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THOMASJU 06-23-2014 07:29 AM

Gravity furnace problem
 
During last winter I purchased my "new" 1972 Compact Jr. Have almost completed my re-doings and repairs. One major thing left to do. The furnace is the original gravity "Comfy" brand (don't have the model number with me right now). Need to take care of this before winter here in Colorado.
The pilot light works fine and the main burner comes on in quite a normal mode. (BTW, I have already removed the furnace, cleaned the burner, and reinstalled it). However, the main stays on for only about 10-12 seconds and goes off, and so does the pilot light. Since it is a gravity furnace there are no moving parts. I can re-light it repeatedly and it does the same thing.
I have to believe that the thermocouple for the pilot light is okay. However, at the bottom of the furnace there is a HUGE thermocouple-looking device that appears to be an integral part of the gas valve, unlike the pilot-light thermocouple which is easily replaceable. Don't know exactly what that is but I think that part is the culprit. Anyone have a solution for me, other than replacing the main gas valve? If replacement is the answer, GOK if there is a direct replacement is available. Tnx for any help/advice....Tom J.

Darwin Maring 06-23-2014 08:27 AM

I think that big thermocouple gets hot and actually moves a device at the opposite end (The mechanical thermostat). I had one like that on a small tabletop oven.

mszabo 06-23-2014 10:30 AM

Thomas, there is clean and there is clean. I'll bet you have spider nest in both the burner and thermocouple tubes.
Those nest are so sticky gas won't flow through them.
Air alone won't do it.

Roy in TO 06-23-2014 11:03 AM

Quote:

Originally Posted by THOMASJU (Post 466020)
Need to take care of this before winter here in Colorado.
The pilot light works fine and the main burner comes on in quite a normal mode. (BTW, I have already removed the furnace, cleaned the burner, and reinstalled it). However, the main stays on for only about 10-12 seconds and goes off, and so does the pilot light. ...

I have to believe that the thermocouple for the pilot light is okay. However, at the bottom of the furnace there is a HUGE thermocouple-looking device that appears to be an integral part of the gas valve, unlike the pilot-light thermocouple which is easily replaceable.

Winter in Colorado? I just heard some parts got snow last week!

Will the pilot light stay on for extended periods of time? Like an hour or so?

The HUGE thermocouple is what calls for heat, that shutting off the gas to the main burner should not affect the pilot light. Some furnaces have an adjustment screw for for the pilot light, you might want to try adjusting that. It is usually a needle valve, that may need cleaning.

THOMASJU 06-23-2014 06:29 PM

OK....just left the pilot light on for thirty four minutes. Then turned the gas valve to the "on" position and the main burner came on. Then it went off after 10 seconds or so and took the pilot off with it. Don't see how that could be spider webs/nests, but I see what you are saying. Any point in taking the gas valve apart to perhaps find something? Probably too hazardous when I put it back together again. Local RV parts guy said he could get a new valve for approx. $200.00. OUCH ! ! I AM "mechanically inclined", having been in home/appliance maintenance nearly all my working life (now 76 years old).

steve dunham 06-23-2014 07:46 PM

Check the size and position of the pilot flame (main burner off) and compare it to the size and position of the pilot flame when the main burner is on . Low/high gas pressure / exhaust draft can cause the pilot flame position to vary . The pilot flame may not be hitting the thermocouple properly when the main burner is on

THOMASJU 06-23-2014 08:46 PM

Thanx, Steve.....I'll ck that tomorrow morning.

