Furnace Shutting Off Before Heat Levels Reached

Problems related to residential installations.

Furnace Shutting Off Before Heat Levels Reached

Postby wetzelja » Fri Jan 14, 2011 6:48 pm

All right, I'm a total rookie, but I enjoy troubleshooting (and hate spending money I don't have to) so I'm trying to figure out what's going on with our furnace. We have a Ruud electric ignition gas furnace (UGDA-10EC-ER), which as far as I can tell is about 20 years old. It's certainly rusty enough to be that old. Not sure when the last time it's been inspected.

About a month ago we started having issues where the furnace would operate fine all day, but when we woke up in the morning, the house was cold and below the called-for temperature on the thermostat. I narrowed it down to what I thought was the problem...I discovered the limit switch, and when I would push the manual button in and then pull it back out and turn the power back on, the pilot light would kick on, the blower would come on, and everything operated normally. Until a night or two later when it would do the same thing. Each time, I would just go reset the switch and it would work again.

I did some research about what the limit switch did, and that it was getting too hot in the plenum. Possible causes were lack of intake air, and closed registers. We only have 5 registers, so I made sure they were all open. We only have one return air vent, so I made sure that was clear. I replace the filter regularly, so it wasn't that.

Well, now it's to the point where even during the day, it will go on for about 5 minutes and then shut off. I will go down, and even without turning off the power, hit the side of the furnace (professional, I know), and the flames will pop on, the blower will come on, and it will heat for about 5 minutes, and then shut off again.

So now I'm wondering if it's a faulty connection somewhere? Or I read something about the valve for the gas not staying open?

Any thoughts? I live in Michigan...and it's January...so it's cold!
wetzelja
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- Furnace Shutting Off Before Heat Levels Reached

Postby wetzelja » Fri Jan 14, 2011 11:06 pm

Well, after finally getting to the point of posting on the forum, within a couple of hours I discovered the problem, and it was consistent with the original theory. I happened to follow the ductwork in my basement, and discovered that there were dampers on the 4 or 5 ducts, and each one of them was about 90% closed! My guess is that was causing the plenum to heat up, which would cause the limit switch to trip. I opened them up, and the furnace has been working perfectly all night. So not only is the furnace problem working, but we're getting a lot more heat out of our registers.

Thanks for the willingness to help!
wetzelja
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Joined: Fri Jan 14, 2011 6:02 pm


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