Furnace Noise

[azflycaster]

For the past two years when ever I ran the furnace (rarely) It sounded like the motor bushing were dry. The noise would start after a few minutes of operation and never stop. Today I pulled the furnace and ran it on a bench with no propane for an hour and it never made any noise at all. Could the heat from the furnace made any difference?

It is a Suburban NT-22A and the cross over tube is fine. I have a used NT-22 for parts that has a good motor in it (thanks Zep!) but it will take some work to swap out the motor since the blower assembly is different.

Any suggestions?

[overlander63]

It may be the squirrel cage is barely touching the housing. The heat could possibly cause the metal parts to expand just enough to cause the noise.
It's more likely a dry bearing, but I've learned it's not always the likely that's the problem. It's also very possible your moving everything around to get it out of the trailer stopped whatever it was that was making noise.

[eubank]

Humm. Leave the thing out of the AS and wait until a good, hot day in Phoenix. Put it in some enclosure (like an uninsulated garage) so that it gets too hot even to touch. Then run it.

Lynn

[Lumatic]

My NT 30 screeches sometimes when the blower comes on. Sometimes a well placed kick will get it to run right again. I see 2 options replace the blower motor for a hundred and change or replace the whole furnace for about $350. So far I have not gotten around to pull it. What would you do?

[GCinSC2]

Originally Posted by azflycaster

For the past two years when ever I ran the furnace (rarely) It sounded like the motor bushing were dry. The noise would start after a few minutes of operation and never stop.

Preface: I have never worked on one of these units.

.02$ info: If the unit has actual brass bushings rather than a ball bearing on the blower motor, brass bushings like oil, not grease. If you see a spot that looks like an OIL HERE hole, or if you see an oil wicking pad, try a few drops of light oil, 3in1 for example. Just don't use grease on a high speed bushing. If you have some old power hand tools, you might see the same on the ends. Brass bushings in need of lube chirp and I think you can really hear it at shut down when it is no longer powered, stops a bit abruptly.

If ball bearings, time for bearings.

Good luck.

Gary

[eubank]

I put a new motor in our NT24 (I think) a few years ago. The motor had brass sleeves, not ball bearings.

But from the sound of Richard's, it doesn't quite sound like a bearing problem. When I've dealt with these before, they may start up with a screech, but as they heat up (practically overheat), some lubricant becomes more liquid, flows in, and stops the noise.


Lynn

[nickcrowhurst]

When the furnace is on the bench and running, it doesn't have to work against the back-pressure of the ducting and vents, thereby lesening the end loading on the bushes. When this occurred on my furnace, I dismantled the motor completely to gain access to the bushes, and a few drops of light oil cured your exact sysmptoms completely. I could not get oil to the bushes without total dismantling. Details of the work involved, and the tools required are detailed here:
http://www.airforums.com/forums/f427...ace-60957.html

Nick.

[eubank]

Good idea, Nick! Richard, when you do the bench test, plug the out-vents.

Lynn

[Lumatic]

I think Suburban does not have any way to service the bushings/ bearings and the whole motor unit must be replaced. Do I have this right?

[azflycaster]

Blocking the air flow did not change the symptoms, going to tear it down to get to the motor.

[eubank]

Originally Posted by Lumatic

I think Suburban does not have any way to service the bushings/ bearings and the whole motor unit must be replaced. Do I have this right?

I 'spect that you could probably get new bushings pushed in/on at a motor rebuild shop, but I generally go with a new motor if they're available and still fairly inexpensive. (Parts for the old tractor -- 1948 -- are rebuild-as-needed-purchase-if-absolutely-necessary, but Suburban furnaces are still petty common and easy to buy parts for.)


Lynn

[tpi]

I've had motors over the years without oiling ports where disassembly and oiling the bushings was completely successful. Some have had a felt type oil retainer. I just cleaned everything up and oiled them. On my house central AC the motor has run longer after I oiled it than it did originally before making noise.

OTOH if the motor is hard to get at, you may want to play it safe and just replace it.

[Inland RV Center, In]

Originally Posted by azflycaster

Blocking the air flow did not change the symptoms, going to tear it down to get to the motor.

Rich.

The motor can be disassembled so that you can easily lube the bearings, the source of the noise.

Or, sometimes you can stand the motor up on one end, and place the oil around the shaft. Repeat that a few times. However that only gets you to one bearing.

If the motor was run for many hours with the dry bearings, the bearings at this point, may be shot.

Replacement motors, however, are available.

Andy