Shop furnace won't maintain pilot

[rslaback]

My shop came with a wall type Williams 625DVX-N furnace. It is realistically undersized for the space but it keeps the chill off. This year it seems to have a problem maintaining the pilot. I have started it 3 separate times now and each time when I come back the next day the standing pilot is out.

It starts like normal and maintains the pilot. I turn the valve to on and it lights the burner and heats the shop.

I'm not usually in there long enough to know yet if the furnace turns off and it loses the pilot or if it loses the pilot sometime and then shuts off. I do know that it ran for over an hour while I was working on tires today before I left.

If it is losing the pilot I usually attribute that to a thermocouple issue but it seems weird (at least to me) that it would hold the pilot the entire time that it is running before losing it later. I know the thermocouple is a lot cheaper than the control valve so I am going to start there unless somebody tells me otherwise.

 

[Jim greengo]

Check flame sensor/thermal couple would be my 1st thought,then go from there.

 

[Rc_Guy]

Check flame sensor/thermal couple would be my 1st thought,then go from there.

That is what I would guess also

 

[Wrench97]

Does the pilot flame just go out or does it get blown out when the gas valve clicks on?

 

[bonneyman]

Thermocouples are "consumable" parts, meaning they deteriorate over time and are meant to be changed every 2-3 years. Especially the new ones. Being in a flame your whole life probably has something to do with it. lol For the $8-$10 they added to the bill I'd change them every year - especially if the customer was a far distance. Didn't want to have to do a recall a week after the cold snap because of a borderline thermocouple.

Check the pilot. The top third of the thermocouple needs to be enveloped by a stable, blue pilot flame. A weak yellow flame, or a lazy flame won't keep the thermocouple hot enough to generate the max microvolts to maintain the safety circuit. Dirty pilot hood or clogged pilot orifice might be present.

Then check the pilot holding switch. Though they could be changed without removing the main valve, there were so many kinds it was just easier to replace the valve. But pilot hold circuits don't go bad often.

Rarely air patterns around the burners could blow a pilot out. From inside or outside the unit. A pilot "shield" might have to be constructed to prevent such air movements form blowing the pilot flame around.

 

[rslaback]

Does the pilot flame just go out or does it get blown out when the gas valve clicks on?

That's the weird part. It stays on.

 

[rslaback]

Thermocouples are "consumable" parts, meaning they deteriorate over time and are meant to be changed every 2-3 years. Especially the new ones. Being in a flame your whole life probably has something to do with it. lol For the $8-$10 they added to the bill I'd change them every year - especially if the customer was a far distance. Didn't want to have to do a recall a week after the cold snap because of a borderline thermocouple.

Check the pilot. The top third of the thermocouple needs to be enveloped by a stable, blue pilot flame. A weak yellow flame, or a lazy flame won't keep the thermocouple hot enough to generate the max microvolts to maintain the safety circuit. Dirty pilot hood or clogged pilot orifice might be present.

Then check the pilot holding switch. Though they could be changed without removing the main valve, there were so many kinds it was just easier to replace the valve. But pilot hold circuits don't go bad often.

Rarely air patterns around the burners could blow a pilot out. From inside or outside the unit. A pilot "shield" might have to be constructed to prevent such air movements form blowing the pilot flame around.

I wish it was ten bucks. Cheapest I am seeing a P254000 is $55.

 

[The Cobbler]

sounds to me like a weak thermo couple and i would gamble on that , if it was mine.
many years ago My ex's friends had a water heater that would randomly lose pilot . they had a repair guy in & he was going to sell them a new heater. I replaced the thermocouple for them and it lasted at least several years , then lost contact with them

 

[isb cornbinder]

Check flame sensor/thermal couple would be my 1st thought, then go from there.

I totally agree. If the Thermocouple can become weak and not generate enough electricity to hold the pilot circuit open. We often light the pilot with a Bernzrite propane torch. This may make more heat than the pilot. The thermocouple make enought current for a short time. When it cools to the temperature the pilot can maintain, the current drops and the pilot circuit shuts off.
A NEW THERMOCOULPS IS LESS THAN $20.

 

[bonneyman]

I wish it was ten bucks. Cheapest I am seeing a P254000 is $55.

 

[PoorUB]

I wish it was ten bucks. Cheapest I am seeing a P254000 is $55.

Do you have a Menards or a real hardware store nearby? Run over and see what they have for thermocouples. A generic Honeywell/Resideo might fit, for a lot less money. Menards has them for under $10.

 

[isb cornbinder]

Do you have a Menards or a real hardware store nearby? Run over and see what they have for thermocouples. A generic Honeywell/Resideo might fit, for a lot less money. Menards has them for under $10.

Run over? Never, in this traffic. My first option is to check their stock with
Google

 

[Bert_]

I wish it was ten bucks. Cheapest I am seeing a P254000 is $55.

