Noisy forced air heater fan


Old 02-08-10, 02:04 AM
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Noisy forced air heater fan

Hi everyone,

I have an old Holly forced air gas wall furnace in my house with a blower fan at the top and it's started being obnoxiously loud recently. It makes kind of a cross between a rattling and a buzzing noise that varies in intensity but is always present.

The intensity of it seems to go up and down in a cycle. I ran the fan with the grille off and it seems to be doing a very subtle thing where it is either moving forward and back ever so slightly on the shaft or in some other way changing position (possibly very slightly wobbling). For all I know this is natural but I thought it might be related to the noise. I can't see anything else about it that suggests a problem (it isn't visibly scraping anything).

I'm looking for pointers on what I can do without calling a service person in in terms of improving the noise problem if not eliminating it. I thought about trying to tighten the set screw but it is partly blocked by one of the fan blades and I only have allen wrenches that are all together like a pocket knife and I couldn't get one to fit in there. If I buy an individual allen wrench I'm not sure what size to get so I'm wondering if there's any point in messing with the set screw anyway.

I can't see a belt, nor can I see the motor but I'm thinking it's direct drive with the motor behind the fan. I looked all over the net for some kind of information or cutaway drawing to show the construction of this old thing but nothing doing.

If it's the bearings, is there any hope of doing anything other than a full replacement of the entire fan assembly?

Any suggestions would be most appreciated!

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Old 02-08-10, 10:33 PM
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Hi again. I think I've gathered more info so maybe someone can help deduce the problem/fix now.

I'm convinced it's not the bearings because I can spin the fan freely and easily with no noise whatsoever when the power is off. It's also not a "grinding" noise like I've been told is an indication of bad bearings.

I've noticed that while the fan spins at full speed it is drifting a tiny bit under the hub. It's not wobbling. There is a rubber ring or gasket between the fan blade unit and the hub and I can see the fan blade unit drifting back and forth about a millimeter, maybe two, and as it does that the noise changes, increasing and decreasing as the fan moves around slightly.

The sound is hard to describe and is almost like scraping but not quite. It's sort of right in between squeaking, scraping, and vibrating. I've looked really carefully and there is enough clearance around the fan so that I don't think the blades are scraping anything as they drift around. I'm leaning towards the noise having something to do with the blade unit rubbing or somehow vibrating against the hub.

If I hold the fan blades with one hand and grab the motor shaft, I can move the hub (at least in one direction) independently of the fan. The hub is tightened securely to the motor shaft but the fan isn't fastened completely to the hub, it seems (though the fan doesn't move easily under the hub).

Problem is I can't see any way to tighten the fan against the hub. Is that possible and if so, how is it done? The only thing I see which tightens anything is the set screw fixing the hub against the motor shaft, but that is tight (I found the right allen wrench and made sure).

This noise is really loud and obnoxious and it would be lovely if I could find the cause and at least make it quieter without a costly replacement.

Sorry to be long winded about this but the answer to this isn't obvious to me so I wanted to include as much detail as possible.

Thanks again,
Old 12-30-13, 03:08 PM
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Did you ever Solve this?

I have something similar with my Empire wall heater. Sounds like the fan is scraping but I can't figure our where.

Thanks for any info you can provide.

Old 12-30-13, 04:52 PM
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I would guess it's aged rubber grommet motor mounts and it vibrates when the motor comes up to speed. Seen it quite a few times.
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