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Old 11-27-2011, 11:52 PM   #1
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suburban NT - 30sp problems

I have a suburban NT 30 sp propane furnace. I have been having issues with it lately. It will come on, light supply heat for 2 to 10 minutes. It then shuts down and will not come back on unless I turn it off let it rest for thirty minutes restart and repeat. This evening I took the front panel off the furnace and ran it. I remained on, ran fine and cycled normally. I placed the panel cover back on and the problem started again. Any ideas as to what the problem may be?
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Old 11-28-2011, 12:31 AM   #2
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Did you take the panel off the furnace, or did you just remove the grill off the wall for the air inlet? There is a difference. If you just pulled the grill off the wall, and it ran fine, sounds like the grill is to small, too small of an opening behind it, or something causing a restriction. Do you have anything in front of the grill like a trash can, clothes basket, etc,?

These furnaces must get a lot of air. Don't seem like they move much, but the intake can not be restricted. Sounds like you have a restricted intake causing an overheat, and the shut down. After it cools, it works again. Good Luck.
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Old 11-28-2011, 12:47 AM   #3
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I did not remove the fresh air intake panel I removed the actual metal cover panel from the interior. Once removed it runs fine. Which after what you've said makes a lot of sense. With the panel removed the furnace receives a lot more air and doesn't have a chance to over heat.
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Old 11-28-2011, 12:50 AM   #4
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And to clarify more I removed the metal panel on the front of the furnace not a an air register.
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Old 11-28-2011, 01:23 AM   #5
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On the flip side. The furnace must not only receive air, but be able to get rid of it. Put the intake panel back on, and remove the the duct work and see if it runs. Could be some blocked ducts preventing air from getting out. Once again, are any of the ducts blocked by trash cans, clothes baskets, etc. ?
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Old 02-19-2014, 11:42 PM   #6
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I have a suburban NT 30 sp propane furnace. I have been having issues with it lately. It will come on, light supply heat for 2 to 10 minutes. It then shuts down and will not come back on unless I turn it off let it rest for thirty minutes restart and repeat. This evening I took the front panel off the furnace and ran it. I remained on, ran fine and cycled normally. I placed the panel cover back on and the problem started again. Any ideas as to what the problem may be?
I am having this exact same issue with mine, it is only a year old and just started happening suddenly, I had a torn duct which I thought was the issue so I replaced the duct and the issue is still happening, but runs perfectly fine with the front cover off, I see this is an old post, please post what the solution was that you came up with to fix this ssue
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Old 02-20-2014, 04:44 AM   #7
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Your problem is that you are cycling off the limit switch. The torn duct has nothing to do with it. You could just need a new limit if it was working for a year. I would replace the limit and add a duct to your basement or cut another register in. The front cover you are talking about is where all your ducts connect correct? If you can with all the ducts connected wait til it happens again and put a meter 12v to the terminals on the limit should be 1 wire on each side. If it is open without a volt reading the limit is bad and needs replacing. It very well could have gone bad by not having a enough air flow being exhausted. As your unit starts to heat up the return air will increase in temp and cause that. But when it is cold inside the return temp will allow it to stay on. Sounds like a weak limit switch. THe cause is either not enough square inches of duct work coming off or just that your limit took a crap and just getting weaker. Also make sure that the blower is running at full speed, meaning full batteries. With the duct off you would reduce the static pressure and the blower would run faster moving more air over the limit switch. The limit is a Normally Closed switch. I would start there. I repaired HVAC for over 20 years and that is where I would start, always follow the power.
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Old 02-20-2014, 09:45 AM   #8
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now it is happening with the front cover off, less frequently but still happening.

with this model, the ducts r in the front sides right near the front cover

[edit] I found a 231244 limit switch for the furnace for $16 and will be replacing it and hopefully that fixes the issue
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Old 02-20-2014, 09:38 PM   #9
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I changed the limit switch and the same issue is happening
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Old 02-20-2014, 11:38 PM   #10
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Hmmm .... sounds to me like Suburban makes garbage appliances. In my case is the oven and tonight I note that my right rear burner is not lighting and is flopping around on the stove top. Will look at that in the light of day tomorrow. The oven will have to be addressed by the dealer. I my old Jayco I had a Atwood stove - what a champ! Worked workout any hitch from day one. Love my Flagstaff otherwise.

Now I will have to watch out for this furnace issue too, as I believe I to have a Suburban unit.
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Old 02-21-2014, 08:42 AM   #11
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IMO, the issue here could be the "sail switch" in the output plenum is not getting sufficient air flow to keep the furnace going and is opening as the volume of air decreases during use (the initial flow is enough to close it but eventually the flow slows down enough to open it).

Causes (in order of most likely):

1) Low or bad Battery/Converter.

The unit needs a LOT of 12 volt power and a weak or low battery's voltage will decay when there is a load on it. This lower voltage will slow the fan below the cut out flow required for the sail switch. If this happens on shore power, the converter or battery could be bad. ELIMINATE this as a possibility before proceeding. (Note: Battery voltage will recover some when the load is removed!)
2) Mud Dauber nest or debris in squirrel cage intake fan or clogged grill on the output fan plenum.

Since the output fan is co-shafted with the burner intake fan, anything that slows down the fan motor will affect the air flow output and kill the furnace. Removing the inflow air restriction (dirty or clogged grill) could allow less drag on the fan allowing it to turn faster and keep the sail switch closed.

3) Clogged filters in the distribution system.


If you have filters in the floor heater grills (like I do to keep dirt and junk out of the hoses), when these get clogged they cause back pressure to build up in the distribution system, dropping the air flow at the sail switch.

4) Rusted/Corroded motor shaft bearings.

Motor already running slow (low flow) and the drag from a clogged intake grill filter drops output volume below the sail switch cut out once the air in the plenum is used up.

