Strange Furnace Problem - Fiberglass RV
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Old 11-18-2021, 02:36 AM   #1
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Name: Tom
Trailer: BigFoot 25B25RT
Massachusetts
Posts: 536
Strange Furnace Problem

My 25-foot rear twin BigFoot has a strange furnace problem. I got home to Massachusetts late last March and my furnace promptly failed. I knew about the sail switch and little else. So, I watched some videos and looked at the wiring diagram which is quite simple.

All it has is a gas valve, igniter, fan, circuit board, and a couple of switches. The switches are in series. They are a sail switch which senses whether the fan is running and a temperature limit switch which will open if it gets too hot. So, if either switch opens the furnace will shut down.

I got out my ohm-meter and found almost 10 ohms in the limit switch. I found one at a RV dealer and replaced it. Easy fix. I ordered a circuit board and a sail switch to have spares but I couldn't find another limit switch right away. I remeasured the faulty limit switch and it came in at 0 ohms like a new one. Did it fix itself?

Well, it has started to get cold and the furnace failed again. The limit switch was about 7 ohms. So, I installed the old self-healing one and ordered a couple of new ones. I have heat and everything is fine. The new ones come in and I start ohm checking everything. Well, the second faulty one healed itself and has a zero-ohm reading now.

Don't you just love electronics. If this was an old-fashioned Bi-metal switch this wouldn't be happening. I left the first resurrected switch in there and we will see where this goes.
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Old 11-18-2021, 09:38 AM   #2
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Name: Dave W
Trailer: Trillium 4500 - 1976, 1978 (2), 1300 - 1977, 1973, and a 1972
Alberta
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Registry
I hate gremlins.
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Old 11-18-2021, 09:59 AM   #3
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Name: Tom
Trailer: BigFoot 25B25RT
Massachusetts
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Originally Posted by David Tilston View Post
I hate gremlins.
Yeah this thing is a 2020. Is my furnace going to be problematic all of the time?
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Old 11-18-2021, 10:15 AM   #4
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Name: Perry
Trailer: 2018 Escape 5.0
Lanesboro, Minnesota, between Whalan and Fountain
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Originally Posted by ThomasC View Post
Yeah this thing is a 2020. Is my furnace going to be problematic all of the time?
Sail switch problems in the first three months of ownership of our Escape 5.0 caused us to purchase a Martin catalytic heater that is 2,800 btu's on high and about 1,400 btu's on low. It keeps our Escape at about 50-60 F all night in cold weather (<40 F).

Yes, we purchased a second CO detector, always have the MaxxFan open a half inch, and open another window 1/4" when running the Martin so there is plenty of ventilation. We use it quite often since it's quiet and efficient, way more efficient than our furnace. If you don't understand the chemistry involved here don't use one, but for those who do it's great!

We have friends who have multiple sail switches, replaced a circuit board and carry a spare, and had to replace their gas solenoid valve (they probably have a spare valve as well). Those Dometic furnaces are not exactly state-of-the-art.

We also carry a Mr Buddy to warm up the camper when we get in at night with the outside temp at 40 F or less. Takes about 15 minutes to warm the camper, but the Buddy is just too hot (4,000 btu's) to use all night.

Enjoy,

Perry
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2018 Escape 5.0 TA - 2019 Ford F-150, 3.5 V6 Ecoboost,

Previous Eggs - 2001 Scamp 16' Side Bath, 2007 Casita 17' Spirit basic, no bath, water or tanks, that we regret selling, 2003 Bigfoot 25B25RQ, that we also regret selling
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Old 11-18-2021, 02:10 PM   #5
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Name: Tom
Trailer: BigFoot 25B25RT
Massachusetts
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Originally Posted by Perryb67 View Post
Sail switch problems in the first three months of ownership of our Escape 5.0 caused us to purchase a Martin catalytic heater that is 2,800 btu's on high and about 1,400 btu's on low. It keeps our Escape at about 50-60 F all night in cold weather (<40 F).

Yes, we purchased a second CO detector, always have the MaxxFan open a half inch, and open another window 1/4" when running the Martin so there is plenty of ventilation. We use it quite often since it's quiet and efficient, way more efficient than our furnace. If you don't understand the chemistry involved here don't use one, but for those who do it's great!

We have friends who have multiple sail switches, replaced a circuit board and carry a spare, and had to replace their gas solenoid valve (they probably have a spare valve as well). Those Dometic furnaces are not exactly state-of-the-art.

We also carry a Mr Buddy to warm up the camper when we get in at night with the outside temp at 40 F or less. Takes about 15 minutes to warm the camper, but the Buddy is just too hot (4,000 btu's) to use all night.

Enjoy,

Perry
The BigFoot furnace also heats the plumbing and tanks. If you have too much alternative heat available you could freeze your pipes.
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Old 11-18-2021, 02:22 PM   #6
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Trailer: 2018 Escape 5.0
Lanesboro, Minnesota, between Whalan and Fountain
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The BigFoot furnace also heats the plumbing and tanks. If you have too much alternative heat available you could freeze your pipes.
Good point! Being a former Bigfoot owner I should have remembered.

OTOH, at 25 F and above I'd have no problems using the Martin, in fact we've had many nights in our Escape with the nighttime temps down to 15 F and nothing's frozen (yet). Below 15 I usually use my air compressor to blow the lines clear and drain the fresh/grey water tanks. Our black water tank is in the camper.

