Slight noise from a Suburban furnace

[nickcrowhurst]

Im trying, without success, to trace a slight noise from our Suburban NT30-K furnace. The noise was originally intermittent, but it is now apparent all the time it is running. There is a light fluttering knock, or tinkling or click all the time the blower is operating. The noise is not regular, and it would not be apparent to one not accustomed to the normal sound of an efficient furnace.It occurs on average a couple of times a second, but it is not regular.
I suspected a problem with fan motor bearings, fan blades, broken sail on the sail switch,or a large rust flake or insect dancing around in the heat exchanger. There is a screen over the intakes and exhaust, so anything large should have been excluded.
Four years ago I replaced the sail switch, limit switch, time delay relay, gas valve and installed a Dinosaur board, as preventative maintenance.
Today I serviced the furnace, stripping the heat exchanger and blowing it out with compressed air.There was just a small quantity of very small rust particles. The visible interior of the exchanger looks in very good condition. The bearings of the motor shaft had little side play. The sail was intact. Nothing was fouling the fans. I ran the motor on the bench, and it ran smoothly and without any noise. I rebuilt the furnace, reinstalled it, and, of course, the noise persists.
Any ideas?? (I'll post a separate thread on possible replacement furnaces)
Thanks in advance. Nick.

[Inland RV Center, In]

Originally Posted by nickcrowhurst

Im trying, without success, to trace a slight noise from our Suburban NT30-K furnace. The noise was originally intermittent, but it is now apparent all the time it is running. There is a light fluttering knock, or tinkling or click all the time the blower is operating. The noise is not regular, and it would not be apparent to one not accustomed to the normal sound of an efficient furnace.It occurs on average a couple of times a second, but it is not regular.
I suspected a problem with fan motor bearings, fan blades, broken sail on the sail switch,or a large rust flake or insect dancing around in the heat exchanger. There is a screen over the intakes and exhaust, so anything large should have been excluded.
Four years ago I replaced the sail switch, limit switch, time delay relay, gas valve and installed a Dinosaur board, as preventative maintenance.
Today I serviced the furnace, stripping the heat exchanger and blowing it out with compressed air.There was just a small quantity of very small rust particles. The visible interior of the exchanger looks in very good condition. The bearings of the motor shaft had little side play. The sail was intact. Nothing was fouling the fans. I ran the motor on the bench, and it ran smoothly and without any noise. I rebuilt the furnace, reinstalled it, and, of course, the noise persists.
Any ideas?? (I'll post a separate thread on possible replacement furnaces)
Thanks in advance. Nick.

Nick.

Using air to blow out the combustion chamber is ok, but it usually will not remove all the stuff. Many dealers do the same, with little to no practical results.

When the furnace is disassembled, tapping, not too hard, on the combustion chamber with a hammer, will loosen things.

Then turn the combustion chamber from one side to the other, moving the "stuff" thru it's double "S" shape, and out the large opening.

I think you will be surprised of what all comes out.

Andy

[Mike Leary]

The only thing that could be causing that is the motor, which Suburban is infamous for. Not too many moving parts in that assembly, unless the fan is hitting the cage after it warms up, or the housing is not bolted together correctly.

[nickcrowhurst]

Andy, thanks for taking the time to reply. I did knock all over the heat exchanger with the rubber handle of a screwdriver, but when I next strip it down I'll be a little more vigorous. That may be soon if the noise gets worse. Before blowing it out with the air, I rotated the exchanger several times to route the debris in the required directions, until there was no rattling when I shook the exchanger.
Mike, thanks for your ideas. The fans were well clear of the housing, and it ran as smooth as a new unit when on the bench without the heat exchanger and burner assembly, although, as you point out, there would have been no heat to expand the parts.
Nick.

[Inland RV Center, In]

Originally Posted by nickcrowhurst

Andy, thanks for taking the time to reply. I did knock all over the heat exchanger with the rubber handle of a screwdriver, but when I next strip it down I'll be a little more vigorous. That may be soon if the noise gets worse. Before blowing it out with the air, I rotated the exchanger several times to route the debris in the required directions, until there was no rattling when I shook the exchanger.
Mike, thanks for your ideas. The fans were well clear of the housing, and it ran as smooth as a new unit when on the bench without the heat exchanger and burner assembly, although, as you point out, there would have been no heat to expand the parts.
Nick.

Nick.

If your going to tear it down again lube the motor very well.

As the motor heats up, sometimes it can do strange things.

Andy

[nickcrowhurst]

Originally Posted by Inland RV Center, In

If your going to tear it down again lube the motor very well.Andy

I assume that lubing the felt washers by the bushes involves removing the blower wheels. The manual says to use a long 1/8" hex wrench. Does this go down through the fan blades, or along the center of the wheel, and then down into the locking screws? I assume these screws will be corroded in, and will need some PB Blaster. Anyone have some tips on dismantling and oiling these motors? Thanks. Nick.

[Inland RV Center, In]

Originally Posted by nickcrowhurst

I assume that lubing the felt washers by the bushes involves removing the blower wheels. The manual says to use a long 1/8" hex wrench. Does this go down through the fan blades, or along the center of the wheel, and then down into the locking screws? I assume these screws will be corroded in, and will need some PB Blaster. Anyone have some tips on dismantling and oiling these motors? Thanks. Nick.

