• Greg F
    304
    We have a Suburban SF-35Q forced air furnace that came with our new 5th wheel.

    Here are the symptoms:

    Using T-stat furnace starts up normally. Fan starts and then a short time later the igniter fires the burner. This is what I have experienced as normal in every RV I have owned.

    Furnace runs fine for a few minutes and them flame stops. Fan blower continues blowing as air temp out of vents drops. A short time later the igniter can be heard and the flame lights, air gets hot again. This repeats indefinitely every 3 or so minutes. This is well before t-stat temp and room temp is met.

    Things I have tried:
    1. Inspect sail switch. Clean and seems fully functional. I may just replace it and see if it could be a problem. Having a spare wouldn't be a bad thing anyway. I assume that if it was the sail switch the furnace would not light at all. The switch remains in the 'on' position always as the fan is running.
    2. I replace high temp limit switch (my first thought) no dice..
    3. Remove furnace and check for any obstructions in both the burn and blower sides
    4. Bench test blower motor for abnormalities.
    5. New RV main gas regulator. Our old was sort of funky anyway
    6. Inspected ductwork for blockage that could be causing furnace over temp due to back pressure.

    Possible culprits:
    1. Bad mother board (have an e-mail into dinosaur asking for help)
    2. Something bad in the gas side of the system.
    3. Bad flame sensor. This is something I need to do more research to see if symptoms fit but seems like a possibility
    I keep coming back to the fact it seems temperature related. Furnace gets hot, flame shuts off, furnace cools down, flame is re-ignited. I may also re-install the furnace and leave all the ductwork off the supply plenum and see if the problem continues. That should eliminate any 'overheating' due to back pressure of ductwork condition.

    Intersted in any thoughts any of you tech savvy folks might have. The trailer is under warranty but I prefer to handle things like this on my own and better familiarize myself with roadside repairs.

    Anyway, hope all are having a nice Sunday. Thanks!

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  • Ray
    821
    Sounds like you've checked pretty well everything, would be nice to have a board to try. I wonder if the sail switch maybe is intermittent. Maybe you could jumper it and see.
    I do remember way back the first year I had the trailer I had a problem with one of the cheapie OEM pigtail hoses off one LP cylinder. It played havoc with the furnace and water heater lighting and quitting. I narrowed it down as I noticed on the other tank things worked fine.
    Guess the little safety valve they have in them was flakey. A propane guy showed me how the Marshall hose he sold me to replace it had a much better quality valve with a spring, so I ended up swapping all my two pigtails and regulator to Marshall stuff. Never a problem since.
  • Greg F
    304
    Good Idea to try jumping the sail switch. I just ordered a new one. Even if it isn't the issue it would be a good part to keep with us. Hopefully Dinosaur will also have some advice. I have been considering buying their fridge board anyway. The stock boards seem to be a common failure issue.

    I did try 2 different propane sources and a different regulator so if it's a propane issue I think it is within the furnace unit.

    It's always something. :grimace:

    Thanks! :)
  • Jim Litz
    4
    Please keep us posted on what you find. Also, check out the My RV Works YouTube videos with furnace repair and troubleshooting tips .... https://www.youtube.com/playlist?list=PLk44vSl27TRCaZIQbm-0utYK7sVOKwXVY
  • Leftie Canuk
    4
    Any chance the flame sensor thermocouple (assuming there is one) is the culprit ? In our first rig, those triggered all kinds of troubleshooting. i finally just swapped it out, and the safety valve did its thing properly, and the 3 minute cycling stopped. It didn't actually make sense, because in theory the safety valve should not have opened if the thermocouple was faulty, but it corrected the problem.
    Not being an electronics guy like Ray, I could only assume the electrical properties of of the flame sensor (thermocouple) changed improperly with a constant flame on the thermocouple. Anyway, might be worth a try.
  • Greg F
    304
    Thanks for the reply! Yes the flame sensor is on my possible list. I haven't had a chance to work on the furnace as we are in there midst of getting our house ready to sell. I will dig a little farther into what's involved. The furnace is still on my workbench and now would be great time to replace parts.

