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Old 12-16-2022, 04:40 PM   #1
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Atwood Furnace 'Long' Short Cycle Problem Fixed

My Atwood 8940iii DCLP furnace would blow, then ignite like it's supposed to, but would only 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. 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 hose going to 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 it 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. The problem was the OSB board the furnace was setting on had an 11 1/2-inch x 6 1/2-inch opening for the floor duct. There was aluminum duct tape on the short side of the plenum, but the rear of the plenum had 1 inch gap. I removed the furnace and discovered the plenum opening for the floor duct is only 10 inches x 5 inches! I shimmed up the floor duct hole to 10" x 5" and reinstalled the furnace. Now the furnace performs as it should!
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.
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Old 12-16-2022, 05:05 PM   #2
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Well done! And thanks for posting. Someone is going to be happy to find a solution to their similar issue down the road.
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Old 12-17-2022, 12:00 AM   #3
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That is information that can't be had at any price. Thankyou.
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