Troubleshoot Gas Furnace

Problems related to residential installations.

Troubleshoot Gas Furnace

Postby hakunama » Thu Jan 30, 2014 10:15 am

I am a homeowner with two nearly identical units, one for 1st floor, one for 2nd floor (Goodman GMS30703ANA). Both units are about 7 years old.

Our 1st floor unit (3 burners) was intermittently failing to ignite (startup sequence made it to the ignitor glowing hot, then failure). Always the same - call for heat, inducer start, ignitor light, no flame.

Because our other unit has identical parts, I started - Pressure switch first. Switch worked fine on the other unit. "New" switch installed into the problem unit, and then the problem on the "bad" unit - now it will not even start up the inducer. It gets power to the circuit board (red LED light steady red), I can turn on the house fan, but no startup sequence whatsoever (unlike previously when it failed to generate a flame).

So, what are the possible culprits for the original problem followed by the new problem? Which would be the most likely culprit?

I - "circuit board". So I swapped circuit boards between the two units (they were identical). Nothing different on the "bad" side; "good" side works fine with the swapped board.

Then, I - maybe the ignitor itself. So I swapped that. - works on "good" side; nothing different on the "bad" side.

My next guess would be ground wire (inspected it, and nothing obvious wrong with it). I plan to replace it, just in case, but I am ready to call in reinforcements!

I cannot find my multimeter to test anything; decided just to swap potential problems to see if I could hit pay dirt. I've had enough of that and would appreciate helpful suggestions.

If it does not even start the inducer now, while it did previously, what would make sense to explain that new condition?
hakunama
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- Troubleshoot Gas Furnace

Postby Freon » Thu Jan 30, 2014 11:58 am

The fact the 'bad' unit had the ignitor glow should have told you the pressure switch was not the problem. But for the current problem I would turn off all power to the furnace for 30 minutes. The goal is to reset the control board. With the power off I would check all connections you made when switching the pressure switches. Since this problem happened after that one event, it seems logical the switch switching is a very probable cause.

There may be another way to reset the control board that's shown in the installation manual.

When you did the pressure switch changing, did you have the power to the furnace turned off? Whenever you work on anything electrical always turn off the power. Control boards are easy to fry by accident.

When you were watching and listening to the bad furnace before any changes, did you hear the gas valve click after the ignitor was glowing?
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- Troubleshoot Gas Furnace

Postby hakunama » Thu Jan 30, 2014 1:58 pm

Thanks, Freon. I jumpered the thermostat inputs at the furnace (R to W) to bypass the thermostat and that got it running, and it successfully fired and ran. I will remove that jumper and try at the thermostat end to see if it is the wiring or the thermostat itself.

I am still puzzled at the original problem--call for heat, startup sequence all the way to ignition glowing, no flame. I am thinking that the problem will come back at some point, but let me not be too pessimistic! Let me know if you have any further thoughts about all this.

Thanks again.
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- Troubleshoot Gas Furnace

Postby Freon » Thu Jan 30, 2014 4:30 pm

You wrote, "...call for heat, startup sequence all the way to ignition glowing, no flame". When the ignitor glowed, did you heat any clicking at the gas valve? Usually you can hear the gas valve solenoid opening.

Is your thermostat only battery operated (there in no C (common) wire connection at the thermostat)? If yes, when did you last replace the batteries?
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- Troubleshoot Gas Furnace

Postby hakunama » Thu Jan 30, 2014 5:22 pm

Thanks, Freon. Regarding the gas valve--we did not hear a (2nd) click, that I can remember. My original thought was that the valve was bad, so we replaced it (the valves were reported to be especially prone to failure). Since I did not have my multimeter, I was not able to check for 24V at the valve, and just replaced it.

Not sure about how the thermostat is wired. I will check the batteries to be sure, when I get back to working on this furnace issue after my other work.

Thanks again for continuing your thoughts and suggestions.
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- Troubleshoot Gas Furnace

Postby hakunama » Sat Feb 01, 2014 2:43 pm

Took off the R/W jumper at the furnace, and connected the R/W wires prior to the thermostat, and it fired right up. So, I decided to hook up a common (C) wire from the furnace to the thermostat to ensure power to the thermostat. Working fine now.

Still do not know what the original problem was. Any guesses?
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- Troubleshoot Gas Furnace

Postby Freon » Sat Feb 01, 2014 4:11 pm

I can only speculate that the thermostat power from the batteries was insufficient for proper thermostat operation. Possibly the R-W connection (relay) at the thermostat failed to stay closed long enough for the furnace to complete the lighting sequence. Also make sure all thermostat wiring at the furnace and thermostat is secure. Good work.
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