Looks no different than a Honeywell that I could get for $15.

That's all I carry is a few different length Honeywell versions. Haven't had something it wouldn't fit.

 

[Jackfre]

Pilot assemblies also accumulate a lot of dust. I’d hit the area with a bit of air and see if it helps any.

 

[nadogail]

Pilot assemblies also accumulate a lot of dust. I’d hit the area with a bit of air and see if it helps any.

I have solved more than a few problems with a simple cleaning of the equipment I a working on.

 

[Yankeefarmer]

As @bonneyman wrote, make sure the pilot flame is properly engaging the thermocouple. When I had a problem with the pilot in my gas furnace, I solved it by adjusting the pilot nozzle orientatio.

 

[DGersic]

I had a weird failure many years ago. The furnace had electric ignition, so not exactly like yours, but maybe similar.

The failure was in the pilot flame. The cycle was supposed to be:

1. pilot valve opens
2. electric spark fires to ignite pilot
3. pilot lit confirmed by thermocouple
4. main valve opens

What was happening was that the heat of the main burner was expanding the metal around the pilot orifice. This slowly closed off the orifice until the pilot flame went out. The thermocouple was only monitoring the pilot flame, so when the pilot went out, the furnace would shut down both gas valves (safety). Because the thermostat was never satisfied, the house would get cold.

I replaced the thermocouple several times in tracking this down. In the end, it took me sitting on a bucket watching the furnace cycle until I saw the failure happen. There are few things more boring than watching a furnace. Even when I saw it happen, I didn‘t initially understand the failure. I could see that the main burners were running, and the pilot went out, but it took a while to realize that the “flame sensor” was really only there to monitor the pilot flame, not the much larger flames of the burners.

I ran a tiny number drill bit through the orifice, opening it up slightly. I don’t know if a bit of corrosion or whatever got in there, but after that, the furnace ran fine for a few years until it was replaced (heat exchanger failed).

 

[fitter30]

Looks like this is a wall furnace? Uses a powerpile instead of a thermocouple. They put out 250 - 750 mv. 1/4 - 3/4 volt Gas valve needs 140 mv minimum.
At gas valve red and green wires comes from the powerpile disconnect from gas valve clean terminals and fork connectors. Lite pilot check voltage. Before pulling pilot and cleaning it turn heater on watch flame before and after blower starts. If flame changes might have a cracked heat exchanger. $17

Amazon.com

 

[venne]

Thermocouples are "consumable" parts, meaning they deteriorate over time and are meant to be changed every 2-3 years. Especially the new ones. Being in a flame your whole life probably has something to do with it. lol For the $8-$10 they added to the bill I'd change them every year - especially if the customer was a far distance. Didn't want to have to do a recall a week after the cold snap because of a borderline thermocouple.

Check the pilot. The top third of the thermocouple needs to be enveloped by a stable, blue pilot flame. A weak yellow flame, or a lazy flame won't keep the thermocouple hot enough to generate the max microvolts to maintain the safety circuit. Dirty pilot hood or clogged pilot orifice might be present.

Then check the pilot holding switch. Though they could be changed without removing the main valve, there were so many kinds it was just easier to replace the valve. But pilot hold circuits don't go bad often.

Rarely air patterns around the burners could blow a pilot out. From inside or outside the unit. A pilot "shield" might have to be constructed to prevent such air movements form blowing the pilot flame around.

I like hanging with you guys because I learn things. great post.
Bud

 

[rslaback]

So now I'm struggling to even figure out how to service the pilot or the thermocouple. I was expecting the assembly to pull out the bottom once I got the small plate off but that just sealed the holes that the thermocouple and pilot tube run through. Do I have to take the whole heat box off from the top to work on this thing?

 

[bonneyman]

The whole burner/valve assembly will have to come out in order to get access to the pilot assembly. Not overly difficult, just tedious.

 

[rslaback]

The whole burner/valve assembly will have to come out in order to get access to the pilot assembly. Not overly difficult, just tedious.

I finally got it. Had to fight a stuck gasket and then take the end of the gas line off at the elbow to get the clearance. Now that it's out, what should a guy use to replace the gasket that looks like it was torn the last time it was serviced?

 

[The Cobbler]

what should a guy use to replace the gasket that looks like it was torn the last time it was serviced?

if you get silicone in there, I wonder if high temp red silicone would be good?

 

[4x4Pete]

Fibreglass gasket material, like what's used on wood stove door. There's a glue that works with it.

 

[bonneyman]

Do a good cleaning with compressed air on the burners. And the fiberglass gasket stuff should work fine.

 

[Ben Buck]

I cleaned the igniter & thermocouple on my Procom heater in the garage the other day.

Used a green scotch rite pad, started right up , propane fed.