Atwood 8531 furnace blower cycling when shut off

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Russ+Chris

Russ+Chris

Well-known member
Joined
Sep 4, 2016
Posts
268
Location
Brownsville, Pa.
Hi all.
Have a '09 Fleetwood Discovery 40X, with the Atwood 8531 furnace. The front furnace runs fine. The problem is the blower cycles after the furnace shut down. The blower is to run for about 2 minutes then shut off. It don't. It will cycle not stop, until the furnace is called on again to run. Then the blower will run like it is programmed to. So far, I have replaced the circuit board, and the limit switch. Traced the wiring. Don't tell me it's the circuit boards. I' have tried them in my rear furnace and they work fine. When the furnace is running. No fault light. When shut down. 1 flash every 3 seconds. Air Flow/Limit. I removed the duct work from the rear of the furnace housing. No difference. I'm open for answers. Here's a video. The blower is quite. The gopro makes it noisy.

https://youtu.be/8-psyXja1HQ
 
Does the bad furnace have a two speed blower?  If so, the circuit board switches the blower to low speed during the cool-down cycle and if the sail switch is defective it may be signalling that the fan has failed, causing the board to return to high speed.

Just a shot in the dark ... there isn't much else in the circuit other than the board.
 
Single speed blower. This all started at last camping trip. Before we left. All was good. Get to camp and set up. Turn on hot water. Both electric and gas to heat the water up faster. No hot water. Played around with everything. Finally found a blown 20 amp fuse. Replaced that. Got hot water. At the same time. The front furnace blower starts up, and is making all kinds of noise. Had to take the side cover off and turn off the switch mounted on the blower housing. The 12v control power for both the hot water and furnace is on the same fuse. Looking around the blower area. Seen plastic pieces. The blower fan cage had exploded. Plastic pieces everywhere on that side of the furnace. Got home. Installed new blower fan cage and sail switch. The old one was beat up. Now the blower cycles. Replace the circuit board, and limit switch. Same thing. Blower cycles. Tried all the parts in the rear furnace. Every is fine.
 
Is there a hi-temp switch that would cause the fan to turn on to cool the unit down?  Not sure if they have them but a possibility.
 
The 3-second flash is your clue - the burner remains off and the fan runs as long as the controller thinks it is too hot.  Usually the problem is either constricted outflow through the ducts or inadequate air intake. The intake is usually just openings in the furnace case, so can get clogged with dust bunnies or maybe some stored gear that gets in the way.

Poor combustion can also make it overheat, and that is harder to diagnose. Combustion air is drawn in from outside by the fan and exhaust leaves thru an adjacent tube. Blockage in either one can disrupt the air/fuel ratio and result in a variety of problems. So can a gasket leak between or around the combustion air path.  Low or high gas pressure (not 11 W.C. inches) can have a similar effect.  Since your other furnace works ok, presumably the LP gas regulator is fine and gas pressure is probably not the issue.
 
I can go out to the motorhome right now. And turn the switch on at the blower housing and the blower will start to cycle. The furnace hasn't been on since yesterday. The furnace is cold.

Is there a separate timer delay/relay that is not on the circuit board?
 
Russ+Chris said:
Hi all.
Have a '09 Fleetwood Discovery 40X, with the Atwood 8531 furnace. The front furnace runs fine. The problem is the blower cycles after the furnace shut down. The blower is to run for about 2 minutes then shut off. It don't. It will cycle not stop, until the furnace is called on again to run. Then the blower will run like it is programmed to. So far, I have replaced the circuit board, and the limit switch. Traced the wiring. Don't tell me it's the circuit boards. I' have tried them in my rear furnace and they work fine. When the furnace is running. No fault light. When shut down. 1 flash every 3 seconds. Air Flow/Limit. I removed the duct work from the rear of the furnace housing. No difference. I'm open for answers. Here's a video. The blower is quite. The gopro makes it noisy.

https://youtu.be/8-psyXja1HQ
Click to expand...

Russ+Chris
Sounds like the AIR FLOW TO and/or FROM that furnace is INSUFFICIENT , (aka: restricted/blocked).
Are both the cold air return and the the hot air ducts clear... and the heat registers open.

Russ+Chris said:
I can go out to the motorhome right now. And turn the switch on at the blower housing and the blower will start to cycle. The furnace hasn't been on since yesterday. The furnace is cold.
Is there a separate timer delay/relay that is not on the circuit board?
Click to expand...

However because it cycles when the furnace is cold I suspect the problem lies in the thermostat... (or in the wiring).
 
I can go out to the motorhome right now. And turn the switch on at the blower housing and the blower will start to cycle. The furnace hasn't been on since yesterday. The furnace is cold.
Click to expand...

What switch is that?  The one that turns all furnace power on/off? If the fan comes on whenever there is 12v power to the furnace, that is a major clue. Basically the fan runs whenever the thermostat calls for heat, so +12v on the thermostat wire into the furnace triggers it. Not sure why/how it would cycle off, though, or why the furnace wouldn't go ahead and light after 15-20 seconds of the fan start.

I'm wondering if the wiring harness or the connectors to the circuit board are damaged or corroded. A shorted pin on the connection can produce all sorts of weird behavior. Inspect closely and clean with a pencil eraser.

Is there a separate timer delay/relay that is not on the circuit board?
Click to expand...
No, not in the models made since about 2000 or so. The separate fan delay relay was eliminated when the electronic control board was introduced.
 
I shot a video of the furnace. In the video, you can see what switch I shut off. Also you can see that I removed the duct work so I wouldn't have a flow restriction. The blower starts to cycles everytime I turn the switch on.

Here's the video.

https://youtu.be/8-psyXja1HQ
 
That's a weird pulsing of the motor. Not what I was imagining when you said "fan cycling".

The switch is the main power I referred to earlier - it removes all 12v power to the furnace. Naturally, the fan cannot run without it being on.

Find the thermostat wire to the furnace board - voltage there triggers the fan to run. Disconnect it to see if that changes anything. If it does, look for any DC or AC voltage on that line. Nominally the t-stat triggers the furnace with +12v there, but the furnace isn't all that fussy about the voltage and might start on most anything.

In most installations the duct work handles only the outflow from the furnace and the circulation air intake is direct thru the furnace casing (no ducts). If yours is like that, verify that the intake is also clear. It can't blow air out if it cannot also suck it in the other side.  However, that's not the issue here since the fan cycles even when cold.
 
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