Thread: Lennox G40UH Furnace Combustion Air Noise

A friend has noticed that his furnace produces a noise when it has been running for 10-15 minutes. When combustion stops, and just the blower runs during cool down, the noise stops. I have not been over to witness this problem, but he seems to think that it's coming from the draft inducer blower area.

The unit was installed in 2002.

Here's a quote of the problem in his words:

"When this happens, I hear a noise that sounds like maybe a bad bearing or intermittent thumping noise. The noise seems to be coming from the combustion air inducer fan near the chimney. The fan itself seems to be ok - shaft not binding and doesn't have any sideways play that I can feel."

He says he can hear the noise 2 stories up, via the vent stack.

Anyone seen or heard of a similar problem? He is thinking about going ahead and replacing that fan unit.

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Lennox made a (Pulse Gas Furnace) Some of them are a little noisy and make a pulsating sound when operated. This may be the cause of the noise if it's a (Pulse Gas Furnace).

From researching that model it doesn't look like Pulse model. It's just a standard efficiency (80%ish) type.

If it's a new noise, the inducer fan (assuming it has one, I didn't research it that much) has/is failing. If it's done it for any length of time it may just be the way it is.

I've never been one to mess much with furances. The money spent on a pro is about the best you can spend.

The fan may have shaft end play instead of side-to-side play like when a bearing goes out.

If there is end play, the fan could be moving back and forth hitting the squirrel cage against the metal housing.

That could also be caused by someone not tightening the squirrel cage to the shaft properly to start with, allowing the set screw to wear into the shaft, allowing the run out.

It's more likely the squirrel cage than the blower motor shaft having end play.

It also could be the fan is not moving back and forth but the fan unit is causing the side of the furnace to oil can, that would make a noise too.

Is the fan housing plastic? If so, the fan housing could be cracked, which would allow the fan to move with the housing as it moves at the cracked area.

I have the same furnace and Im on my third inducer motor. That is probably what is is from experience.

Thanks guys, really appreciate your thoughts. He is going to check on the things you've suggested, and then probably go ahead and replace it if required.

Thomas, have you found a good (reasonably priced) place to get a replacement?

I live in Illinois and they have run me with instalation around three hundred

Thanks Thomas. How long have you had your unit? I would have thought we'd get more than 7 years out of that motor, especially down here in GA. That's probably about 2-3 years for you in IL.

There's a chance we may get snow here on Sunday morning!!!! Just the thought throws the whole state in a frenzy.

Originally Posted by Brent Crouse

A friend has noticed that his furnace produces a noise when it has been running for 10-15 minutes. When combustion stops, and just the blower runs during cool down, the noise stops. I have not been over to witness this problem, but he seems to think that it's coming from the draft inducer blower area.

I too have a noisy exhaust vent noise on a new Lennox condensing furnace. It is a whine, like a bad bearing. The neighbor can hear in all through their house and in the street in front of the house. It is a hi pitched whine when the furnace first comes on and runs at 100% then quiets some as the furnace goes into 'maintain' mode.
The dealer has replaced the induced draft fan thinking it was a noisy bearing.
I have read the forums but no one has a solution for the noise. I tried adding a 6' piece of 3" shop vacuum hose and all it did was move the noise. It is like an organ pipe or one of the 'speaking tubes' in old paddle wheelers. What ever we try it just moves the whine but not removes it.
We cannot move it too far as on zero days ice will form a plug. I tried a tee fitting but it is still a problem. The neighbor called this morning about 7:00 for me to come out an listen.
Does someone have a solution, a muffler, a larger vent pipe, or what?
Thanks for a creative solution.

Originally Posted by Jerry Peck

The fan may have shaft end play instead of side-to-side play like when a bearing goes out.

If there is end play, the fan could be moving back and forth hitting the squirrel cage against the metal housing.

I have seen this end play- where the plastic fan blades hit the cage- in about 3 or 4 of these units over the last few years. Seems a fairly simple fix to correct the plastic fan blade by pushing it back onto the shaft. This may not be a long lasting fix though....