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Old 01-13-2020, 10:45 PM   #1
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Odd problem w/Suburban NT-34SP furnace

We have been renovating a 29' 1976 Airstream. 18 months ago I installed a Suburban NT-34SP furnace. Did all of the measurements for the ducting and installed per the instructions. First time we fired it up it worked and has kept us warm even when temps get into the single digits (F).

It has continued to work for the past 18 months but yesterday it started doing something odd. When I move the thermostat up to 75F the fan goes on and air can be felt coming through the ducting at both ends of the trailer. After about 15 seconds there is a click. The air and ducting actually get slightly warm and then cold as it cycles through for 10 minutes before the fan shuts down.

Took the front cover off. No extra parts laying around and fan is pushing lots of air out of the open end. Looked through the flame "port hole". The flame is visible but then it stops and then it restarts. It goes through this cycle for 10 minutes and then the furnace shuts down.

The flame looks like the drawing of the "Incorrect Flame" in the Installation Instructions. Conclusion is that it is getting 12V and propane (tank is full) but for some reason the flame is not burning like the image of the "Correct Flame". Thinking that it may be an air bubble I have restarted the furnace many many many times. Does not seem to have any effect on the flame.

Any suggestions welcome. Fortunately we have two 110V heaters that are working so we are not desperate - yet.
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Old 01-14-2020, 08:43 AM   #2
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Try turning on the stovetop for a few minutes to clean any air in the lines. If the stovetop burns poorly look at your propane bottles, regulator and the flex lines that connect the bottles to the regulator. Buy or make a nanometer to check gas pressure (YouTube for how to videos) Low gas pressure means adjusting or replacing the regulator, or if the flex hoses are over 5 years old, they may be leaking and need replacement.

If everything is normal on the propane supply side, the furnace sail switch and control board are likely suspects... and I chicken out and call a pro at this point.
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Old 01-14-2020, 09:06 AM   #3
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Most propane cylinders have an overflow protection valve. If the tank valve is opened too quickly the flow of propane will be restricted. Try shutting the cylinder valves, try to light stove, and then reopening the cylinder valves slowly. This has happened to me a couple of times. Your furnace has a lot more flow than your fridge or stovetop. If that doesn't solve the problem, a check of the burner orfice should be considered. Spiders love propane, as well as other critters. If all of the above don't solve the problem, then "Foiled Again's" suggestions should be looked at. I always look to the most simple options (cheapest) first!!! Good luck.
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Old 01-14-2020, 09:36 PM   #4
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CBWELL "Try shutting the cylinder valves, try to light stove, and then reopening the cylinder valves slowly."

I loosened the nut on the hose that connects it to the regulator (cylinder that is currently providing propane to the furnace). Let a few seconds of propane escape, tightened the nut that holds the hose onto the regulator and then closed the primary valve on top of the propane tank. Opened the valve and went back inside. Lit a stove top burner, made sure it had a good flame and then turned the burner off. Set the furnace thermostat to 80F. I have no idea why but the furnace is now working. This strategy saved me from buying a new regulator only 3 years after I installed the current one. Thank you both for responding.
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Old 01-15-2020, 07:55 AM   #5
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Glad things worked out. Sometimes it is the simple things that happen and are easily repaired. I always look for the easiest and cheapest way first. A lot of my fixes are from past experience
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