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Old 03-28-2020, 02:03 PM   #1
mikectx
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Furnace Problem. I think or ???

2020 Keystone Montana Model 3121RL Purchased new October 2019
(Have owned two other Montana’s in past 20 years with zero furnace problems)
2017 Silverado 3500 6.6L Diesel

Commentary: We are full-timers. It is a real pain to take the RV to service centers. Wait all day and then be told “we will order the part, Have a good day”. Then call in two weeks and be told “it is working like it is supposed to”. They don’t seem to like warranty work and want to do everything they can to discourage customers. Okay I’m officially off soapbox, onto the furnace problem.

Furnace: Dometic 35000 BTU/Hr Model: DMFD35131
Propane Tanks: Full
• Checked gas stove burners no change in flame output.
• Removed furnace fuse and replaced with a similar fuse.
• No flashing lights on furnace control circuit board

Problem: Furnace will continue to short cycle every 10 seconds or so (does not go in lockout) for about 15 minutes and then starts and runs until temperature set point is reached. Blower goes into cool down and stops. When temperature drops below set point it will short cycle then restart in a shorter time and sequence normally. Subsequent times same thing.

Action with Dealer (60 miles one way): Technician observed problem and said it was flame sensor. Told me they ordered flame sensor. Called two weeks later they said Keystone would not cover cost as “it was working as it was supposed to”. Had to leave town so didn’t go back to dealer.

Action with Mobile repair tech: Observed problem. Said it was gas valve. They changed gas valve, checked and tightened all propane connections, then changed electrode. It started while they were here then checked two days later when it was cold and same problem.

Action with Dometic: Sent this information to Dometic. Response: We do not provide technical support over the phone as we need a tech to diagnose the unit and test components. Sent me service manual.

This is a sequence I found on an internet site. I added my observations, actions and results so far.

1) The thermostat triggers a request for heat, sending a current to the RV furnace.
Action: Relocated Bedroom Thermostat to Living Room Thermostat
Results: No Change
2) The current powers the time delay relay which in turn passed power to the RV furnace blower. The blower spins up and pulls air from the Air Return creating a flow of air through the heat exchanger and out of the ducts. Another fan pulls air into the combustion chamber to feed the furnace.
Observation: Blower continues to run the entire time. Air is coming out of
all ducts.
3) As the blower speed increased the fan air causes the sail switch to close.
Action: Changed sail switch
Results: No Change
4) Power then flows through the High Limit Switch and onwards to the Control Circuit Board.
Action: Changed limit switch and Changed control board
Results: No Change
5) As power is delivered to the control board it opens the gas supply and causes the DSI igniter to spark. The gas lights and creates heat within the combustion chamber.
Action: Replaced gas valve and electrode
Results: No change
Observation: Burner lights then goes off. This action repeats for 10-15 minutes then the burner remains lit until thermostat reaches set temperature and then burner goes off and then after a short period fan turns off
6) Heat will pass into the heat exchanger, heating the air passing through the Ducts.
Action: Removed ducts from heater to determine if duct was problem
Results: No change

Observation: Ambient temperature seems to affect the short cycle startup time. Example It was in the 80’s here the other day. Short cycle time was about 2 minutes and then subsequent time even shorter. Starting to think propane is warmer therefore more pressure. Could it be propane regulator?
Action: Adjusted regulator screw clockwise ¼ turn then ½ turn. Seemed to effect a change but then tried again in the morning when outside temp was in the 40’s
Result: Same old problem.