    Thanks! I will update this thread with final results
  • John J
    2
    I would be curious if the furnace continues to drop out if you bypass the thermostat.
  • Greg F
    304
    Quick update:

    I will hopefully be reinstalling and firing the furnace up later today. While it's out I went ahead and replaced the following:

    Previously replaced the high temp limit switch
    yo34dkqs2nyi92e9.jpeg


    New igniter.
    u7gnvvqwdgieo6ze.jpeg

    New sail switch
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    New control board
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    ubabakxvki3vh3rj.jpeg

    Ready to be reinstalled. The old parts will come along with us as spare parts.
    wqd4p0jvjyjgi54v.jpeg

    Let's see what happens. :brow:
  • Greg F
    304
    Well, none of the new parts made a difference. I did isolate the issue to the hight temp limit switch and feel that the problem is a fundamental design issue with too much duct back pressure in the system. I relocated the ductwork on the back of the furnace to better direct air to the higher volume ducts, removed some curves in the ductwork and the system is running better with much less cycling. So that points to the issue.

    The basic problem seems to be that the constraints of the factory ducting isn't allowing a proper air exchange through the furnace. I suspect this issue is somewhat normal as I see many complain about lack of heating in these units on a couple different forums. The cycling of the furnace by way off the limit switch isn't an obvious flaw since the blower never stops and there are no detectable alerts. Unless you are a dork like me listening all the time for any bump in the night it would go unnoticed.

    At some point I will drop the plastic undercarriage and go farther with this via improved/additional ductwork. As we are able to do much of the heating through our inverter and solar and travel in fair weather I think I am done with this for a while though. *shakes fist*
  • Willie
    30
    Gosh, not unlike the problem I had develop with my in-home furnace a few years ago with the same symptoms, shutoff after a short run time. The code thrown indicated that a vacuum sensor on the intake side was tripping. The tech did two things to solve the problem. He made the furnace intake more efficient by replacing the older registers with a more efficient design and he replaced the restrictive paper filter with one that is washable, reusable and allows for a more efficient flow of air. He said because of the fire prevention features (vacuum and back pressure sensors) that the key to a happy furnace is unrestricted air flow, both in and out.
  • Columbus
    12
    Had the same problem so pulled the plastic belly skin off and the duct work was a joke. Some of it was squeezed mostly shut off and had to cut off 4’or 5’ of extra duct just wadded up an thrower loose in there. Where it goes by the tanks it it was smashed mostly flat. Most of the heat was going to the bathroom in a short direct hose. Not engineered just thrown in. Good luck.
  • Greg F
    304
    I suspect you guys are correct. I should bite the bullet and pull the bottom off. I can install some SeeLevel sensors on the tanks while I'm at it. We are going to be listing our house for sale soon so now is the time while I have a nice place to work on it. *sigh*
  • Colibabas
    41
    I just came across this article if you haven’t already seen it.
    https://www.doityourselfrv.com/rv-furnace-repair/
  • Drew
    34
    Willie,

    What kind of rv furnace to you have? I've never heard of one with a filter in it.

    Thanks, Drew
  • TerryMac
    17
    I have an Attwood furnace, old obsolete etc. It failed a couple of years ago during the bomb cyclone in the Texas Pan handle. I went to Walmart and bought a Mr Heater Little Buddy and a case of propane tanks. It worked so well that when I got home and fought with the furnace (As an FYI the sail switch is to make sure the fan is blowing.) Apparently over time the combustion chamber got clogged with soot. Cleaned it out, replaced burner parts, finally got it to work on a bench outside the RV but when re-installed, wouldn't work.

    I got a Mr Heater Big Buddy and using a Natural Gas hose, direct connected it to the RV propane system. (The connector in the heater uses Natural Gas fittings instead of Propane connectors.) At the end of heating season, I disconnect the quick connect, slide the hose into the furnace area through a little door, and the heater goes under the bed or in the garage. We like to sleep cool so we always have a window cracked and I invested in a carbon monoxide detector as well as the normal smoke alarm.

