Day & Night (Carrier) Natural Gas Furnace Lights Sometimes

Problems related to residential installations.

Day & Night (Carrier) Natural Gas Furnace Lights Sometimes

Postby shutchin007 » Tue Nov 01, 2011 11:38 pm

Day & Night (Carrier) Natural Gas Furnace Model 394JAZ036065AAFA with intermittent pilot (3-wire model 740A).

Problem - Sometimes it works fine, but more often than not the pilot will not light unless I blow on it with a blow dryer while the electrode/ignitor is sparking. After just a couple of seconds of blowing, the pilot lights and then everything usually works ok.

Problem - Sometimes before the pilot has a chance to ignite or even shortly AFTER it has ignited, the ignitor shuts off after a fairly loud "clunk" sound. This can happen before the first spark or after just a couple sparks. Whatever makes the clunk sound seems to be shutting off the ignitor and the gas before it should.

I've already replaced the thermostat, the electrode/ignitor, the pilot assembly along with the aluminum supply line and compression fittings on both ends, and the gas valve, but none of that helped. I also inspected the circuit board inside the ignitor/lockout control box, but found no obvious burnt out components. I measured the spark electrode wire resistance at 0.5 ohms, which seems pretty good.

What could be causing these problems?
shutchin007
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- Day & Night (Carrier) Natural Gas Furnace Lights Sometim

Postby Freon » Wed Nov 02, 2011 9:50 am

Since you have replaced all but the control board, it seems, by the process of elimination, that's the problem. And here's one explanation. I do not know your furnace but most use a method of rectified current flow to prove pilot flame. The pilot acts like a conductor allowing current to flow through it. The current is very small and the control board is able to 'sense' this current and then allow the ignition process to continue. In your case, and I have had this problem too, the control board is not sensing the current or there is not enough potential provided by the board to generate the necessary current. Many times this happens especially with boards close to the burners... heat kills electronics quickly.
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- Day & Night (Carrier) Natural Gas Furnace Lights Sometim

Postby shutchin007 » Thu Nov 03, 2011 4:46 pm

Freon, thank you for your reply. I came to a similar conclusion, and just now replaced the igniter/lockout control board, but I'm not sure yet if it fixed the problem. On the first try, it sparked for about 30 seconds before I decided to apply the blow dryer, after which the pilot ignited immediately and everything worked fine. So...

Here are a couple easier related -
How long should it take for the pilot to ignite once the spark starts?
And how long should it take for the lockout control to shut off the spark if the pilot fails to ignite?
shutchin007
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- Day & Night (Carrier) Natural Gas Furnace Lights Sometim

Postby Freon » Thu Nov 03, 2011 5:21 pm

Time to use your ears. The second the spark starts, you should hear a distinct 'click' in the gas valve as it opens the gas to the pilot light. The pilot should light immediately. Now if you hear the click of the gas valve (as someone calls for heat and you watch and listen) and don't see the pilot, then you may need to adjust the pilot gas flow since it may be too low. I doubt your new valve comes from the factory all set to go. So read the gas valve manual about setting the pilot gas flow... it's very simple, usually a screw to turn to increase flow and flame.

If the pilot does not light, you'll hear the sparking for maybe 10 to 15 seconds before all shuts down. Some controls may retry after a minute wait. But you need to first get a well-established pilot flame before going further.
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