Stuck draft cap assembly of Atwood Furnace

[Chef Guy]

Tried posting this on the RV Appliances section with little response. Hopefully I will get some good info here.

Been trying to troubleshoot both of my Atwood 8500 series furnaces as both have given me the lockout codes (3 quickly flashing lights followed by a pause) and the next step is getting to the electrodes and gas burner assemblies sitting behind the gas valve to check them out.

One problem: the outside draft cap assembly (aka exhaust tube)which sits inside of another tube at the back of the gas valve is proving to be a SOB to remove. I suspect rust has set in place. The one wing nut has been removed and I have tried PB Blaster over several days and a lot of wiggling and waggling with no luck, along with a fair amount of brute force and gently tapping at the join with a long screw driver.

Heat may help, but I cannot get heat that far back in the area without torching some wires and rubber grommets along the side.

Anyone else had this problem and what did it take to remove the draft cap assembly?

Thanks for any usefull tips, as usual

ps, it will not help to remove the furnace from the MH as the area I need to access is completely enclosed by sheet metal.

[MrMark52]

Several YouTube videos showing removal of the burner for that furnace.

If you have the wing nut loosened than it is very likely the draft cap tube is rusted into the heat exchanger.

You might try a 50/50 mix of transmission fluid and acetone. I’ve found that to be better than PBlaster.

I couldn’t tell how much of an overlap there is between the two tubes or exactly how deep into the unit the “seam” of the two tubes overlapping starts - but it looks pretty deep - I’m not real sure the PBlaster with the straw will reach where you need to get to.

And your right, there is no external or rear access to the burner. It’s all accessible from the front.

[Chef Guy]

Mr Mark,

Thanks for the reply...after 4 days of PB Blaster, a lot of wiggling and finally my 1800 watt heat gun fastened to a 1/2" piece of long pipe to reach into the small area, I was able to extricate the piece. Rust was everywhere and deep AND the bottom of the rusted piece of pipe that I took out was bent at 90 degrees inside of itself! Must have been that way from the factory...as if someone had stepped onto the end of it with their steel toe boots, decided to throw it in anyways and the furnace must have passed the check test 'cause it worked for a bit years ago (this is the rear furnace which does not see a lot of use, and it quit years ago).

Ordered a new draft cap assembly for about $50 CDN ($32 USD) and I have taken out a ton of rust! I also noticed while I took out the gas valve, that the electrodes were way out of adjustment both to themselves and to the heater element, so I hope after adjusting them and installing the new draft cap assembly that she will be good to go for awhile longer...if not, then on to more troubleshooting! Nothing to get bored about working on these legacy coaches.

[MrMark52]

With that amount of rust - you might watch closely for holes in the heat exchanger. Problem is, no easy way to see the heat exchanger well enough, and the holes usually start our as pin holes.

You might try plugging one end of the heat exchanger tube and then applying some air pressure to see if it bleeds off.

With holes in the heat exchanger - it allows carbon monoxide to enter the coach. Besides the fact that these are pressurized heat exchangers. And you never know when one of those pin holes all of the sudden flakes into a 1/4” hole.

[Old-Biscuit]

Electrodes out of adjustment......s/b 1/8" spark gap

Also clean up those electrodes with some emery cloth to remove any soot/carbon buildup (inhibits the flame proving milliamp return signal)

[Chef Guy]

Originally Posted by Old-Biscuit

Electrodes out of adjustment......s/b 1/8" spark gap

Also clean up those electrodes with some emery cloth to remove any soot/carbon buildup (inhibits the flame proving milliamp return signal)

Done!