Older Heil Forced Air Furnace--flame or sensor problem


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Old 01-17-13, 04:23 PM
C
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Older Heil Forced Air Furnace--flame or sensor problem

Hello,
I have a (going on 25 years) old Heil Gas, Induced Draft Forced Air Furnace: 75K Input, 59.7K Output, Model Number:867.768140 NUGO75AGO1. I also have the Serial Number, but figured at this point, it isn't needed.

The issue: It has worked fine for all these years, but this year when I went to fire it up, the inducer operated correctly, the pressure switch worked, the gas valve opened and the burners fired nicely...the flames were nice and blue, standing straight up, just like usual. Then, after about 3 seconds, the flame shut off and about 3 seconds later, it cycled again. It continued doing this until I shut off the main switch.
There is a little roll-out of the flame (mostly down past the burners, not out the front) when the burners first light, but doesn't seem excessive, though I know there shouldn't be any roll-out at all.

It doesn't stay on long enough so that the fan will come on, so after a few cycles I activated the fan via the limit switch--wondering if it would make any difference, but it didn't; the burner still went out after about 3 seconds and continued to cycle, until I shut off the main switch.

Since then I have checked the fresh air intake; it was plugged solid, so I cleaned that out, thinking maybe it wasn't getting enough air, but that made no difference at all.

I also removed the vent stack on top of the furnace to make sure the inducer is actually blowing out, and that the there is good exhaust flow when the burners do light; there is. Very good.

I'm out of ideas so I'll stop here for now, and hope somebody can give me some advice on what this sounds like.

Thanks all.

Mr. C.
 
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Old 01-17-13, 05:38 PM
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If the intake was plugged, the exhaust could be too. You need to check that as well. If the furnace has a flame sensing rod, take it out & clean with a Scotch Brite pad or fine steel wool.
 
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Old 01-17-13, 06:26 PM
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Ok. Thanks. I'll double-check the exhaust, but I am going to guess it is clear, as it is also the pipe for our water heater, and that is exhausting fine.

As for the flame sensor, I see one indicated on the schematic for this model number, but I don't recall seeing one when I was looking around. I will also check that out to be sure.

Could that sensor--if there is one--be a culprit seeing as it does fire up initially, or is the sensor in the flame sequence after the initial fire up, and its input is needed in order to keep the gas valve open and the burners firing?

I will check it out this weekend, and respond here.

Thank you for your help.

Mr. C.
 
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Old 01-18-13, 01:03 PM
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Please post the make and model of the ignition module. I think you have a Honeywell intermittent pilot ignition system, but I might be wrong.
 
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Old 01-18-13, 06:35 PM
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I know some Heil furnaces used Honeywell ? S8610 and it was a local sense not remote sense. Meaning the pilot was also the flame rod.
 
 

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