Atwood 8520 furnace will not start

[StephenHale]

I have a 2011 Fleetwood terra with an Atwood 8520 furnace that will not start.
The circuit board gives one flash in three seconds indicating air flow/limit fault.
When turning the thermostat to gas I hear the converter box come on for a few seconds but nothing else. The air intake and exhaust vents are clear. On several tries turning the thermostat to gas, the on/off switch in furnace compartment would flip to off and the the thermostat would go dead. Turning the switch to on in the furnace box turns the thermostat back on.
I have two furnaces, so switched circuit boards which showed the circuit board is working and not the problem.
The limit and sail switches are not visible in the furnace box or on outside of the box. Looks like there is a short somewhere to me.
Anyone have ideas of what the problem is?

Thanks

[shiggs68]

When you let it run as normal, does the fan run for about 30 seconds then shutdown. If so, that is an indication that the sail switch did not see enough air flow to continue to the ignition process. Dust around the sail switch, bad switch, or restricted registers will cause this.

You can jumper across the sail switch wires to prove it is not working. The furnace should then ignite.


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[StephenHale]

The fan never comes on. The sail switch must be closed.

[G Schulz]

I had a similar problem with my coach. My furnace would try to light but wouldn't. The problem turned out to be a gas flow problem. When my slide was in the propane line kinked and would not allow full gas flow. Got rid of the kinking problem and all is well.


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[shiggs68]

Originally Posted by StephenHale

The fan never comes on. The sail switch must be closed.

If the fan never comes on, it has nothing to do with the sail switch open or closed. You need to check the connections to the fan and if voltage is being applied to the fan. No voltage, the board or fan relay on the board are bad. If voltage, then the fan motor is bad.



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[StephenHale]

I solved the problem by using my air compressor to blow air into both ends of the shroud surrounding the motor. This allowed the paddle on the sail switch to open from being in a stuck closed position. Not sure if the sail switch was in the combustion blower or blower on other side of the motor.
Atwood has a trouble shooting guide for "when the fan plus the gas ignition doesn't occur" , but doesn't mention this solution.
The furnace works perfect now.

Thanks to those who tried to help!