Pilot valve not being commanded to open

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Old 01-02-12, 02:21 PM
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Pilot valve not being commanded to open

This morning I noticed my heat was not working. The exhaust fan (which forces the combustion gas out the flue) turns on when when the thermostat is set to "heat", but the pilot valve never opens, so nothing else ever happens. I have confirmed that the control module (I have a Honeywell S8600M intermittent pilot control module) is receiving power (should be 24VAC, but I am reading 28.1VAC), but no voltage is ever sent to the pilot control valve. I can't see the spark, but it sounds as if the igniter is trying to spark. I just don't have a good view of it.

The furnace is an updraft Heil, model NUGE100AG02, probably 20+ years old.

Does this mean the controller has gone bad? I'm not sure what else to look at to diagnose the issue.
 
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Old 01-02-12, 04:31 PM
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Could be the control but also could be any of several other things. One of the most common is a draft switch failing to close & the cause is very rarely the switch itself. More often than not either the venter is at fault in that the nipple where the hose connects is plugged with rust or other debris. Remove the hose & clean out the nipple with a small drill bit.
If you have a volt meter you can measure voltage across the switch. Before the venter starts you should get +/- 24 vac across the switch then very shortly after start up the voltage should drop to zero.
 
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Old 01-02-12, 05:16 PM
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Well, if the spark is turned on, there should be 24 VAC or thereabouts being applied to the pilot valve as well. Both turn on together and both should stay on until the pilot light is lit, at which point the pilot gas should stay on, the spark should shut off and the main burner gas valve should be energized and turn on.

So if you have measured the voltage being applied to the PV contact on the ignition module while the spark is on and gotten zero, that indicates that the ignition control is no good and needs to be replaced.

Another thing I'd do as a double check if it were me would be to use a jumper wire to apply 24 VAC to the PV connection on the gas valve while the spark is operating. That should cause the pilot to and main burners to light.

You should be able to observe the spark on the furnace. If I'm recalling correctly, there's a flap that can be moved up and down above the burner opening to eliminate flame rollout, but which can be raised to inspect and maintain the pilot burner and burners.

I'd move that flap up and carefully inspect what is happening at the pilot burner, and you should be able to see the spark operating.

Perhaps Grady can correct me if he sees any mistatement on my part.
 

Last edited by SeattlePioneer; 01-02-12 at 05:37 PM.
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Old 01-03-12, 08:09 AM
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Well, I replaced the control module this morning, and everything is working fine now. Thanks for the helpful tips folks!

Grady - I did check the draft switch last night, and it did close when the fan was switched on. (In fact on my furnace it is wired in series prior to the +24V input on the control module, so since the control module has power, the switch had to be working.)

SeattlePioneer - there is in fact a shield that can be moved to better access the ignitor. It gave me a better view, but was still hard to see anything sparking. Last night, I used some jumpers to open the pilot and main valves, and lit the gas with a lighter, so that I could get some heat in the house. I made sure to only leave it operating as long as I was paying attention to it, since if the flame went out, the gas wouldn't shut off.

After hearing the spark igniter operate with the new control module, I should have known the it wasn't igniting properly before. It was clicking about once every second or two. With the new module, it fires a series of sparks until ignition (maybe 10-20 per second).

Thanks again to you both for your help. My house is now warm again!
 
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Old 01-03-12, 07:12 PM
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Mekaneck,
Good to hear you got fixed up &, as usual, S/P was right on the money. Once in a while we'll disagree but 99+% of the time he's right to begin with. Maybe one day I'll learn not to second guess him.
 
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Old 01-03-12, 08:35 PM
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Hello Grady,

Personally I think it's great that there are other people out there who can offer other ideas and suggestions. It's quite easy to make a mistake in trying to understand what is happening from the descriptions people give. I always find your contribution useful and well worth reading.
 
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