Troubleshooting my Suburban NT-30SP furnace

[Bear816]

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?

[SeaDog]

Welcome to the forum its a great place to get your questions answered. That electrode getting red is part of the flame proving circuit.

[TitanMike]

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.

[nayther]

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.

[Bear816]

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.

[Larry-NC]

Originally Posted by Bear816

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.

[ElyWhy]

Originally Posted by Bear816

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.

[Larry-NC]

Originally Posted by ElyWhy

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.

[TGO Bill]

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.

[nomad297]

Originally Posted by Larry-NC

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.



Bruce

[Dwilcox]

Sail switch moves to. Open/ close.

[MR.M]

Originally Posted by Bear816

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?

Chances are it's sail switch since it's intermittent . the igniter also acts as a flame sensor and sends signal to the board telling it there is flame . it will get red hot since it sits in the flame . the igniter is spark not hot surface type . sail switches can be intermittent . i've seen them covered in dog hair which can make them not work , or they will work one time and not another because the internals will not make contact all the time .

[Scrapper]

There is a Service Manual in the Library section of the forum which covers this NT-30SP and every other furnace Suburban makes. Suburban does not have a diagnostics LED on the control board mentioned by nather.

I may be wrong but I believe the NT-30SP has time delay fan control. Suburban made 12vdc and 24vac controls, the 24vac control needs a minimum of 18vdc which is supplied from 120vac step-down transformer. You need to verify the board number, the 12vdc Fan Control Module Board is Part Number 520820, the 24vac Fan Control Module Board is Part Number 520947.

I have attached a pdf for the Suburban Service Manual for your convenience. Sequence of operation description and troubleshooting starts on page 23.

[MR.M]

Originally Posted by Scrapper

There is a Service Manual in the Library section of the forum which covers this NT-30SP and every other furnace Suburban makes. Suburban does not have a diagnostics LED on the control board mentioned by nather.

I may be wrong but I believe the NT-30SP has time delay fan control. Suburban made 12vdc and 24vac controls, the 24vac control needs a minimum of 18vdc which is supplied from 120vac step-down transformer. You need to verify the board number, the 12vdc Fan Control Module Board is Part Number 520820, the 24vac Fan Control Module Board is Part Number 520947.

I have attached a pdf for the Suburban Service Manual for your convenience. Sequence of operation description and troubleshooting starts on page 23.

The furnaces in the RV are all 12 volt ! attwood and surburban do make them for small cabins and will be 120v with transformers but not the ones you find in RV's

[Scrapper]

Here is a service manual specific to the NT-xxxx Series furnaces. As I suspected the fan has a time delay relay which works in conjunction with the control board.

It may be the time delay relay is experiencing failure when it gets hot.


You can download other documents for this furnace from https://pdxrvwholesale.com/products/...sp-tune-up-kit

[MR.M]

Originally Posted by Scrapper

Here is a service manual specific to the NT-xxxx Series furnaces. As I suspected the fan has a time delay relay which works in conjunction with the control board.

It may be the time delay relay is experiencing failure when it gets hot.


You can download other documents for this furnace from https://pdxrvwholesale.com/products/...sp-tune-up-kit

all time delay is built into the control board there is no separate relay . the firing sequence is call for heat , blower comes on , sail switch makes contact , then igniter and gas valve turn on , flame is sensed telling control board to continue operation , then heat satisfied t-stat breaks contact blower keeps running for cool down ! all this is controlled by the board , sail switch , igniter .

[Scrapper]

Originally Posted by MR.M

all time delay is built into the control board there is no separate relay . the firing sequence is call for heat , blower comes on , sail switch makes contact , then igniter and gas valve turn on , flame is sensed telling control board to continue operation , then heat satisfied t-stat breaks contact blower keeps running for cool down ! all this is controlled by the board , sail switch , igniter .

Not according to the service manual (see Figure 6, top left), the parts list on the site in my post also lists a replacement number for the time delay relay (bottom, page 10).

[MR.M]

Originally Posted by Scrapper

Not according to the service manual (see Figure 6, top left), the parts list on the site in my post also lists a replacement number for the time delay relay (bottom, page 10).

not on the RV units . 12 volt all is incorporated into the board , no separate time delay switch unless you're dealing with a 30 yr old unit .

[MR.M]

Originally Posted by Scrapper

Not according to the service manual (see Figure 6, top left), the parts list on the site in my post also lists a replacement number for the time delay relay (bottom, page 10).

SEQUENCE OF NORMAL OPERATIONThe electronic ignition system is made up of three main parts: the module board, theelectrode assembly, and the electrode wire. The module board is the brain of the electronicignition system and it has several functions.1.When the blower reaches approximately 75% of the normal rpm and sufficient airflow isestablished, the sail switch engages and completes a 12-volt circuit through the limitswitch to the module board. (NOTE: Low voltage power supply will not providesufficient motor rpm to

[MR.M]

Originally Posted by Scrapper

Not according to the service manual (see Figure 6, top left), the parts list on the site in my post also lists a replacement number for the time delay relay (bottom, page 10).

After a 12-18 second delay, 12-volt current will pass through the module board to thesolenoid valve. The current to the valve opens it and allows gas to the main burner,simultaneously, the module board sends high voltage through the electrode wire to theelectrode assembly. The voltage seeks a ground between electrode and ground probeand a spark occurs. The spark.



The time delay is built into the control board.......