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Old 01-14-2021, 01:13 AM   #1
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Troubleshooting my Suburban NT-30SP furnace

I have been fighting with this furnace for days. It will start and will burn for one cycle. It shuts down but the next time it starts it is just the blower motor.....no flame. I checked the electrode assembly and it seems to be fine. I did notice that when it does start, the electrode sparks and when the burner runs, the electrode gets red hot. Is this the electrode still trying to run?
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Old 01-14-2021, 10:07 AM   #2
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Welcome to the forum its a great place to get your questions answered. That electrode getting red is part of the flame proving circuit.
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Old 01-14-2021, 01:26 PM   #3
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Sounds like an intermittent problem. Most common of these is the Sail Switch.

Might need to see if the furnace has had a lot of pet hair or dust sucked into the inside air intake.
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Old 01-14-2021, 02:02 PM   #4
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there should be a LED on the circuit board that blinks a code for why it shut down. I'd suspect a sail switch or circuit board, most common problems I've read about. Has this just occurred or has it been a problem in the past. Some furnaces are incorrectly ducted and don't get enough air across the heat exchanger and fail on high limit. Put a meat thermometer in the supply duct next time and see if the burner is shutting off before it reaches temperature.
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Old 01-14-2021, 05:45 PM   #5
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Unfortunately I do not have a history for anything on the motorhome. I just purchased a few months ago knowing it was a fixer upper. And I am learning a lot. I did notice that when I have the front panel is off it starts a little better. I read somewhere that if it were the sail switch to gently bang on the top when the fan kicks in. I will certainly look for the flashing LED.
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Old Yesterday, 01:59 PM   #6
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Aha!

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Originally Posted by Bear816 View Post
Unfortunately I do not have a history for anything on the motorhome. I just purchased a few months ago knowing it was a fixer upper. And I am learning a lot. I did notice that when I have the front panel is off it starts a little better. I read somewhere that if it were the sail switch to gently bang on the top when the fan kicks in. I will certainly look for the flashing LED.
When our regulator (on the small SOB trailer) was failing, it would operate perfectly--if the front panel was off. It turns out that with the front panel was off there is less resistance to the blowers. (In a Suburban furnace, there is one double-ended motor--shafts out both ends. There's a squirrel cage blower on each end. One circulates air inside the RV. The other draws outside air into the firebox and exhausts combustion products to the outside.)

It could be, as Mike suggests, that there's enough dust and pet hair inside that the blowers cannot operate fast enough to set the (safety) sail switch. There was enough dog hair in ours to make another dog.

It could also be (as was the case with ours) that the propane regulator is failing. The Suburban furnaces are very fussy about pressure. If it isn't exactly right (11" w.c.), it won't ignite.
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Old Yesterday, 03:59 PM   #7
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Quote:
Originally Posted by Bear816 View Post
I have been fighting with this furnace for days. It will start and will burn for one cycle. It shuts down but the next time it starts it is just the blower motor.....no flame. I checked the electrode assembly and it seems to be fine. I did notice that when it does start, the electrode sparks and when the burner runs, the electrode gets red hot. Is this the electrode still trying to run?

Similar problem with another older Suburban furnace in a '94 trailer getting little use, except mine will scream like a banshee (my neighbors should have called the sheriff) until I beat on the furnace box with a garlic press, the nearest tool at hand. That seems to release some part that gets hung up...perhaps the sail switch? When I start the furnace regularly/weekly, it always cooperates without complaint.
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Old Yesterday, 04:15 PM   #8
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Two possibilities

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Similar problem with another older Suburban furnace in a '94 trailer getting little use, except mine will scream like a banshee (my neighbors should have called the sheriff) until I beat on the furnace box with a garlic press, the nearest tool at hand. That seems to release some part that gets hung up...perhaps the sail switch? When I start the furnace regularly/weekly, it always cooperates without complaint.
There are only two moving parts in a Suburban furnace:
  1. The blower motor which is double-ended and has a squirrel-cage blower on each end.
  2. The gas solenoid which turns the gas on and off in response to commands from the control board. It makes a single "clunk" each time heat starts or stops.
So the noise is clearly coming from the blower assembly. Once again, there are two possibilities:
  1. One of the squirrel-cages is loose on the shaft and is either slipping or has slid along the shaft and one side is scraping on the housing. This should be easy to find and fix. You can clean out a lot of dust, pet hair, and gunk while you're in there. A toothbrush is helpful.
  2. The motor shaft bearings are dry. I think these bearings are porous bronze bearings. When they are manufactured, they are heated and dipped in oil. As they cool, oil enters the pores. Eventually all the oil is dispersed and the bearings run dry. Suburban states that the motor is permanently lubricated and should not be oiled.

Good luck and let us know how it comes out.
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Old Today, 06:04 AM   #9
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Just a simple thought

If you have 2 propane tanks, make sure that it is switched to the one that has propane in it. As we all know, sometimes its a simple mistake that drives us crazy.
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Old Today, 08:00 AM   #10
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Quote:
Originally Posted by Larry-NC View Post
There are only two moving parts in a Suburban furnace.
Just messing with you, Larry, but technically, there are many more moving parts. Consider how many moving parts are in just one bearing and how many vanes are on a squirrel cage and all of the moving parts that make up the motor — the list goes on and on — there are hundreds of moving parts when you think about it this way, and just one of those parts could be causing the problem.

But, when considering how many moving parts there are that are actually replaceable, just like you stated; there’s not too many.



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