Atwood Furnace 'Long' Short Cycle Problem Fixed

[MiKeMo]

My Atwood 8940iii DCLP furnace would blow, then ignite like it's supposed to, and would blow hot air for approximately 4-5 minutes when first started, depending on the outside temperature. Then, the flame would go out and blower runs for approximately 1 minute, then ignition would occur, and hot air flows for approximately 2 minutes, then flame would go out and the blower would continue to blow for another minute or so then it would ignite again. That's the way my furnace cycled. There was very little air flow from the floor registers, but good air flow from the register under the sink. This is how my furnace operated from day one of my purchase. Due to the age of the MH-(2005), I installed a new sail switch, high-limit switch, and electrode. The burner was good as new. My furnace continued to have a 'long' short cycle. I blew out the floor ducting with an air hose and only seen a little bit of sawdust. The duct work was not restricted. The register under the sink was fed from a 4” hose coming from the plenum/heat exchanger housing. I used an infrared thermometer and the air around the furnace-(area under the refrigerator)-varied from 156 degrees to 175 degrees depending on where the beam was pointing. I knew then that air was escaping from the plenum into the furnace bay area instead of being directed to the floor ducting. The air inside the furnace bay area was too hot and was causing the high limit switch to work overtime. I removed one of the 4" plates and shined a light around the heat exchanger, then on the 'back' side of the plenum and I could see light in the floor duct below the heat exchanger! This was not supposed to be so, I removed the furnace and discovered the plenum opening for the floor duct is only 10 inches x 5 inches! The problem was the OSB board the furnace was setting on had an 11 1/2-inch x 6 1/2-inch opening thus allowing hot air to escape the floor ducting into the furnace bay area. There was aluminum duct tape on the short side of the plenum, but the rear of the plenum had 1 inch gap which was 11 ½” long. I shimmed up the floor duct hole to 10" x 5" and reinstalled the furnace. Now the furnace performs as it should! The burner only shuts off when the thermostat setting has been reached.
Note: I have spent hours on the forums and did not find any info regarding a 'long' short cycle problem, but it's very possible I missed it. Sorry for rambling on but wanted to add this info in hopes it helps someone looking for a ‘long’ short cycle solution.

[BradHend]

Great find Michael.  
I too will be investigating a furnace duct leakage on my ‘05 this winter hopefully.  
My furnace has very similar symptoms and only the flexible duct hose that runs to the dinette feet area has good air movement.  Bedroom, bath and the two other main vents have next to no air movement.  I have sent an inspection camera in the ducts and they are clear, next is as you did with the thermal camera.  I have a descent one that connects to Apple devices.  There’s gotta be a leak or questionable connection somewhere around the furnace.    

 

[CaymanXL]

I found a similar situation on our 2007 Cayman. the floor was HOT just outside the louvered vent under the fridge. Not much coming out of heat registers. Upon inspection t he furnace had a huge gap where it should have been flush/sealed to the ducting! I fabed up some wooden shims and duct taped until I have a chance to remove furnace and fix properly.

[MyronTruex]

I just found my bedroom ducts have very little air flow compared to the front heater. We have owned the rv since new and know this is not normal. The heat coming out does not seem to be as high either.

Normally that heater only needs to run a few minutes and then you can bake cookies in the small space of the bedroom. So I guess I will have to do some checking when it warms up a bit in the next few days. It does not seem to be cycling heat on and off, just low flow and not as hot as normal.

I suspect a duct has popped loose. Fortunately it is easy enough to get to.

Thanks for the diagnosis. That gives me some encouragement.