Troubleshooting My Suburban NT-30SP Furnace

dmulligan · 03-05-2018, 12:16 AM

My Suburban NT-30SP furnace is not working. My current hypothesis is that it is most likely a faulty control board, but I would appreciate your advice & review of my troubleshooting so far. Thank you to those who posted the Suburban NT Furnace manuals in other threads - very helpful.

I have a 2007 FR Flagstaff 831BH. I am the third owner, but I believe there have been no furnace problems previously.
The last time I used the furnace was likely 9-10 months ago.
At one point my LP tank was emptied (my fault, no leaks).
I fill both tanks.
When I turn on the furnace, it is not working.

My AC and furnace are controlled from the same thermostat. When I set the thermostat to "furnace" and adjust the temperature, the blower turns on and runs for about 25 seconds and then stops. That is all. I do not hear any clicking, or iginition, or feel any heat (sigh).

The battery is well-charged at 13V. I have continuous 120V residential AC to the camper.

I try again and check the outside intake & exhaust.
The exhaust is strong; the intake is not quite as strong as the exhaust.
I visually inspect the intake & exhaust with a flashlight. I don't see any bugs, crud, blockages, or soot.

All range burners work, nice blue flames, even at lowest setting. My understanding is that this is a poor man's way of possibly diagnosing a faulty LP regualator.

I try several more times. It is a fine line between persistance and insanity.

I do not immediately see a fuse specifically for the furnace, but I recall that it might be on the same circuit as the fridge. I pull that fuse and check it - it's good. I seem to recall that pulling the fuse may reset the control board. I don't know that for sure, but that makes sense to me.

When I turn on the furnace & the blower is running, I don't see anything happening in the burn chamber and I'm definitely not seeing any voltage spikes on the ignition lead.

It is my understanding that thermostat --> limit switch --> sail switch --> module board --> electrode.

The thermostat appears to be working because the blower turns on. Everything else is obviously suspect, but it seems like many people have trouble with the board - it is a likely culprit.

From what I can tell I'll have to pull the furnace assembly to get to the module board. If I do that, I will attempt test the limit and sail switches.

Before I pull anything, I will go out and try the other tank just for due diligence....and wait for some responses here.

Is there anything obvious that I've missed or should try? Any tips or advice? This is my first foray into furnace troubleshooting...and not too excited about it.

HangDiver · 03-05-2018, 12:27 AM

Quote:

Originally Posted by dmulligan

Is there anything obvious that I've missed or should try? Any tips or advice? This is my first foray into furnace troubleshooting...and not too excited about it.

Tips or advice? None. Following this thread.

dmulligan · 03-05-2018, 01:29 AM

Switching tanks had no effect. Not surprising, as the problem appears to be ignition related.

The return air opening in the living area of the trailer is wide open - door off - so, no restrictions there either. But it did not help.

Section C of the "Suburban NT Furnances Services Manual" PDF posted in the forum library is very helpful - 16 steps to check when the blower runs but the burner does not light.

The setup and replacement time for the module board is listed as one-hour of labor. That doesn't sound too bad - ha - it will likely take me eight (hours? brewskies?)

TimVWulp · 03-05-2018, 07:00 AM

Below is a link to the suburban furnace manuals in the library at the top of the page. Go to page 23 for sequence of operation/trouble shooting.

Forest River Forums - Downloads - Furnace - Suburban Operation (Various models)

Hope this helps Tim

dmulligan · 03-05-2018, 10:11 AM

Thanks! I did not have that document.

lmiken · 03-05-2018, 01:21 PM

Sail switch sticking? My runs 30 seconds then stops. Sail switch usually clicks at 15 seconds. Occasionally I have to bang on the top of the furnace then it will ignite. Going to replace it 1 day as soon as I figure out how to get the furnace out from a tight place. Only use it twice a year so not a high priority

djemgoss · 03-05-2018, 01:26 PM

Sounds like the same thing that mine did. It turned out to be a "high limit switch" in the furnace. The trailer was new and the dealer diagnosed the problem, had the part and it was good to go in about 1.5 hours.

overeasy · 03-05-2018, 09:19 PM

The field repair man from that (wood) other furnace company told me to loosen and retighten all ground wire’s I could see. The other part he suggested was test all spade connectors and use a continuity tester on the fusable link just above the igniter. That guy was on the Mark a lot. Good luck.

TimVWulp · 03-05-2018, 09:59 PM

The module board will then verify that the sail switch circuit is closed and motor is up
to speed. If this circuit remains open for 30 seconds after the blower motor starts, the
module board will go into lock out and shut down the blower motor

If the furnace is shutting off after 25-30 seconds I agree that the sail switch is not working or stuck in the open position. it is not proving that the fan is providing the right air flow.
The other possibility is that mud dabber wasp have built a nest in the furnace restricting its combustion air flow.

Hope This Helps Tim

dmulligan · 03-06-2018, 08:41 PM

Thanks for the advice. I won't get a chance to pull the assembly and test the connectors/switches/board until later this week. I'll post an update when I do.

TimVWulp · 03-06-2018, 09:04 PM

try giving the furnace housing a good thump. care full not to dent it. Do this when or before the furnace is running to maybe jar the switch loose.

dmulligan · 03-11-2018, 09:00 PM

Thanks to all who chimed in, and especially lmiken & TimVWulp who diagnosed the issue.

It was (I believe) a stuck sail switch.

I decided to give it one more try. I unscrewed the furnace vent near the side of the assembly (next to the bunk beds), managed to get my hand and a screwdriver in there - and gave the assembly housing a few sharp raps. Two more on the front - just for good measure.

I turned it on - and it worked like a champ. No more problems (or needing to bang on it) since.

I'm guessing the sail switch was just stuck from dis-use (or accumulated dust).

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