Furnace Short Cycles Off and On

[rogerck]

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

[dougrainer]

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

[rogerck]

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.

[trailrider]

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]

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.

[rstanek]

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.

[dougrainer]

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

[pirlbeck]

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."

[time2roll]

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.

[Charlie_D_]

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?

[rogerck]

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.

[DrewE]

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]

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.

[jplante4]

Is there a filter somewhere?

[dougrainer]

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