Any help would be greatly appreciated.
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Old 03-28-2020, 02:19 PM   #2
jimcol
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Think a similar issue was recently addressed on this forum. If memory serves me correct the issue was not enough gas getting to the furnace. You need the proper test equipment to measure gas pressure. Furnace requires 11 inches water column. I'll try to find the post.
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Old 03-28-2020, 02:47 PM   #3
mikectx
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Thanks jimcol. After reading this are you thinking like I am that it might be the propane regulator?
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Old 03-28-2020, 03:22 PM   #4
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Here is the post I was thinking of: https://www.montanaowners.com/forums...ad.php?t=80218
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Old 03-29-2020, 10:39 AM   #5
DQDick
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If you try everything else and can't find the problem, replace the sail switch. Even though sail switches aren't supposed to cause the problems some of us have been having replacing the sail switch has fixed a wide variety of problems. Dometic had a bunch of bad ones and knows it but isn't issueing a recall or even a service bulletin. You can't call and get anyone who knows about it, but when our mobile tech called, the real service people, they told him right off.
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Old 03-29-2020, 10:55 AM   #6
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I also had problems with my HC 371FL. Purchased it in September 2018 and used it till the beginning of January 2019. During that time, the furnace would attempt to fire 3 times. On the 3rd attempt it would finally kick in. I thought it was just the nature of the thing.

By April 2019, we were back in the camper, only now the furnace would not fire at all. Fan blew, but no fire. Lots of research, YouTube videos, and trial and error, turned out to be the Sail Switch. Attempted to make an appointment for warranty, but the wait time was so long, decided to just fix it myself. I bit the bullet for the cost of the switch, hoped this was the real problem afterall, and installed it myself. Problem solved. Never a problem again. Furnace has run great ever since. We made it through Winter 2019-2020.

I know that sounds simple, and yours may be a completely different situation, but it's worth it to pull the Sail Switch and at least make sure there are no dust bunnies preventing it from working correct.

Here's the YouTube video that changed my world:

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Old 03-29-2020, 11:22 AM   #7
Daryles
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I know you said you replaced the self resetting furnace fuse with a "similar " one.
Maybe try a regular blue 15A fuse. The self resetting ones are notoriously unreliable.
The blades are thinner. You may have to give it a twist to make good contact.
Jim Vititow made a video about bad boards.
Intermittent furnace. Bad circuit board, heat gun troubleshooting. By Jim Vititow.
https://www.facebook.com/groups/2740...7964195593987/
His tech put a heat gun (hair dryer would work) on it to heat up the circuit board and it worked fine. They had 3 brand new boards do the same thing right out of the box.
It is also known Dometic had bad batch of sail switches. I know you said you changed it already. Some have had success bending the sail a little.
And or course, regulator pressure. I'm sure a gas tech would have the test equipment for that. Or switch the diverter valve to the other tank/regulator. Very unlikely both are bad.
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Old 03-29-2020, 11:29 AM   #8
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sail switch

We've been full timing in Horn Lake Mississippi since late January. We too were having furnace problems. I bent the sail switch as had been posted elsewhere. It then would cycle normally, most of the time. It took about three weeks to get the switch due to backorder. We're happy campers now with no further problems. The people on here are so helpful!


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Old 03-29-2020, 05:08 PM   #9
mikectx
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Thanks for your help. Will try all these suggestions. Some of them I have already tried but will recheck. Once I finally get it fixed I will let you know the outcome.
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Old 03-29-2020, 06:45 PM   #10
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Since you've changed/replaced most things other than the regulator it would seem that it might be the regulator or a component that has already been replaced.

Replaced the control board on a Dometic 35k unit 3-4 years ago. Failed again right after replacement - installed Dinosaur board and no more problems. This winter in FL furnace started acting flaky - similar to yours but not exactly. Turned out to be the regulator. Seemed over a few minutes of running the regulator could not keep up. I finally turned on all the burners on the stove and fired up the furnace. I could see the difference in the flames when the furnace fired and over a period of a few minutes you could see the flames start to dwindle. Doesn't sound like that is your issue - maybe control board.

I will throw this out as well; I doubt it's your problem but this winter while in FL my furnace would click, fire up and run for a period of time and start to warm the unit up then shut off. After that it would just try to ignite, click and time out. Figured it was the limit switch and was about to get one but thought about the process for it - getting too hot? Come to find out I had installed a mesh screen over the exhaust for mud daubers and such and removed it.....fixed the issue. Seems really crazy but guess the 35k puts out enough heat, and the mesh screen provided just enough blockage, that it caused the limit switch to kick in. Just something to think about in the future. You've done a lot of work so far and wishing you good luck in getting it going.
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