Enjoy,

Perry
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2018 Escape 5.0 TA - 2019 Ford F-150, 3.5 V6 Ecoboost,

Previous Eggs - 2001 Scamp 16' Side Bath, 2007 Casita 17' Spirit basic, no bath, water or tanks, that we regret selling, 2003 Bigfoot 25B25RQ, that we also regret selling
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Old 11-18-2021, 08:57 PM   #7
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Name: Derek
Trailer: 2007 Bigfoot 25B25RQ
Washington
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I had a problem where the furnace would occasionally decide not to turn on. After trying many different things I finally found the problem when I wiggled some wires and it turned on. There was a bad connection at the quick-splice that is staring you in the face when you open the outside cover.
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Old 11-24-2021, 02:10 PM   #8
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Name: Rob
Trailer: 2020 BigFoot 25RQ
Colorado
Posts: 14
We have a 2020 25RQ Bigfoot and are also having furnace issues, especially at higher elevations. Wish I had an answer.
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Old 11-24-2021, 04:24 PM   #9
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Trailer: BigFoot 25B25RT
Massachusetts
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Originally Posted by Moots View Post
We have a 2020 25RQ Bigfoot and are also having furnace issues, especially at higher elevations. Wish I had an answer.
I think I have figured it out. I just haven't posted because it will be a long post and there are modified photos involved. I'm just a little lazy. Could you give some details of the problems I might be able to help now.

I think my problem was restrictions in the air flow. Like having anything in front of a vent even a little bit.
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Old 11-24-2021, 08:06 PM   #10
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Name: Rob
Trailer: 2020 BigFoot 25RQ
Colorado
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We have been pretty careful about putting anything in front of the air intake.

We live in CO and when we first went above 7,000 feet (when the BF was brand new), we couldn't get the furnace to light at all. It would cycle all night long if we would let it. Then we replaced the sail switch. That helped. Now the furnace will light (eventually) even at high elevations, but when it takes five our six cycles to light, it doesn't inspire much confidence for cold weather camping.

We have a solar installation and have an inverter and lithium battery located close to the furnace under the seat cushions. I have frequently wondered if we have crowded the furnace too much for it to function properly. However, when I search online, I read that a number of people have had problems at high elevations with these Dometic furnaces.

I've read your posts before and you are very good at diagnosing and solving problems. I am less so. We have an appointment in December with a fellow who is pretty handy with these furnaces. Hoping he can locate our issue. If he figures it out, we will post our solution.
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Old 11-25-2021, 01:49 AM   #11
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Name: Tom
Trailer: BigFoot 25B25RT
Massachusetts
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Originally Posted by Moots View Post
We have been pretty careful about putting anything in front of the air intake.

We live in CO and when we first went above 7,000 feet (when the BF was brand new), we couldn't get the furnace to light at all. It would cycle all night long if we would let it. Then we replaced the sail switch. That helped. Now the furnace will light (eventually) even at high elevations, but when it takes five our six cycles to light, it doesn't inspire much confidence for cold weather camping.

We have a solar installation and have an inverter and lithium battery located close to the furnace under the seat cushions. I have frequently wondered if we have crowded the furnace too much for it to function properly. However, when I search online, I read that a number of people have had problems at high elevations with these Dometic furnaces.

I've read your posts before and you are very good at diagnosing and solving problems. I am less so. We have an appointment in December with a fellow who is pretty handy with these furnaces. Hoping he can locate our issue. If he figures it out, we will post our solution.
You should have some information that came with the RV on the furnace. There is a led on the circuit board that will blink out several codes depending on what is going on. You have to remove the outside cover and at my age get down on my hands and knees and look up to see it.

There is a limit switch over the exhaust pipe but it is hard to get at. I have some extra-long needle nose pliers to get the wires off. Mine were originally a little loose, so I carefully tightened them with a pliers.

The first time I had problems the quilt was hanging in front of the bedroom vent. I corrected that because the bedroom felt cold. The furnace was also shutting down. The second time I put a 6-pack of Coke in the doorway that kind of blocked that vent.

My blink code said limit switch or air restriction. I am thinking it was the 6-pack that was in the way. It would crop up on colder nights when the furnace had to work harder and get warmer and maybe hot enough to trip the limit switch. It would shut down then retry in an hour. By then it had cooled and would work a while then repeat. I replaced the limit switch yet again even though I think it is alright and I moved the Coke. It has been good for a week.

High altitude,,, there is less air so it would burn rich but it should burn. I wonder if the thin air makes the sail switch iffy. Did you get your BigFoot from Denver. They are a mile high maybe they know. If you didn't get it there, I could call next week for you. Let me know.
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Old 11-25-2021, 08:29 AM   #12
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Name: Rob
Trailer: 2020 BigFoot 25RQ
Colorado
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Happy Thanksgiving, and what a thoughtful reply.

You have given us some good things to consider and look into. Thank you!

We know Dave and Jordan at Trailer World Denver well. Great people!

We actually bought our BF from British Columbia because Trailer World was sold out. Get Away RV had to pull our trailer to the U.S. border because we couldn't cross into Canada at the time because of Covid.

Thank you.
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