Atwood 8520 Furnace Won't Start

[boguslaw]

I looked for similar topics in the forum but nothing addresses my issue exactly. I have a 2007 RV with two Atwood 8520 furnaces. The front one won't run. I get the single blinking trouble light which means low air flow. The blower does not start, hence the low air flow. I put 12v directly to the blower and it runs fine. I checked voltage to the unit and get 13.5v and I switched the control board with the rear heater that works fine and no change( the rear still works, the front still doesn't). Since I get the single blinking trouble light after turning it on at the thermostat, I assume the thermostat circuit is working. I did hear a noise once during startup, like the blower was maybe trying to turn but when I went outside and tried to duplicate the noise(by removing the cover and listening while my wife turned on the unit inside), it won't start at all, so I'm not sure of the noise.

I might suspect the sail switch or the high temp limit switch but I don't get far enough in the start cycle for those to be the problem. There just aren't any other parts and very little wiring to know what else to look at. Any ideas?

Jim Boguslaw

2007 Country Coach Tribute

[hermanmullins]

Jim,

What has Atwood said? They should be your best source of information.

Herman

[boguslaw]

They said to check the voltage. I'm not sure where they meant to do that so I checked the power coming to the board and it was 13.5 v. I intended to check it at the motor during the start cycle, before I got the low air flow light but it read zero.

[boguslaw]

Not much interest in this one but I did call Atwood who suggested it was the sail switch. I bought another and installed it and it worked for a while. When it wouldn't start, I started messing with the wiring (the only thing left) and it is working now. I believe it was a ground wire in the safety circuit that has both the sail switch (which verifies air flow) and the high temp shutdown.

[wildebill308]

Glad you found your problem.

Merry Christmas and a Happy New Year.

Bill

[millersocialnutrition]

I’m having the same problem. What did you do to fix?   

[wolfe10]

8 hours ago, millersocialnutrition said:

I’m having the same problem. What did you do to fix?   

Please be aware that this thread is 2 years old.

[dickandlois]

12 hours ago, millersocialnutrition said:

I’m having the same problem. What did you do to fix?   

What is the Model, Year and make of your coach?

The sail switches do go bad, The standard sequence for the 8200 series start up.

         1- The blower starts up -THE sail switch MUST make contact!!  There is a delay of 4 seconds ,the delay is built into the TDR(time delay relay) then the control board enables the ignition / spark

          2-  Timing circuit built in the control board then energizes the LP valve. for about 6 seconds and is also timed from the time that the sail switch closes.

           3- If the gas valve does not open in a few seconds,(12 volts is supplied through the sail switch) the board stops triggering the ignition spark. The Blower will continue to run with no heat.

           4- you need to turn off the furnace - The blower will run for about 1 - 1/2 min. and then the blower stops(this delay is built into the TRD to purge any LP from the furnace.

            5- The TDR's do fail. They can be purchased from places like Dinosaur.com - cost is around $35.00  . Replacement control boards now have the TDR built on the boards, but one needs to do some rewiring between the board and the original TDR location. Proper wiring is needed so the TDR is not damaged. And the replacement board price is around 30% more. $180.00 range.

The sail switch is inside the blower assembly and is difficult to get to - Testing the switch is also difficult. No real good access to the wire terminals.

Not what I would consider a DYI project for owners.  

Hope this helps.

Rich.

Note, not all sail switches are created equal. Bought some from a parts supplier that failed in lees then a year. Most likely made across the big pond and no real quality control. Just a cheep price ! 

[bm02tj]

Is this like newer house furnaces that will not start if proof of fan is closed before it starts as protection to the safety being jumped or shorted 

[dickandlois]

22 minutes ago, bm02tj said:

Is this like newer house furnaces that will not start if proof of fan is closed before it starts as protection to the safety being jumped or shorted 

Kind of ! The older RV furnace control boards are much simpler then the newer high efficiency residential units.

The  system makes sure that the burner is not lit if not enough air is flowing through to close the sail switch.

To much heat in one spot can start a fire.   

Rich.  

[Jquerry]

Having somewhat the same kind of problem, except my blower comes on the gas comes on and the gas ignites but after about 5 to 10 seconds the gas shuts off and the blower continues to run until the exhaust cools down.  I have two furnaces and the other furnace works just find. 

Any suggestions?

[abyrd]

Sounds like the thermocouple is not recognizing that burner has ignited.  I would check that it is not just dirty also check for any loose wires.  On most units the thermocouple and igniter are one unit.

 

Jim

[tbutler]

The sail switch on our furnace is easily accessible.  It is in the fan housing mounted on the side toward the outside opening.  I think it was four screws to remove the cover from the fan and the switch was right on that cover.  The coach isn't in the driveway right now so I can't give model and year information.  Ours failed once and I had it replaced by a repair shop.  The second time I removed it myself.  I was showing Louise how it worked, flipping the switch, and a ball of fuzz fell from the switch housing.  I put the same switch back in the furnace and it is still working today, something like ten years later.

Depending on your furnace, it might be a user serviceable or replaceable part.