TJ

THOMASJU 06-24-2014 07:36 AM

Steve...It is now "tomorrow" morning and I checked the furnace in the manner you suggested, three times. Pilot light stays "exactly" the same after main burner is on. Same results....after a few seconds the main burner goes off, taking the pilot light off with it.
If the HUGE t'couple is the one which controls the t'stat, I wonder why the main doesn't just go off and leave the pilot alone...
Something internal to the gas valve I guess.
I'll wait a day or so before I pull the furnace, clean the burners again (both pilot and main) before I think more about taking the gas valve apart and check it out/attempt to clean its innards and see what
trouble I can get into. I WILL NOT compromise my safety just to save $200.00. TJ

steve dunham 06-24-2014 09:22 AM

Does the main burner light right away when calling for heat or is there a slight delay and noise when the main burner lights ? If the combustion chamber is warped or damaged sometimes gas builds up in the firebox and when the main burner finally lights it will screw up the pilot flame and shut down the fuel supply.

Carol H 06-24-2014 09:30 AM

Quote:

Originally Posted by THOMASJU (Post 466173)
Local RV parts guy said he could get a new valve for approx. $200.00. OUCH ! ! I AM "mechanically inclined", having been in home/appliance maintenance nearly all my working life (now 76 years old).

Be thankful (then again maybe not) that they are willing to get you a new valve regardless of the cost. We have a number of RV shops here who have flatly refused to work on or bring in parts for old gravity feed furnaces (assuming yous is the same age as the trailers) for fear of liability issues.

David Tilston 06-24-2014 10:27 AM

I have never taken apart a gas valve in a gravity furnace. The temperature control seems to use gas pressure to turn the fuel on and off. I assume it is at least as complex as an old clock. It would be nice if they could be repaired.

What I would like to try is transplanting the gas valve from another gravity furnace. I suspect that the orifice of the valve is dependent on the Btu rating of the furnace, so I would try to match up the Btu values, or find a way to change the orifice.

mszabo 06-24-2014 01:21 PM

Thomas, when you check it again make sure the bees haven't setup shop in the exhaust side of the furnace.
If the heat exchanger fills with CO2 it will put out the flame also.

THOMASJU 06-26-2014 10:38 AM

Re post #9
No, Steve, that doesn't happen. I'll take the furnace out and re-clean the burner (poking something down into all the burner orifices) and see what happens. TJ

Darwin Maring 06-26-2014 11:57 AM

A pipe cleaner works really well. The kind of pipe a person smokes.

THOMASJU 07-05-2014 07:05 AM

Well, I ordered a new (been sitting on a shelf for approx. 40 years in original packing) thermostat control to replace the one on my gas valve. Will install it tomorrow or so and let you know what happens. Keeping my fingers crossed. Tom Justice

THOMASJU 07-08-2014 06:07 PM

New thermostat is now installed onto the main gas valve body and the furnace works just fine ! ! However, I have a concern regarding the fresh air intake and the exhaust (burned gas). The intake and the exhaust, on the street side of the unit, are immediately next to each other. This indicates to me that the intake will be taking in fresh air AND exhaust fumes. This appears to be the original furnace and intake/chimney system.
Doesn't make sense to me, from a safety standpoint. Any other CJr owners out there who can verify that this is the factory-built system ??

David Tilston 07-08-2014 06:21 PM

I don't own a Compact Jr. but I have several Trilliums with gravity furnaces. They use a concentric vent. Exhaust in the center, and fresh air intake surrounds that. I assume that exhaust does end up in the fresh air intake. But since, after combustion, it all comes back out the exhaust, I don't see a problem. None of it should come in the trailer.
As long as there is sufficient oxygen for combustion everything should be fine.

Darwin Maring 07-08-2014 06:38 PM

ThT is the way the fresh and exhaust air is. One tube is inside a bigger one. As long as the exhaust goes outside you R Golden. Don't reengineer the thing. I had one years and it was the same way. BE Absolutely Sure You Have A Carbon Monoxide Detector inside the camper and in good working order.

AlanKilian 07-10-2014 07:55 AM

Quote:

Originally Posted by THOMASJU (Post 469068)
New thermostat is now installed onto the main gas valve body and the furnace works just fine ! !

I could use a replacement on my furnace. Where did you find one?

THOMASJU 07-10-2014 05:30 PM

ALAN, check here on E-bay..
T5286A1007 Honeywell Adatrol Thermostat | eBay


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