Flame Shutoff

[Crusty]

A new problem just appeared with my NT35 furnace. After the heat came on (fan, flame normally) and the temp setpoint was reached, the furnace went through it's shutdown cycle and the fan finally stopped. In a few minutes I noticed a burnt smell and I investigated and found that the burner was still burning (evidenced by smoke out of the exhaust). The furnace wouldn't respond to cycling of the thermostat or Off/On switch and I finally stopped it by closing the propane shutoff valves and ran the fan until it cooled back down.

My question now is: Do I need a new control board or a gas shutoff solenoid valve?

Opinions anyone.

[TG Twinkie]

Sounds like you gas valve is not closing off completely. It might have dirt or something in it. Doesn't sound like a safe situation. If that flame should go out, you could get raw gas and that could lead to a big bang if it is ignited.
It is normal for the fan to continue to run; it will run until the plenum temperature is cool enough to be considered safe. with your little fire going in there it probably takes longer to cool it down.

[Crusty]

It's not a little flame - the burner appears to be running flat out. The fan does eventually stop after the flame quits when I shutoff the propane.

I agree that it's not safe and I intend to keep my propane valved off until the problem is fixed and I've already broken out the electric heaters to get me through this cold snap.

I guess there's not really a way practical way to troubleshoot the problem and maybe I should just shotgun it and replace both the control board and the shutoff valve.

[Jammer]

If you are comfortable working on furnaces you could put a meter across the gas valve and see whether it is getting 12 volts when it does this.

With a 20 year old furnace I guess I'd replace the whole thing rather than replace a major component like a gas valve.

[jdalrymple]

Could be either the gas valve or board. The above suggestion is what I would do. The gas valve should have 12 volts across the two terminals when open and flowing gas to the burner. The flame should go out when the 12 volts quits.

Test by allowing the furnace to light normally, then pull one of the wires from the valve, if the flame stays lit, its the valve.

[Crusty]

The NT35 is an oddball size furnace and the replacement is a special order $1200 furnace. Newer cheaper models won't fit in the space. Since it otherwise works fine I'm going to devote some time and money to fixing it.

That's a good idea about pulling a wire to the valve - I might be able to rig a way to do that. Thanks for answering my question.