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Discussion Starter · #1 ·
I have an 80's Bryant gas furnace, 376BAW. It's a down-flow. On this furnace a pilot is spark-ignited at the start of each cycle.

Let me review how this furnace operates to the best of my understanding. Experts can skip ahead.

When the thermostat calls for heat, it turns on a relay that sends 120VAC to the inducer (exhaust) blower motor. It also passes along a 24VAC circuit (probably to limit the current through the thermostat. When operation of the inducer is proved via a diaphragm pressure switch, power is sent to the pilot gas valve, and to an SPDT switch heated by the pilot. When cold, this switch sends power to the high voltage spark generator. As it heats this is turned off and on continued heating, the main gas valve is turned on. There is also a line going up to the control board which activates a time delay so the blower comes on a short time later (and stays on for a while after burner shut down. When the pilot heat switch sends power to the main gas valve, it takes about six or seven seconds to open; it doesn't just snap open. The gas valve manual says "delay-opening."

Just for the record, the gas valve (White-Rodgers 36E93-301) consists of two valves in series for redundant safety. Pilot gas is tapped off in between. So what I call the pilot gas valve is the first valve. What I call the main gas valve is the second valve.

Here is my issue. A few times now, I have noticed the main gas coming on shortly after the pilot valve opens. When it does this is it is NOT waiting for the pilot heat switch to get hot. However, it does not come on instantaneously with the pilot gas; there is still the several second delay. If it was physically stuck open one would presume it would come on with no delay.

You might think maybe the pilot heat switch is stuck on. I know that is not the case because I added an indicator (an LED with suitable resistor). In fact I have two...one on the hot side output of the pilot heat switch and another at the main gas valve connector...even though I think these are simply opposite ends of the same wire. This also dispels any notion that something odd at the blower control board is sending power down.

When this happens, the pilot heat switch DOES kick on about 45 seconds after pilot start. That's when the LEDs light up even though main gas was already on.

I am hoping an expert will say something like

"Any time you have main gas without the main gas valve input being energized you have a bad gas valve and it should be replaced."

I put that in quotes because I don't know that that's the case. While I've done a lot of work on this furnace over many years (replaced the inducer motor twice, gas valve once, and pilot heat switch soon for the third time), I'm not an expert and do not pretend to be.

Yes, I'm about to replace the pilot heat switch because the sparker has not been turning off. I have the part but I'm holding off for now because given this possible gas valve issue maybe a non stop spark is a blessing in disguise. I do not know how long that's been going on but it hasn't seemed to burn out the spark generator. As I said I know it's not sticking on the hot side as it does kick the usual time after the pilot starts.

The reason I noticed the possible gas valve issue is because the thermal safety switch atop the blower tripped a couple of times. That was a puzzler but I think I figured out how they relate. Apparently the time delay for blower start is timed to burner startup NOT so many seconds after the thermostat calls for heat. The reason that matters is that if the burner is coming on right away that means the burner is on a minute or so earlier than what the time delay was designed for. So the heat exchanger is getting a lot hotter before the blower kicks on. As this is a downflow, with burner & heat exchanger below the blower, the hot air convecting upwards can trigger the safety.

FWIW it has not been doing this weird thing lately. But it def did it enough time for me to notice and see main gas burning w/o my LEDs lit.

Time for a new gas valve?

(Yes, time for an all new furnace and that is in the plans for the future.)
 

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The ignition system is obsolete and you may have issues getting parts.

You can rule out the gas valve by disconnecting the wire which opens the main valve. Could be some kind of intermittent short.

I doubt it - problem is likely upstream.

Need meter to troubleshoot it.

To be honest a unit with 3 wire pilot is not worth fixing - you'll be best off getting rid of it. I think they got rid of 3 wire pilot in the early 90s.

Heat exchanger likely cracked by now.
 

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Discussion Starter · #3 ·
If there is no power on the wire going to the main gas valve connection and main gas has come on shortly after the pilot gas valve opened (the two valves are in series with pilot gas tapped off in between--it's a redundant arrangement so both must be open for the main gas to flow) then how could it NOT be the gas valve? The LED would be lit if 24V was there. And the LED does light up...about 30-45 seconds later when the pilot heat switch gets hot. Under normal circumstances this is when the main burner comes on. At the times it's acting weird the main gas was already on.
 

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If the main valve is not getting power, yet it's allowing fuel to flow to the main burners, it's bad for sure.

Time for a new furnace imo, not gas valve - especially if the 3 wire pilot assembly is also failing.

The parts are obsolete now - super expensive if you can find them.

If you decide to repair, really need to take a good look at the heat exchanger. 30+ year old furnace is most likely cracked.
 
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