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Old 11-23-2011, 10:46 PM   #1
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Furnace ignites twice almost every cycle?

I've spent a few hours trying to debug my furnace. Technically it heats the house just fine, but I noticed the burners ignite twice during a heating cycle. It doesn't do it every time (it knows if I have a multimeter connected!), but let's say it does it 80% of the time.

Goodman GMP 075, 14 years old

time in seconds: status:
0 call for heat
3 inducer fan starts
10 hot surface ignitor glows
15 flame starts
45 recirculation blower starts
55 flames go out
65 igniter glows (again)
67 flames ignite
X thermostat is satisfied, call for heat is stopped, flames out
X+120 recirculation fan stops

This is the most typical behavior. For a while, the flames went out at somewhat random times during the first minute after heat was called for. The flames would go out, and the re-circulation fan would immediately come on.

At one point the diagnostic LED blinked 4 times, indicating an open high-limit sensor, but I put my multimeter across the limit sensor (while it was connected), and read 0 volts the entire time, even during the flame-out event, so it never opened according to my meter (it's a digital meter, which samples every 1/8th of a second or so). The LED did not blink an error code during this observation.

I cleaned the flame sensor at one point, and the problem seemed to go away for a week, at least I didn't notice it.

Bad ground? ground potential goes up when blower current comes on? nullifying the tiny voltage of the flame sensor? I have a two-wire electrical system (neutral is tied to ground at the meter).

Any hints? I've done research and not seen these symptoms anywhere. The high-limit sensor is a pita to replace, gas valve is in the way, and it's a 7" sensor. The flame sensor is easy to replace, but you guys already know that. I already tried a different thermostat too, same problem cropped up after a few cycles.

Flame sensor current is 2.5 to 3.2 micro amps (this measurement was taken when the system cycled properly however).

thanks for any hints!


ps) i have little hvac background, but i did read the service/repair manual for this unit, and I have a degree in computer science, and a background in electronics.


Last edited by edfardos; 11-23-2011 at 10:49 PM.
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Old 11-23-2011, 10:49 PM   #2
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I think i would start by replacing flame sensor since you already cleaned the old one and it seemed to help ...worth a try....


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Old 11-24-2011, 07:02 AM   #3
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The first thing you do on a gmp is pull the blower and check the crimp rings that hold the heat exchanger together. If any are cracked or missing then it's time for a replacement heat exchanger or furnace .

A 2.5 microamp reading is good. Lose wire connection or cracked solder connection on the control board would be my guess. Wiggle wires one at a time and see if the flame drops out. Control boards and heat exchangers are both common issues with the gmp.
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Old 11-24-2011, 09:41 AM   #4
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Try ben's solution first. Part is inexpensive. Cleaning with sandpaper can damage part.
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Old 11-24-2011, 11:02 AM   #5
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thanks for the advice guys!

flame sensor ordered...

I still get the feeling its a real or false high limit sensor reading. I unplugged and re plugged the wiring harness from the control board yesterday, maybe its an intermittent connection, as it hasn't flamed out in the las 24 hours, with maybe 10 cycles. The only issue with the loose connection theory is that once it lights the second time it *always* stays lit until the thermostat is satisfied.

thanks again,

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