Forum Discussion

rogerck's avatar
rogerck
Explorer
Jan 19, 2017

Furnace Short Cycles Off and On

I have a Suburban SF30fq furnace that cycles the heater flame off and on about every three minutes while the coach is reaching the thermostat setting (technically called short cycling on the limit switch). The furnace is a factory install in an a Keystone Cougar. My understanding is this short cycling should not be happening. As a result heating the coach takes much longer. Things I've checked:
- adequate gas pressure including a new regulator
- thermostat works properly
- no obstruction in ductwork, plus still short cycles when front panel removed and all ductwork is bypassed.
- fully charged batteries, blower fan seems to be working fine
- replaced limit switch
- no air obstruction on intake side
- no obstructions at intake and exhaust outlets on trailer exterior
- The limit switch shuts flame down when furnace air temp is 185 degrees (at limit switch) and restarts flame when air is 135 degrees.

Any suggestions? I'm finding it very hard to find a local tech that really knows much about this.
Thanks, Roger
  • DrewE's avatar
    DrewE
    Explorer II
    Jan 20, 2017
    If the motor is running very much slow, the combustion air would be insufficient to activate the sail switch and the furnace would not light at all. The two fans are run by the same motor on the same shaft in RV furnaces. Getting insufficient airflow on the duct side but not the combustion side would imply some physical problem with that fan rather than a motor problem, and typically such damage would be rather obvious.

    Looking at the documentation for the furnace, the limit switch and the sail switch are wired in series. It might be worthwhile to check that the sail switch is behaving properly. Depending on how one is checking the limit switch, the sail switch opening may appear to be the limit switch opening. Follow Doug's advice first before checking the sail switch as he has far, far more experience and knowledge than I have.
  • Chris_Bryant's avatar
    Chris_Bryant
    Explorer II
    Jan 20, 2017
    What strikes me is calling this model quiet- I wonder if the motor is running at full speed. I *think* if one brush is bad it will run slow.
  • dougrainer's avatar
    dougrainer
    Nomad
    Jan 20, 2017
    rogerck wrote:
    I removed the inside furnace panel facing the trailer, then replaced the hi limit switch. After testing, the new switch also cycled with front panel removed. Maybe front panel with ducts attached is required for proper air flow across limit switch. Will try.


    Removing the panel removes case pressure. That means the fresh air can move over the chamber freely without restrictions like a restricted duct. USUALLY, if the Hi limit is GOOD, the ductwork is inadequate(not enough for spec) or it is restricted. I would suspect insect nests INSIDE the combustion chamber restricting the flow inside the chamber creating hot spots. If I had the unit, I would be testing on the work bench to see if the problem stays when on the bench. I would take a rubber mallet and hit the chamber and then see if I hear nests inside the chamber. Doug
  • rogerck's avatar
    rogerck
    Explorer
    Jan 20, 2017
    I removed the inside furnace panel facing the trailer, then replaced the hi limit switch. After testing, the new switch also cycled with front panel removed. Maybe front panel with ducts attached is required for proper air flow across limit switch. Will try.
  • dougrainer's avatar
    dougrainer
    Nomad
    Jan 20, 2017
    IF you have removed the back panel and this allows the heat to blow inward to the RV and the Hi limit short cycles. REPLACE the Hi limit. Doug
  • rogerck's avatar
    rogerck
    Explorer
    Jan 19, 2017
    I've held meter probes on the limit switch and it is shutting on and off. Haven't yet looked at combustion area but there is no soot in the exterior exhaust. It does seem heat exchange is not keeping up. The blower fan runs steady but I wonder if it's at specified rpm's? It seems unlikely that it would run well but not at proper speed. However, the blower on this rig is much quieter than my preview trailer. I bought my 2012 Cougar 6 months ago so have no prior experience with it.
  • DrewE's avatar
    DrewE
    Explorer II
    Jan 19, 2017
    It would appear that the heat exchange isn't keeping up with the burner. I think you've already covered most of the reasons why this would reasonably happen, at least the reasons I know of as a moderately handy guy who isn't at all a furnace specialist.

    Have you verified that the limit switch is indeed what is shutting the flame down (and not something else in the control system)? I can envision that something like a poor connection or bad solder joint on the control board could act similarly to the limit switch cycling under the right (wrong?) conditions.

    I'd also check that the baffles etc. in the "house" part of the furnace are all properly in place, and that the combustion chamber area is clean.

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