Goodman Furnace - power cycling


Old 01-16-16, 08:11 AM
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Goodman Furnace - power cycling

The house has two 20-year old Goodman/Janitrol (GMP series) forced air gas furnaces.

The upstairs furnace will run for about 3 minutes putting out good heat and then the power to the thermostat shuts off. The blower fan will continue to run and after a minute the power to the thermostat returns and the cycle repeats.

What could turn off the power to the thermostat? (I am using an ICM280 controller board). I would suspect the limit switch, but the LED on the controller is not blinking. I suspected the 24V transformer, but I replaced it with no change in the problem.

Any tips on where I should be looking would be appreciated.
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Old 01-16-16, 08:47 AM
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Possible bad thermostat so try just shoring R and W to see if you get constant heat.

How are you measuring power at the thermostat? If it is between R and C and you see it going to 0 then your controller board is faulty or you have a bad connection. Try measuring directly at the R terminal on the board. The ICM280 is a replacement so it is time to plan on replacing the furnace. Or maybe it was installed wrong? Verify all of the connections as maybe someone did not install it correctly.

If it is a bad board you can temporarily move the R wire from the board to the transformer.
Old 01-16-16, 08:57 AM
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Welcome to the forums.

Many furnaces are set up so that if a high limit trips...... the R power is interrupted and the blower keeps running to cool the furnace.

The first thing to check/change is the air filter.
Old 01-16-16, 09:12 AM
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PJ - Is it common for a limit trip without a blink code?
Old 01-16-16, 04:28 PM
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Many, many thanks to Astuff and Pete.

I verified that the thermostats were working by swapping the thermostats. The upper and lower zones are identical, and the thermostats just plug in. The problem stayed in the upper zone. The thermostats are WiFi connected Honeywell units and I can turn on the backlight. Wwhen the power is lost, the screen goes dark.

But, I think the problem is fixed. And it was the limit switch doing it's job.
As Pete said, above, "The first thing to check/change is the air filter."

I did. It was one of the first things I did and it looked good (it's one of those permanent washable versions), but I completely forgot about another return air path in a room over the garage that we rarely go into. The filter there was very full. I've replaced it and I am testing the system again.

In hindsight, I should have realized the limit switch was working correctly when I used my IR thermometer to see if I was getting hot air at the registers. I read 192F at one register, and somehow the alarm didn't go off in my mind that 192 was way too high. (DOH!) I cleaned the main filter and replaced the secondary return filter and that same register is now reading a reasonable 115F. It's been running for more than ten minutes and everything is stable and working properly.

I am embarrassed that it was as simple as the second return filter, but I post this in hopes that it saves someone else of hearing hoofbeats and thinking zebras.

Again, I really appreciate your input.


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