Natural gas furnace keeps shutting off with immediate restart

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Old 01-02-13, 01:53 PM
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Natural gas furnace keeps shutting off with immediate restart

This is a split-system AC/furnace with the air-handler and gas furnace in the attic.

Recently, it is shutting off every ten minutes or so, but immediately starting again about 10-15 seconds after it shuts off.

So, it's actually heating the house, but obviously something is not right.

I jumped the thermostat, and it still did it. I replaced the thermostat and it still did it.

I disconnected the blower capacitor and it measured 14.7 uF, so was within tolerance of the 15 uF label.

I took off the doors and watched it for 20-30 minutes with the door switched clamped in. Would you believe it never once shut off during that time, but within ten minutes of me coming out of the attic, it shut off and restarted again!

I about went crazy.

Any ideas?

I'm an electrician, but have also learned how to fix air conditioners. Furnaces are a little trickier.
 
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Old 01-02-13, 01:59 PM
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The furnace might be overheating and shutting off on the high temperature limit switch.

That's most often caused by a dirty filter, return air vent covered over or too many warm air vents closed.

If none of those things appear to be an issue, use a multimeter to see if the limit switch is ahutting off the 24 VAC to one side of the limit switch, which will shut off the burners.
 
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Old 01-02-13, 02:13 PM
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I first suspected the limit switch when the problem was described to me as well, but when I saw that it would immediately restart and run again, that didn't make sense to me. I will test it anyway next time I go up there. Thanks!

Here's something I just thought of:

Would having the doors off affect a rollout? Maybe the rollout safety is causing it, but when the doors were off, the airflow was different and no rollouts were occurring. I'm using the term "rollout" to describe the flame not going properly into the heat exchanger. I think I'm using it correctly, but am describing it to make sure.
 
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Old 01-02-13, 03:32 PM
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Flame rollout switches usually must be manually reset. So that's not likely.


Having the burner compartment door the cover the controls would typically not affect the furnace operation. Having the fan compartment door off would be a problem, so there is usually a switch that prevents the furnace from operating if the door isn't properly installed.

The limit switch typically ashuts the burners off but allows the furnace fan to continue to run to cool off the furnace. When it cools off, the limit switch closes, the furnace burners light again and the furnace will overheat again and shut off. That's the cycle you appear to be describing.

If aso, checking the limit switch is the likely thing to test to see if it's opening and shutting off the burners.
 
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Old 01-02-13, 03:44 PM
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Now that makes perfect sense. The blower is working fine, and the inducer is working fine (that I can tell), so I will have them check the filter and registers for air flow issues!

If nothing appears obvious, how likely is it to be a bad limit switch?

And the reason it probably did not go off while I was in the attic is because the door was off and the blower was sucking cooler attic air instead of warm air from the heated house.

This is starting to at least help me think I'm not crazy.

I will keep you posted!

Thanks!
 
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