe it would be the gas control box. Any ideas? I'm at a loss.

[Raspy]

Try another thermocouple, they're cheap. While changing it make sure the connection is clean and tight at the gas valve. Blow it out and spray in some WD-40 if you see some corrosion. The pilot flame must bathe the thermocouple and sometimes the replacement kits don't hold them directly in the pilot flame. Finally, the propane orifice gets dirty over time and the flame gets smaller. The tip of the thermocouple must reliably glow red.

If all of this checks out, and it's not blowing out from wind, the gas valve is bad.

[Byron Kinnaman]

It's that time of the year. The furnace is about to get used after sitting all summer or longer. The nights are cooler and the spiders are looking for a place to build nests. The combination makes for trailer furnaces that won't work. The spiders looking for a sheltered home like the small orifice of furnace to build nests, thus disrupting the flow of propane.

It's been my experience that clearing the nest the propane devices work great. However newer furnaces have electronic ignition thus eliminating the problem with pilot light. A good spider spray applies every 30 days seems to stop the problem. I use a spray that's supposed to be effective up to 90 days after spraying, but I try to remember to use it every 30 day.

Also yellow jackets, wasps, etc like to get into the furnace this time of year. A wasp screen over the fresh air intake and the exhaust ports will prevent those nasty bugs form entering.

[Hettinga]

As mentioned previously, the thermocouple is good, the pilot light itself is good since the thermocouple is pumping out 30mv.
I like the suggestion to clean out the end where the thermocouple goes into the gas control box. I had made sure that it was screwed in well but never cleaned it out.
Spiders. That does not explain why the pilot doesn't stay on. It could explain an issue of the main flame not firing up. But as I mentioned, the pilot light itself seems to be good. I am hoping its the connection of the thermocouple to the gas control...fingers crossed.
Thanks for the ideas.

[Captleemo]

Have you tried blowing out that piece of copper tubing on the chance that there may be a fleck of crust in it.

[Darwin Maring]

Is the thermocouple over the blue part of the flame?

[Hettinga]

The thermocouple is good since it is throwing 30mv when the pilot is on.
Also, if the issue was a fleck of something in the curvy copper tube, then I should be having problems getting the pilot going and I would not be getting 30mv. 20mv is the minimum needed to open up the gas control from what I understand.

Could there be any sort of limit switch or rollover switch or anything like that that would prevent the pilot from staying on? The limit switch is easily accessible. But would that affect pilot or simply not allow the main burner to fire up?

I'm gonna head out now and try to clean out the hole the thermocouple hole/connection at the gas control box.

Thanks for tossing me ideas. Eventually it will help me nail this issue.

[Hettinga]

Another idea I'd like feedback on:
The thermocouple threw up to 31mv, when I was checking it, it might sometimes throw even more. Is there an issue with that much voltage? Is there any chance that the gas control somehow locked out by getting too high of voltage from the thermocouple?

[Raspy]

One more thing you might try is to lightly tap on the gas valve as you are holding the button to keep the pilot flame lit. There is a mechanical valve in the main gas control that must cycle with an electro-magnet. Yours might be stuck from gas contaminants or old grease in the mechanism. Later, if you get it going, blow out the pilot and after a few seconds you'll hear an audible click from this valve as it shuts off.

I doubt you can get too much voltage from a thermocouple. My dad used to get our stubborn home furnace to go by connecting an old flashlight battery to the thermocouple terminals. Not advising that, but
it did exercise the pilot valve.

[Hettinga]

Originally Posted by Raspy

One more thing you might try is to lightly tap on the gas valve as you are holding the button to keep the pilot flame lit. There is a mechanical valve in the main gas control that must cycle with an electro-magnet. Yours might be stuck from gas contaminants or old grease in the mechanism. Later, if you get it going, blow out the pilot and after a few seconds you'll hear an audible click from this valve as it shuts off.

I doubt you can get too much voltage from a thermocouple. My dad used to get our stubborn home furnace to go by connecting an old flashlight battery to the thermocouple terminals. Not advising that, but
it did exercise the pilot valve.

THanks Raspy for your ideas. However so far I have not discovered the issue. I cleaned everything out in case that was the issue, including where the thermocouple goes into the gas control, then wiped out with alcohol. I tried the tapping option a load of times with no success either. I think its coming down to the gas valve.

Any leads on finding a gas control valve for this? ONline I keep coming up with an Atwood 8012 II, which seems to be the same on the outside, but the gas control it uses looks quite different. Likely cause its all electric start these days. Gone are the piezos. I will also simply keep my eye open this winter for a cheap used furnace to simply swap out. That might be the cheapest option in the end.

[Raspy]

I don't know where you might find one, but they can't be too hard to find.

[Talia]

Did you remove and clean the gas orifice?

[threepine]

I have the same model Bigfoot and furnace as the original poster and have a similar pilot light problem which I haven't been able to solve. The reason for my post is I am also looking for either a gas control valve or another furnace to swap out, preferably with electronic ignition if such a swap is possible. So Hettinga, please update us if you do find a solution.

[Carl V]

Originally Posted by Hettinga

Any leads on finding a gas control valve for this?

No idea where you can get a replacement valve, but looking at your pictures, that valve looks VERY similar to mine. My furnace is a Coleman ST-200, model 4312, of the same vintage ('81 Trillium 5500). Compared to mine, looks like yours has the valve mounted sideways. Here's mine:


Looking for an ST-200 valve may enlarge the number of hits in your search.
Also, there are a couple ST-200 manuals in the document section. One shows that the flat head screw just besides the red knob is actually the pilot burner adjustment.

The issue I had with my furnace was a bit different, the pilot wouldn't lit at all. Turned out the pilot burner was clogged with rust, dirt, etc. I took it out, cleaned everything up, and afterwards everything worked normally, the pilot staying lit after I released the red knob.

Your pilot going out when you let the button go reminds me of an issue I've had a few years back with my popup fridge. Now you problem may be entirely different, but I'll tell you the story anyway, just in case.
My fridge just would stay lit, just like your furnace pilot. I checked the thermocouple, regulator, switched tanks, no change. I my search for an answer, one of the possible causes was what is called an "interrupter". Basically, the thermocouple screws in this interrupter, then the interrupter screws in the main valve. The interrupter has to blade-type terminals, on which a switch can be wired. The switch allows to cut the electrical path from the thermocouple to the gas valve, shutting it off. Mine looked like this, but it may look differently on other systems:


Now as I said, this may not relate at all to your furnace issue, and I have no idea if your furnace is designed with such an interrupter, but if it does you may want to check it out (you can simply bypass it to test it) with its related wiring and switch.

[threepine]

I may have missed this in the comments but have you tried the pilot adjustment. It is that screw cover just above the knob. Underneath is a smaller slotted screw for adjusting the pilot as far as I know.

[Carol H]

Originally Posted by Hettinga

Spiders. That does not explain why the pilot doesn't stay on. It could explain an issue of the main flame not firing up. But as I mentioned, the pilot light itself seems to be good..

Trust me all it takes is one tiny little spider web in there and all bets as to how things should work are off!

I know I am not the only party who might bet a beer or two on whether or not a spider or its web may be part of your issue.