As I recall, the center of the wheel.

Oiling the motor???

Two word advice.

"Use patience."

Andy

[Mike Leary]

When was the last time the motor was replaced?

[Ag&Au]

I am wondering if the the vane that detects airflow and allows the furnace to start might be rattling around.
Ken

[nickcrowhurst]

The motor is probably the original, from 1987, but it has had light use, being based in Florida. I did inspect the sail switch, but could find no likely cause of noise. I could replace it with the one I took out 4 years ago, and have kept as a spare.
Interestingly, when I started the furnace this morning, the noise had disapeared completely. When I started it later, at about lunchtime, the noise was back. This leads me to suspect dryness in the motor bushes.
It looks like I'm going to have another fascinating morning some time next week, when the DW goes to the store, and the weather is warm enough not to need the furnace.
Nick.

[nickcrowhurst]

This morning I stripped out the furnace again, and removed and overhauled the motor. Each of the two blower wheels is attached to the motor shaft by a hex head grubscrew. It took about half an hour of soaking with PB-blaster, and slipping a 15" long tube over the end of the long 1/8" hex wrench to loosen each grubscrew. As I worked carefully away, I felt the spirit of Andy over my shoulder, whispering "Patience, patience, easy does it." On the smaller wheel the wrench had to be inserted through the fan blades.
When the motor was on the bench I tried oiling each bush where the shaft emerged, but no oil would soak through.(motor engine oil was what was available).
Reluctantly, I decided to dismantle the pressed steel motor casing. This involves using a pair of pliers to bend back ablout 6 pressed-over tabs on each end plate. After that, the motor comes apart. There are spring-loaded carbon brushes, which were in excellent condition, as was the armature. I found that the bronze bushes were dry, as were the several shim washers on each end of the shaft. I oiled the bushes, their felt inner inserts, and both sides of each shim washer. To reassemble the brushes on the armature, the springs holding the brushes can be easily released, and then re-attached after the brushes are back in place. The first couple of times I tried to re-install the armature and brushes back into the motor housing, the armature was dragged rapidly in by the surrounding permanent magnet, and the brushes collapsed off the end. I fixed this by locking a pair of grips onto the shaft where it emerged from the end plate. The whole re-assembly then went smoothly, including bending back the pressed tabs on the end plates.
I've run the furnace three times this afternoon, and all is smooth and quiet, so I believe the problem is sorted.Thanks for all the advice.
Government Health warning: This is potentially dangerous work. To carry it out safely, you need to be confident and competent with DC electrical and propane gas work, as well as as general wrenching. If in any doubt, let a trained professional do this sort of maintenance. Explosion or fire could result from any poor quality work.
If you are taking this sort of issue on an old furnace to a professional, fitting a brand new complete furnace would be a good economical choice in the long term, IMHO.
Nick.

[Inland RV Center, In]

Originally Posted by nickcrowhurst

This morning I stripped out the furnace again, and removed and overhauled the motor. Each of the two blower wheels is attached to the motor shaft by a hex head grubscrew. It took about half an hour of soaking with PB-blaster, and slipping a 15" long tube over the end of the long 1/8" hex wrench to loosen each grubscrew. As I worked carefully away, I felt the spirit of Andy over my shoulder, whispering "Patience, patience, easy does it." On the smaller wheel the wrench had to be inserted through the fan blades.
When the motor was on the bench I tried oiling each bush where the shaft emerged, but no oil would soak through.(motor engine oil was what was available).
Reluctantly, I decided to dismantle the pressed steel motor casing. This involves using a pair of pliers to bend back ablout 6 pressed-over tabs on each end plate. After that, the motor comes apart. There are spring-loaded carbon brushes, which were in excellent condition, as was the armature. I found that the bronze bushes were dry, as were the several shim washers on each end of the shaft. I oiled the bushes, their felt inner inserts, and both sides of each shim washer. To reassemble the brushes on the armature, the springs holding the brushes can be easily released, and then re-attached after the brushes are back in place. The first couple of times I tried to re-install the armature and brushes back into the motor housing, the armature was dragged rapidly in by the surrounding permanent magnet, and the brushes collapsed off the end. I fixed this by locking a pair of grips onto the shaft where it emerged from the end plate. The whole re-assembly then went smoothly, including bending back the pressed tabs on the end plates.
I've run the furnace three times this afternoon, and all is smooth and quiet, so I believe the problem is sorted.Thanks for all the advice.
Government Health warning: This is potentially dangerous work. To carry it out safely, you need to be confident and competent with DC electrical and propane gas work, as well as as general wrenching. If in any doubt, let a trained professional do this sort of maintenance. Explosion or fire could result from any poor quality work.
If you are taking this sort of issue on an old furnace to a professional, fitting a brand new complete furnace would be a good economical choice in the long term, IMHO.
Nick.

Nick.

Now see what patience can do for you.

Glad you resolved the issue.

And hey, look at the DC motor rebuilding experience you now have.

You might get requests from others, as to how "exactly" step by step did you do that.

Did you certify the complete repairs, with a "cool one"?

Andy

[nickcrowhurst]

Originally Posted by Inland RV Center, In

Nick.
Did you certify the complete repairs, with a "cool one"?
Andy

An empty bottle of Shiraz has just hit the trash bin! Nick.