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Odd issue with gas furnace: The whole thing turns off mid-cycle then back on

Odd issue with gas furnace: The whole thing turns off mid-cycle then back on

Old 02-11-15, 04:32 AM
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Odd issue with gas furnace: The whole thing turns off mid-cycle then back on

Hi all, working on a RUUD UGVG-010EBRJR gas furnace with A/C.
No pilot light; uses a "glow plug" type igniter.
System has been in place for many years and works fine.

System starts up fine. I watch and hear it go through:
1. Draft motor starts and runs
2. Igniter turns on and glows
3. Gas valve turns on and burners ignite
4. Main blower comes on and begins blowing heat through the house

Here is what happens: Between 30 seconds and any additional amount of time, BEFORE the thermostat target temp is reached, the whole furnace shuts off. By that I mean the blower, the gas valve, and the draft motor all completely turn off. The SECOND after this happens, the startup sequence listed above starts right back up at #1 and I can hear the draft motor start to spool up again. All four steps complete and the heater begins blowing hot air through the house again. This on, off, restart cycle continues until finally the target temp on the thermostat is reached.

I removed the thermostat and jumpered the wires to the heater, in order to rule out the thermostat and the issue still occurred.

I haven't worked on any wiring or anything related to the furnace.

I don't think it's the limit switch because my understanding is a bad or wonky limit switch will cause the gas valve to shutoff, but the main blower motor to keep running and that's not what I'm seeing. As mentioned above, the whole thing is shutting down.

There are a number of sensors wired in series. Two of them appear to be heat sensors which are near the flame of the burners, and one of them appears to be the limit switch. There is also what appears to be a pressure sensor connected to the draft fan but it is not wired in with the other switches mentioned.

Any ideas of what the issue could be? The troubleshooting chart inside the unit says to check grounds and if those are good, to replace the control board. Kind of surprised they jump to control board replacement without instructing checks of the various safety sensors. I have a multimeter and can check whatever needs to be checked, I just need to know what. Oh yeah, the circuit board has two LED lights on it which normally both glow green. I haven't had a chance to run the system and see what the lights do when it does it's premature shutdown, but I also have no chart to look up the results on. If anyone knows where I could find one, I'd appreciate it.

Thank in advance for any tips.
Old 02-11-15, 09:42 AM
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Are your filters clean,could the evaporator coil be dirty, return air ducts are clear?
Just the obvious stuff,
Old 02-11-15, 10:20 AM
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I'd start with the blower section door switch.
Old 02-13-15, 07:06 AM
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Thanks guys.

I was doing some more testing last night and found that I was mistaken about the sequence of events when the shutdown occurs.

Previously I stated the whole unit turns off including the main blower but that is not the case. Sorry for any confusion. What's happening is that some time after the burner lights and the main fan comes on, the burner shuts off and the main fan continues running for a couple minutes. As soon as the main fan stops running, the startup cycle begins again starting with the draft fan.

I checked the status lights; there are two on the board, both green. When the shutoff occurs, the lights don't blink; both stay solid green.

I have discovered this furnace has no dedicated flame sensor. From what I've been able to find online, it seems the igniter in this unit doubles as the flame sensor. Does that sound right? Anyway, I took it off and cleaned it with some steel wool per what I read online, and re-installed it. Issue didn't go away.

All the other safety sensors (except the draft fan pressure sensor) are wired in series so I put a multimeter on them and logged the voltage to see if any of them are tripping (like the limit switch, rollout sensors, etc). Of course as soon as I did that, the problem wouldn't reoccur and I ran out of time to continue testing.

I ordered a new igniter since it's only $20. Will see if that fixes it. I would test the current igniter's flame sensor functionality but can't find any specs on the range of milliamps it should generate, so I'll just replace it.

If that doesn't fix it, I will keep trying to log the pressure sensor and the limit switch to see if they are tripping. If none of that shows anything, I guess I will have to try replacing the board.
Old 02-16-15, 06:12 AM
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After a lot more troubleshooting, it ended up being the draft inducer fan.
Old 02-16-15, 10:06 AM
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It's definitely not the igniter. (but a spare is a good investment)
I doubt it's the draft inducer blower UNLESS it won't start running or squeals with bearing noise.

Your burner is shutting down on overheat that's caused by lack of airflow. A common problem caused by clogged air filter, blocked return vent, too many closed off registers and in extreme cases... a dirty A/C cooling coil.

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