I have attached a wiring diagram and troubleshooting guide to help you.
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Old 02-21-2014, 11:18 AM   #12
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if the problem is with the sail switch wouldn't the fan still turn on but not fire up? also wouldn't this be acting the exact same with the front cover on or off?
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Old 02-21-2014, 12:36 PM   #13
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if the problem is with the sail switch wouldn't the fan still turn on but not fire up? also wouldn't this be acting the exact same with the front cover on or off?
So when it shuts down after running a few minutes the fan stops too and when you try to start it the second time the fan does not run at all; correct (now)?
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Old 02-21-2014, 12:44 PM   #14
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when I turn it on it runs for a time, then cycles off, sometimes will do a couple cycles, but then wont cycle on at all until I turn it off then turn it back on after it has cooled a bit, now if I have the front panel off it runs normally unless it runs for a longer period then it acts the same as with the cover on.

the fan wont even turn on, I had a sail switch problem with my old furnace before I replaced it with this one, and the fan would turn on, but no ignition periodically, this is different, even the fan wont turn on unless I turn the furnace off then back on, but when the fan turns on, it fires up every single time first try with no issue, its acting as if it is overheating, so someone mentioned the limit switch, I confirmed this with 3 RV techs, so I bought a new limit switch and installed it but the problem still persists
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Old 02-21-2014, 01:02 PM   #15
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when I turn it on it runs for a time, then cycles off, sometimes will do a couple cycles, but then wont cycle on at all until I turn it off then turn it back on after it has cooled a bit, now if I have the front panel off it runs normally unless it runs for a longer period then it acts the same as with the cover on.

the fan wont even turn on, I had a sail switch problem with my old furnace before I replaced it with this one, and the fan would turn on, but no ignition periodically, this is different, even the fan wont turn on unless I turn the furnace off then back on, but when the fan turns on, it fires up every single time first try with no issue, its acting as if it is overheating, so someone mentioned the limit switch, I confirmed this with 3 RV techs, so I bought a new limit switch and installed it but the problem still persists
This can still be item 2 or 3 or 4.

If the fan motor is not turning fast enough there is no or low burner air for the fire (too rich a mixture). This can cause a no ignition situation.

If you get 3 no ignition attempts, the control board will lock out the thermostat to prevent an explosion if the mixture becomes "right" when the unit is filled with propane gas.

To clear the lockout, you must switch the thermostat OFF and/or remove the furnace fuse for a few seconds and reinsert it.

It can take a few minutes for the improper mixture to clear and you may get it to work again for a while.

I would try to blow out the combustion side plenum (two round vents on the outside wall) with compressed air. Maybe that will help.
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Old 02-21-2014, 01:15 PM   #16
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I will try that out and see, but the ignition isnt the issue, it cant try to ignite until the fan turns on to open the valve for the propane, it doesnt even get that far, the fan doesnt even come on, yet runs more with the front cover panel off

it's definitely an overheating issue, my batteries r at full power and even when on shore power the same issue happens, the outside air intake is not plugged or blocked, the return air on the side is free of obstructions, I will check and see if its dirty, blow it out and such, but I know its not an ignition issue because whenever it gets the fan running, it ignites every single time without issue
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Old 02-21-2014, 01:31 PM   #17
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I will try that out and see, but the ignition isnt the issue, it cant try to ignite until the fan turns on to open the valve for the propane, it doesnt even get that far, the fan doesnt even come on, yet runs more with the front cover panel off

it's definitely an overheating issue, my batteries r at full power and even when on shore power the same issue happens, the outside air intake is not plugged or blocked, the return air on the side is free of obstructions, I will check and see if its dirty, blow it out and such, but I know its not an ignition issue because whenever it gets the fan running, it ignites every single time without issue
There are only 3 switches in the circuit plus the control module and the fan motor.

If you changed the High limit switch already, and have ruled out the sail switch, it could only be the control module or the gas control solenoid/relay switch.

If you decide to give the control board a whack, check on the upgraded ones from http://www.dinosaurelectronics.com/Ignitor_boards.htm
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Old 02-21-2014, 01:52 PM   #18
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I had a similar problem with my 30... i took it out, inspected it...saw no defects, reinstalled and it worked. Went camping and it worked a while and quit. Mine had a bad connection on the board where the +12 v terminal had been hot. Re-soldered and viola.. worked since. In other words... bad board.
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Old 02-21-2014, 01:55 PM   #19
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I will try that out and see, but the ignition isnt the issue, it cant try to ignite until the fan turns on to open the valve for the propane, it doesnt even get that far, the fan doesnt even come on, yet runs more with the front cover panel off

it's definitely an overheating issue, my batteries r at full power and even when on shore power the same issue happens, the outside air intake is not plugged or blocked, the return air on the side is free of obstructions, I will check and see if its dirty, blow it out and such, but I know its not an ignition issue because whenever it gets the fan running, it ignites every single time without issue
How hot is your motor when this happens, If your motor is starting to drag it will create heat until it hits internal over load then open and will shut down until it cools to reset. If it is still doing it with no restrictions in the intake or exhaust without ducts hooked to it something is shutting it off. you need a meter to find out where the power stops. Start with the power source and work backwards, that includes your stat. Have you tried to jumper the red and white wire out to see if the unit will stay on Rh/RC and W? That way you can rule that out and start tracing the 12V power from there. It's hard to guess what is going on without knowing that, Other wise you will just start replacing parts until you hit the right one hate to say it. follow the diagram Herk sent you on this post. good luck, you will find it....
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Old 02-21-2014, 02:45 PM   #20
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eBay has all the parts you need.

suburban furnace parts | eBay

You can get an entire new one for 360 bucks so don't go crazy.
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