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Old 03-30-2009, 11:41 AM   #1
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Help needed with Suburban NT-12S gas furnace - won't light

Our SMB (bought used last year) has a Suburban NT-12S propane furnace installed, and a thermostat. The problem is that it won't light by itself - the air draw is such that the gas is sucked out the exhaust rather than reaching the sparker. Our temporary solution is to go outside and blow into the darn thing to force the gas over the sparker, and then it lights and works fine. But then when it shuts off as per the thermostat, it won't relight itself, so this is obviously not a permanent solution!

We have taken it out and cleaned it, closed off some of the gas orifices further from the sparker, and reinstalled it to make sure the inlet and outlet air are correct. Nothing helps. Anyone else with this model furnace, and/or this problem? What did you do? What can you suggest?
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Old 03-30-2009, 05:28 PM   #2
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Re: Help needed with Suburban NT-12S gas furnace - won't light

Why is there so much air draw? My furnace has no burner air movement until the fire lights, only the fan moving air in the cabin. Could you have a hole in the heat exchanger allow the blower to move air in the burner?

Try calling Suburban Technical Support 423-775-2131 and see what they have to say.

Mike
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Old 03-31-2009, 01:32 AM   #3
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Re: Help needed with Suburban NT-12S gas furnace - won't light

You want to be very careful when modifying the burner, I've seen orfices on LPGas refrigerators get "cleaned" a little to aggressively and cause a fire. There is something that needs to be corrected in the unit, or with the gas supply, and I would start there rather than risking a fire.

Since you can make the unit ignite by "blowing" into the outside vent (I have to assume you are blowing into the air "inlet"), it sounds like you might have a sail switch that is very dirty, tweeked, or needs to be replaced. When not working properly, the sail switch will not allow the furnace to ignite. The most likely culprit is probably dirt and dust that is preventing the sail switch from moving enough to engage its micro-switch. With goggles on, try using compressed air and a shop-vac to clean the inlet and exhaust tubes and around the sail switch.

Good Luck!

John K.
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Old 04-02-2009, 03:12 PM   #4
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Re: Help needed with Suburban NT-12S gas furnace - won't light

Thanks for the suggestions, guys - I've been sick and my husband's been busy, so we haven't had the time to look into these yet but will do so very soon.
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