Gas and Oil Home Heating Furnaces - Heil gas furnace sparks pilot on for 1 second, then shutdown.

I'm guessing I need a new Honeywell ST9120G fan controller. It's a Heil gas furnace NTC5075BHB1 . When I call for heat, the combustion fan starts, the igniter sparks well, and the pilot lights perfectly for just one second then goes out. System then retries repeatedly, same result. I can feel the gas valve clicking to open, and there's 24 volts at the gas valve terminals when it's trying to light, and I've tried a different valve - Honeywell VR8204M, with same results. The rollout sensors, pressure switch, circulation blower are good according to my ohm meter.

I've read about cleaning the flame detector, but there isn't one in this furnace, just the rollouts and pressure switch. When I disconnect the 6-pin connector from the controller to the spark unit, the circulation fan runs, but no circ. fan otherwise. Has the controller simply failed? Thanks very much for reading...

Pilot probably uses some type of flame proving system. May be right through the pilot hood. You may have to remove the pilot assembly, inspect and polish everything up where the flame comes in contact.

Another common problem is the orifice right down there at the end of the pilot, gets plugged. Those orifice holes are extremely tiny. When they are good, you can barely see daylight through the smaller-than-a-pin size hole. I have effectively reamed the hole back out with a wire plucked out of a wire brush. Needles and pins are likely too large. You do not want to use anything like a drill bit to ream out the hole. The smallest drill bit is too large. If the pilot flame is too small, the flame cannot be proven up against the pilot hood, or whatever. If the pilot is not proven, the gas shuts down right away, like yours.

Can I bypass the flame proving circuit with a jumper to test that theory? Is the flame proved through the sparker, and would I possibly replace the sparker?


I had already tried cleaning the sparker and hood but it looked pretty clean , no luck but I'll try again. I had also blown air through the pilot feed tube and it seemed clear of debris, and he brief pilot flame seems fairly large, engulfing the sparker tip. There are only 2 wires connected to the spark/pilot assembly - a fatter insulated one for the spark high voltage and a thiner one for the spark ground.
Thanks.

Did you polish up both the part that fires the spark and the part that receives the spark?

You have to have a good clean path to ground between the green wire on the board and all metal involved between the pilot assembly and possibly any other metal involved that allows miiliamp current to go through that ground wire.

The only way I could think of trying to fool the flame sense is by temporarily connecting up a bare-end copper wire on a spade connector at the ground terminal, on the board and hold a lighter to the bare copper end and see if the pilot stays burning. I have never tried this, but the theory would make sense, as then you'd be duplicating the effect of the pilot flame.

Did you polish up both the part that fires the spark and the part that receives the spark?

You have to have a good clean path to ground between the green wire on the board and all metal involved between the pilot assembly and possibly any other metal involved that allows miiliamp current to go through that ground wire.

The only way I could think of trying to fool the flame sense is by temporarily connecting up a bare-end copper wire on a spade connector at the ground terminal, on the board and hold a lighter to the bare copper end and see if the pilot stays burning. I have never tried this, but the theory would make sense, as then you'd be duplicating the effect of the pilot flame.


Jumper wire and a screwdriver will work, hold tip of screwdriver in the pilot flame... Have done it many times. Only problem you have to know where to attach jumper. May be ground or the sense terminal.

Thanks for the tips about fooling the flame sense at the pilot ground terminal, very clever... However, now I've replaced the valve and pilot/igniter/sensor assembly and it still won't light the main burner. The pilot lights for a second then shutdown. If I disconnect the pilot ground, the pilot burns fine but no main burner. 24 volts at the valve, rollouts still good, etc...

Just bought a new spark control unit on Ebay, but I'm losing hope. One last question - Could the problem be low gas pressure? The gas meter was replaced 2 months ago, perhaps the regulator there is not set correctly? Thanks again for the help!

The fact you are getting spark means your low voltage safety stuff are all closed = good, and is allowing necessary low voltage to your intermitant pilot ignition control module.

You would have to analyze the pilot flame height/direction to see if the pilot flame is touching any of the metal that allows flame sense. Then deduce from that if you think you have a gas pressure problem.

IF you can see that it is, let's say - then after all you say you've done and replaced, it pretty much have to be the control module.

Replaced the spark control unit, works now, thanks all.