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

Have you tried bypassing the thermastat?

yes, put jumper wire across TS.

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.

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.

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

Furnace Short Cycles Off and On | Good Sam Community

rogerck
Explorer
Jan 20, 2017
I need to remove the furnace, inspect the blower side and bench test it. It seems the chamber (temp prob near limit switch) runs too hot. I can't see any reason on the air output side to cause this. I've run the furnace with duct work detached so air flow is not restricted. Much appreciate all your input.
Charlie_D_
Explorer
Jan 20, 2017
I recently had an issue with my heater fan starting, burner lit for a few minutes, heat from the ducts and then flame would go out, fan ran a few minutes, flame would relight and continue. I checked everything and finally removed the burner. Found a dirt dauber on the end. Once cleaned everything returned to normal. I am assuming there is no bugs of wasp nest anywhere in the system?
time2roll
Nomad
Jan 21, 2017
Mine cycled the burner due to inadequate and poorly installed ducting causing an air flow restriction.

If you have basically bench tested the furnace and it still overheats the firebox I have to speculate internally restricted airflow or the fan/motor is not performing properly. If it is old I would just replace. Check the component warranty if it is not too old.
pirlbeck
Explorer
Jan 21, 2017
Is the blower fan a squirrel cage style? If so, be sure the squirrel cage fan is installed properly. If the cage is installed wrong or the motor rotation is backwards they will still move air, but not nearly as much as they should. I work on farm equipment and all the heating/AC systems use squirrel cage fans and I have seen this problem a LOT of times over the years. What would appear correct by looking at the cage may not be the correct way.

Do a Goggle search for "squirrel cage fan rotation" and start reading.

Here is one comment I found.

"A squirrel cage running backwards produces about 25% of the air flow it should when turning the proper direction."
dougrainer
Nomad
Jan 21, 2017
Also, make SURE the whole house inpellor has ALL the vanes intact. I had this problem a few years ago with a customer. The Impellor had broken 1/2 the vanes and did NOT run unbalanced. But, when you pull the outer door or inner cover you should see the remnants of the broken vanes. Doug
rstanek
Explorer
Jan 21, 2017
I had this happen to me, I reworked the duct work to increase airflow, problem solved., I eliminated 5 90s and 8 feet of duct work. Huge difference, furnace hasn't recycled since.
rogerck
Explorer
Jan 31, 2017
Problem Solved. I finally had a chance to remove the furnace and the problem was very obvious. A round sheet metal cover was wedged directly in front of the blower, substantially reducing the air flow. This is very sloppy on the part of Keystone for not running the furnace and detecting that the air flow was way down. The original owner used the trailer in warm weather and never picked up that the furnace was under performing.

Thank you all for the input. Rogerck.
trailrider
Explorer
Feb 01, 2017
Pictures? What is "directly in front of the blower?" Output side or intake side? I assume you mean between the blower housing output and the heat exchanger.
rogerck
Explorer
Feb 01, 2017
A blank, unused hole plug ( four inch round) for the outlet air distribution openings. This item was wedged directly in between the blower outlet and heat exchange where you've drawn the blue outline. Sloppy assembly. You wouldn't see it if not looking for it.
dougrainer
Nomad
Feb 01, 2017
rogerck wrote:
A blank, unused hole plug ( four inch round) for the outlet air distribution openings. This item was wedged directly in between the blower outlet and heat exchange where you've drawn the blue outline. Sloppy assembly. You wouldn't see it if not looking for it.

What they did was, they knocked out the metal duct hole cover and just let it fall inside. Yes, sloppy install. I do not understand why you did not see it from inside where the hi limit switch is and looking over the combustion chamber to the blower. Unless you were not sure what a normal furnace would look like. Doug

Forum Discussion

Furnace Short Cycles Off and On

About Technical Issues

Related Content

Short

Short?

Furnace

2 cycle generators

Short Term Winterizing?