    Some day I'll just dump the furnace and gain some more storage room. I have to figure out a work around for the stupid fan controller setup. Without the connection to the furnace, the AC won't run. Apparently the two stage fan controller for both the furnace and the AC are built into the furnace boards.

    It also doesn't my batteries overnight like the furnace fan would do!
  • Drew
    34
    TerryMac,

    I was interested in your comment about the ac not running due to a problem with the furnace. Can you explain that? If your furnace is an Atwood that sits at floor level (more or less) and is exhausted to the outside, I don't understand that happening....

    Thanks, Drew
  • TerryMac
    17
    Let me start off by saying that I have a 2005 Glendale Titanium camper that was made in Canada. They are now out of business so other owners are the only source of information. On the Titanium RV owners forum, there is lots of discussions on the Attwood systems. According to the gurus on the forum:

    The Attwood uses a two stage fan logic to determine when you need more fan for the heater and for the AC. I think Ray had a post where he installed a manual switch for low speed or high speed on his fan. The Atwood apparently uses some digital codes to tell the fan to run slow of fast and thus a simple switch will not work.

    The logic board for the fans (heat and AC) are in the furnace and connected to the Attwood thermostat. This also means the thermostat can not be replaced with a standard thermostat. The heater has two circuit boards, one is the normal ignition board which you can get easy and cheaper replacements for but the other is the fan controller is not cheap.

    I'm sure that with work, I could figure out how to isolate that board and scrap everything else but since I not an electronics tech, it would be a lot of trial and maybe error. Currently it is not worth my trouble so it just sits in my RV, wired into the thermostat. I never turn on the heat setting so I get by.

    The forum people tell me that unless you looking to replace everything, it is not worth fooling with. As long as my AC keeps going, I'll just live with it.

    I have tested that if the thermostat connector is not connected to the furnace then the AC will not run.
  • Drew
    34
    TerryMac, Thank you....as they say "You learn something every day."
  • Columbus
    12
    I have a suburban furnace in my 2015 Columbus. The thermostat runs up to the air conditioning unit then back down to the heater. The controller box in the air conditioning in went bad and the furnace quit working but the air conditioning still works. I found the two wires that controller the furnace at the furnace, brought them out and installed a thermostat separate for the furnace. Works great. We only use the heater first thing in the morning for a few minutes. It uses to much gas and battery to be worthwhile when Boondocking. We also have a mr. heater big buddy heater that we use most of the time and love it. Yes we leave a couple windows cracked an would not sleep with it on. It one of the best buys for our trailer that we use the most.
  • Craig Mckenzie
    4
    Have you checked the actual 12v voltage at the furnace. Then check your voltage at the battery. Both voltages should be the same. Now start up your furnace and see what the voltage is. It could be the voltage that the board is seeing when the furnace is running is below it’s shut down voltage. I can’t remember if it’s 10.5 or 11v furnace will shut down. You could have a poor ground or your positive cable has a poor connection. Easy to check. Run a wire from your battery positive to to furnace 12v input. Run a ground cable either from battery negative or from a good known ground spot on the frame to the furnace frame.
    Run furnace to see if that works. First check your 12v voltage at your furnace. Good luck
  • Greg F
    304
    HI,

    Voltage is fine at the furnace.

    I have boiled it down to the high limit switch and that there is too much heat building up in the furnace plenum. Just a crappy design with not enough air exchange. By better routing the ducting with more gentle bends the issue has become much better. I was going to add another register and duct run but after pulling the belly pan the fresh tank runs the entire width of the trailer and no extra room to put the added venting where it makes sense to the rear of the coach.

    Fortunately we don't run the furnace a lot and the heat pump does most of what we need.
  • Craig Mckenzie
    4
    Glad you have it figured out.
    Stay safe
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