Furnace Shutting Off Before Heat Levels Reached

[wetzelja]

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!

[paul52446m]

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!

Different things you say make it hard to see whats going on. You said you turn off power , then reset limit, turn on power and it work. To reset a furnace you turn off power wait a min. and turn the power back on . so how do you know that that limit switch needed to be reset? Next time it quit, turn off power, wait 5 min. and turn power back on and let me know what happens. On call for heat, first inducer fan, then spark and gas to lite pilot,
then after pilot you get main burner. If you take the pilot assembly out you have a flame rod . It looks like a nail. This flame rod get a coating on it so even if it looks clean, use some fine sand paper and clean it real good.
Now look at your pilot. It should be blue, the flame should not left off the burner heat, the flame should go out around the flame rod about 1/2".
If you don't have a good looking pilot like this, then you need to clean out the pilot orifice. If you have a pressure switch, check all hoses for cracks
When you say you banged the furnace, was any thing running before you hit it? Ok i gave you some things to check and start with. A lot of times just a proper cleaning of things will fix the problem. Later Paul Ps I know about cold, Houghton Lake Mi.

[wetzelja]

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!

[brettyg]

Air flow issues will get you every time