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Old 10-16-2020, 06:54 PM   #1
Jtowndc
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My furnace will not stay lit

Hello everyone,

I am looking for some guidance before I start tearing apart my furnace. I have a 2018 Montana Legacy 3950BR. The problem I have is the furnace will not stay lit. I hear it fire up and stays lit for about 10-20 seconds then blows out and re-lites. The ingniter works and it gets gas, just wont stay lit. I think it could be a thermo valve or actuator that is not working correctly. Your help is much appreciated.

Joe
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Old 10-16-2020, 07:30 PM   #2
JDDilly
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First thing I would check is the sail switch.
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Old 10-16-2020, 08:22 PM   #3
Carl n Susan
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In the Files section are two manuals which may help.


1) https://www.montanaowners.com/forums...?do=file&id=54 Page 30 shows startup sequence

2) https://www.montanaowners.com/forums...?do=file&id=26
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Old 10-17-2020, 08:03 AM   #4
Harrisden54
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I had a simialr issue with my 384BR. It would light and go out. I ended up pulling the burner suspecting a bug or cob web. The burner was full of water and corroded. Dometic is familiar with water getting into the burner area on the vertically mounted furnaces and replaced the corroded parts. Water was entering through the exhaust vent. I installed a dryer vent cover over the exhaust but removed it when the techs at the Goshen Rally caulked around the exhaust flange and it seems to have stopped the water entering.
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Old 10-17-2020, 02:39 PM   #5
ChuckS
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Since the furnace actually fires off the Sail Switch ISNT what is causing your issue.. the furnace will not have ignition nor will the gas valve open if sail switch doesn’t make contact when blown closed to complete the circuit...

I’d suspect the flame sensor is not seeing the flame correctly due to dirt, mud daubers, or improperly positioned flame sensor/igniter.

The flame sensor is what tells the furnace control module “I see a flame” and tells gas solenoid valve to stay open...
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Old 10-17-2020, 05:33 PM   #6
Loneoak
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Has it rained lately? Ours wouldn’t stay lit one day shortly after we had a rainstorm where the rain came in sideways. Once the igniter dried out, it worked well.
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Old 10-17-2020, 05:43 PM   #7
WeBeFulltime
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Quote:
Originally Posted by Loneoak View Post
Has it rained lately? Ours wouldn’t stay lit one day shortly after we had a rainstorm where the rain came in sideways. Once the igniter dried out, it worked well.
I had this issue a couple of times too. Sail switches get a bad rap. I have never had a sail switch fail even during 9 + years full-timing.
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Old 10-20-2020, 11:50 AM   #8
Belti Guy
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I just had this problem a couple of weeks ago on our 2007 3000RK. The flame ignited and seemed to burn strong, but then went out. I was convinced that the flame detector was bad, so I ordered a new igniter/ flame detector. But then decided I would just blow out the flame tube incase a spider web or something else was impeding the gas flow. The dust and rust just flew out. Presto, it worked fine and has ever since. So get a source of compressed air and blow through the tube at the air intake. Good chance it will solve your problem.
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Old 10-20-2020, 03:08 PM   #9
beeje
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Just had the same issue. I used my battery blower and blew air in for like 5 mins. Worked great afterwards.
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Old 10-21-2020, 06:51 AM   #10
Butch & Kathy
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Quote:
Originally Posted by Belti Guy View Post
I just had this problem a couple of weeks ago on our 2007 3000RK. The flame ignited and seemed to burn strong, but then went out. I was convinced that the flame detector was bad, so I ordered a new igniter/ flame detector. But then decided I would just blow out the flame tube incase a spider web or something else was impeding the gas flow. The dust and rust just flew out. Presto, it worked fine and has ever since. So get a source of compressed air and blow through the tube at the air intake. Good chance it will solve your problem.
Same situation for me as well but I sucked the junk out with a shop vac.
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Old 10-21-2020, 03:40 PM   #11
Greg n Tracy
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Have you noticed a difference in operation as you find yourself in higher elevations (up in the mountains)? We have the same issue you describe, and it gets progressively worse, the higher the elevation.

I don't understand how the burner gets rusty in the vertically installed units, assuming water enters through the exhaust vent. The burner tube exhaust is at a low point and the tube winds around inside the unit from low point to high point and then back down, around a few bends. I know the saying, "if water can get in, it will", but up hill AND around the bends?
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Old 10-21-2020, 04:05 PM   #12
ejerni
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I had to replace the sail switch for the same reason
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Old 10-21-2020, 09:02 PM   #13
allenclme
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Quote:
Originally Posted by Greg n Tracy View Post
Have you noticed a difference in operation as you find yourself in higher elevations (up in the mountains)? We have the same issue you describe, and it gets progressively worse, the higher the elevation.

I don't understand how the burner gets rusty in the vertically installed units, assuming water enters through the exhaust vent. The burner tube exhaust is at a low point and the tube winds around inside the unit from low point to high point and then back down, around a few bends. I know the saying, "if water can get in, it will", but up hill AND around the bends?
I have the exact opposite problem. When I'm in my neck of the woods between 5000 and 7000', it works great. When I go to the coast at sea level, it won't stay lit.
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Old 10-26-2020, 10:37 AM   #14
decarteaux
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Had the same issue. Opened up the cover and wiggled the switch closest to the cover, assuming it is the sensor switch coming directly from the thermostat. It turned on and off as I wiggled it....pretty clearly the problem.
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