• Greg F
    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!

  • Ray
    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
    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
    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
    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
    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
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