Furnace short cycling

[Flyfishingbc]

Hi everyone,

First time posting but long time reader of these forums. Down to the brass tax here, I have a suburban sf 25 that cycles on the limit switch. I have not bench tested unit yet or checked WC pressure. I know there is not enough vents attached to furnace as the build quality on these RV's are sometimes well below par. If front cover of furnace is on it cycles on limit switch about every 3 to 4 minutes. With cover on the temp out of closest vent (about 3ft away) registers at about 166 degrees when limit switch activates. It runs the cool down cycle then fires back up and repeats process. With all vents off unit and front cover off, the burner chamber directly behind limit switch gets close to 170 then cycles as usual. If I jumper the limit switch the furnace will run non stop. However the temps of burner chamber are 230 after 12min of running with an exhaust temp of over 400. I have replaced limit switch twice so I know it is working correctly but I believe the furnace is being over fired and cannot dispel heat out fast enough. I will WC check my lp pressure to make sure it's at 11 ish but other then that I am at a loss. I checked the ohms of the dual solenoid gas valve on the furnace itself and I'm right at 38 so I am in the specs of 30 to 50 on both sides. Is there a way to adjust flame via the gas valve on the furnace itself? My heating cooling system is run via remote on a carrier V system. Can I remove the furnace from the carrier units thermostat via the wires spliced at back of furnace and splice in a digital wall mount one with no I'll effects? Any input is greatly appreciated. I will follow up with info after I check lp pressure.
Thank you.

[Flyfishingbc]

UPDATE, from factory this suburban SF25 had 3 ducts, 2x4" and 1x2". Way to go on factory build quality. I have the furnace fully installed and running the way it should. I installed 3x4" ducts that come out of kitchen cabinets with about 2 to 3 foot of ducting. Changed the limit switch to the sf25 model prior to a build date if 1992. They ran 175 degree and after 92 they are 160's. The build diagrams between those years look to be identical. They also didnt have the exhaust and intake ports properly fitted into the outside sleeves and hot air was pumping back into the air space around furnace. Cranked the heat to 90 degrees, opended the doors and ran furnace for 1hr straight. Temp at vents was 140 degrees compared to the 167 i was getting before. After 1hr running I can still hold my hand on any part if the furnace shell. No more limit switch cycle, temps are good. Housing is running alot cooler. Thx for the input. I'm calling this one fixed. May switch to a wall mount thermostat with less variance as I find the carrier V has too much temperature swing from what it's set too before furnace kicks back on.

Cheers.

[doughere]

ADD MORE VENTS. Any less than having all the vents called for by the manufacturer is dangerous.

Doug

[time2roll]

How many ducts do you have? I believe the minimum is 36 sq-in or 3x 4" ducts. That is the minimum, best to have at least 4 ducts.

[mchero]

wgriswold wrote:
Here is an extensive (150 replies) discussion of your problem.

https://www.nroa2003.com/forum/forum/message-board/troubleshooting/295882-furnace-problems-you-may-have

The solution is to add more vents.

Need to subscribe to read the post

[Flyfishingbc]

Update,
Pulled furnace again and bench tested with the shell on. Furnace short cycles every 3 to 4 min. Now the strange part is if I pull the housing off the furnace it ran for 20min without short cycling once. Not sure where to go with this one now. Could it be the blower motor is not pushing enough rpms to move air fast enough to cool housing? Did I end up with a limit switch that has too low of trip temperature? All the blades are clean, fully charged battery and a WC of 11. More ducts does not seem to be my issue. With temp probe on the burner chamber by the limit switch it trips at around 167. Fan blades are all clean of any debris/dust.

Cheers.
Cheers

[Lwiddis]

I’ve paid my brass tax faithfully.

[wgriswold]

Here is an extensive (150 replies) discussion of your problem.

https://www.nroa2003.com/forum/forum/message-board/troubleshooting/295882-furnace-problems-you-may-have

The solution is to add more vents.