Coleman 7970C856 pilot won't stay lit

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collinh
Posts: 7
Joined: Fri Dec 02, 2011 10:27 pm

The pilot light in our Coleman Presidential 7970C856 furnace won't stay lit. I light it and get a nice blue flame, and when the thermostat calls for heat everything fires up nicely. But more often than not when the thermostat shuts off the heat, the pilot goes out. Looks like the main burner might be blowing the pilot out?

Sometimes it will run fine and stay lit for a few days at a stretch, but typically I have to re-light the pilot several times a day. Frustrating, to say the least! I've replaced the thermocouple (several times, in fact) and have completely taken out and cleaned the entire burner assembly.

What might be causing this? Need a new pilot burner/orifice, maybe new burner assembly? Could it be the gas valve itself? Any help would be greatly appreciated.

We're running on propane, if that matters. Unit has the Robertshaw gas valve.
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Greg
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Joined: Wed Feb 28, 2007 8:01 pm
Location: Weedsport, NY

Many times pilot light problems can be traced to the roof vent. A missing vent cap or possibly a vent pipe that is too short will allow down drafts to blow out the pilot.

Greg
"If I can't fix it, I can screw it up so bad no one else can either."
collinh
Posts: 7
Joined: Fri Dec 02, 2011 10:27 pm

I really don't think that's the problem. I can light it and get a perfectly steady blue flame, even on windy days. It will stay lit at least until the first time the furnace kicks on. Sometimes the main burner shuts off and the pilot stays lit, other times that's when it goes out. I hear the thermostat click and shut the burner off, and then a short time later (seconds, perhaps a minute; haven't clocked it) I hear the gas valve click due to lack of pilot.

Last time I pulled the burner assembly out to clean it, I saw that the little tab where the pilot burner attaches to the assembly broke at some point. The pilot burner is now held in place by "clamping" the pilot burner and shroud around the remaining piece of the attachment point. Once the screws are tightened it's plenty secure, but the downside is that the pilot is now roughly 1/4" closer to the main burner jet. Could this be the problem?
collinh
Posts: 7
Joined: Fri Dec 02, 2011 10:27 pm

No doubt about it... the main burner is blowing the pilot out. In fact, I sat and watched through the little door as the main burner blew itself out. Zero wind outside at the time. Has to be either a problem with the burner assembly, or else the gas valve is out of adjustment and the main burner is set too strong. Is that something I can adjust properly, or do I need to call a repair guy?
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flcruising
Posts: 606
Joined: Mon Dec 03, 2007 2:18 pm
Location: Florida Panhandle

Does the draft inducer run?
[color=blue]Aaron[/color]
collinh
Posts: 7
Joined: Fri Dec 02, 2011 10:27 pm

Not sure I know exactly what you are referring to?

If you're thinking it's starving for air and thus choking out, I really don't think that's it. It's the force of the flame from the main burner blowing out. That's sure what it looks like, anyway.
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flcruising
Posts: 606
Joined: Mon Dec 03, 2007 2:18 pm
Location: Florida Panhandle

It could be a valve or burner issue, or now the vicinity of the pilot to the main jet. A service tech would know how to check.

Here's a text copy from wikipedia about the drat inducer I mentioned-
The flame originates at the burners and is drawn into the heat exchanger by the negative pressure produced by the draft inducer. The hot gases produced by the combustion of the flame pass through the chambers of the heat exchanger and heat the metal walls of the heat exchanger. The gases cool as they transfer the heat to the heat exchanger and are at about 120 °F (50 °C) as they exit on a high efficiency furnace. The cooled gases then enter the draft inducer blower and are pushed into the venting pipes. The exhaust gases then are directed out of the house through the vent pipes.
It was just a thought of mine that if the draft inducer isn't inducing, then the flame will quickly consume its available oxygen and snuff out. The draft inducer is a small blower that runs for about 30 seconds or so before the main burner/blower kick on. Do you initially hear a quiet motor running when the t-stat calls for heat? Depending on the age of your furnace though, it may not be present at all.

Do you have a way to check your propane pressure level?
[color=blue]Aaron[/color]
collinh
Posts: 7
Joined: Fri Dec 02, 2011 10:27 pm

Based on that description, I don't think this unit has a draft inducer. The only click I hear when the t-stat calls for heat is the gas valve opening and main burner kicking on.

I really don't have a way to check the propane pressure, but our stove/oven also runs off the same tank and we have no issues at all with it.

Methinks it's time to call the repair guy. Thanks very much for your help...
collinh
Posts: 7
Joined: Fri Dec 02, 2011 10:27 pm

Long story short, we've ruled out most everything except the gas valve itself. I can't find a recognizable model number on the valve to determine the correct replacement. The furnace is a Coleman 7970C856 and it has a Robertshaw LP valve with standing pilot. Can you tell me if your 7956-336P valve is the one I need, or maybe the 720-007 universal?
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