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Open Roads Forum  >  Class A Motorhomes  >  Maintenance Issues & Tips

 > Atwood Furnace Problems

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cbr46

Flowery Branch, GA

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Posted: 11/13/19 09:02am Link  |  Quote  |  Print  |  Notify Moderator

I posted this in an existing post a few days ago in Tech Issues forum. No one is responding so (forgive me) I'm also posting here.
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29966083.cfm

My Atwood SF-35 furnace is 17 yrs old. Some surface rust but overall pretty clean.

Fan comes on, a minute or so later gas is turned on and I can hear the igniter frantically clicking (as it should).
I faintly smell propane coming out the furnace exhaust. FAINTLY. I'm not sure how strong of a smell it should be. The smell comes & goes.
Sometimes the furnace will light but then goes out. I can hear a 'woof' or two . . . or multiples.
Sometimes it will run for a few minutes but never reaches a good outlet temp. Maybe 90F. Normal is 120F+.
Other gas appliances work as they should. Batt voltage is 12V+

There were muddobber nests inside the flame chamber. They were cleaned out. Otherwise the inside was pretty clean.

The burner was removed and although it looked clean I ran a quick brass wire brush on it (by hand). I also inserted a rubber coated electrical wire into the gas jet. It went in easily letting me think the jet wasn't clogged. I didn't remove the jet to look for rust inside the tube. The igniter tip was cleaned and is 1/8" from burner (spec).

So where from here?

#1 I don't want to spend $90 for a control board I may not need. Yes, I'll get a Dinosaur board if I replace.
#2 The furnace isn't easy to disassemble - the supply tube nut is so tight I have to completely disassemble the fan shroud to get the tube out. Laborious!

Any experience with gas valves quitting . . . . or rust inside the supply tube (clogging the gas jet)?

Can I apply 12V to the gas valve to actuate it? For testing ONLY!

It's cold out! Fortunately I don't have a trip planned for a week or two.

Thanks,
- bob

opnspaces

San Diego Ca

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Posted: 11/13/19 09:08am Link  |  Quote  |  Print  |  Notify Moderator

The woof makes me think you still have a spider web somewhere in the gas chamber. Are you getting black soot on the outside of the exhaust vent?


1996 Suburban 4x4. 350, 4.10 3/4 ton
2005 Jayco Jay Flight 27BH
1986 Coleman Columbia Popup.

Chris Bryant

Arden, North Carolina

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Posted: 11/13/19 09:17am Link  |  Quote  |  Print  |  Notify Moderator

Remove the orifice and clean that short manifold.


-- Chris Bryant
My RV Service Blog
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cbr46

Flowery Branch, GA

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Posted: 11/13/19 09:19am Link  |  Quote  |  Print  |  Notify Moderator

No webs, gas chamber had the muddobbers and even though it wasn't clogged is now clean.

Exhaust tube is clean and blows hard - no soot inside the tube. If anything the mixture is lean (hence "woofing out").

Tnx,
- bob

opnspaces

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Posted: 11/13/19 12:09pm Link  |  Quote  |  Print  |  Notify Moderator

True, but lean mixture means not enough propane for the amount of air. Since you made no changes to the furnace then the logical suspect is a continued blockage causing less propane than normal hence the lean mixture.

I will be the first to admit that I sometimes jump straight to the weird obscure reason why something is not correct and ignore the simple causes. But more often than not the simple reason is what prevails. I would go with what Chris Bryant suggests since he's been repairing RV's for many years.

dougrainer

Carrolton, Texas

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Posted: 11/13/19 01:17pm Link  |  Quote  |  Print  |  Notify Moderator

On a 17 year old furnace that has Mud nests. It is the ones you CANNOT see in the burner tubes that may be the problem. I would remove the furnace and the motor and circuit board and run water thru the intake and exhaust tubes to flush out the burner chamber. You may be surprised to see what comes out. I also use a rubber mallot to gently tap the burner chamber to dislodge any nests that are hidden when running water thru. DO NOT use that 1/8 gap as gospel. A 3/16 gap is usually a better gap for spark and flame sense. ALWAYS remove the orifice over a white paper or cloth and tap on the Gas valve. debris can be inside and the force of the lp gas and flip debris up and partially clog the orifice. The white cloth lets you see anything that comes out. Doug

A1RVTraveler

North Carolina

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Posted: 11/13/19 04:04pm Link  |  Quote  |  Print  |  Notify Moderator

Chris Bryant wrote:

Remove the orifice and clean that short manifold.


This is what you need to do to solve your problem.

cbr46

Flowery Branch, GA

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Posted: 11/13/19 04:18pm Link  |  Quote  |  Print  |  Notify Moderator

And over a white piece of paper! Workbench is a white smooth surface . . . . :-)

At least no one is jumping on replacing the control board, and all the other stuff is free. If it is the board then it's relatively easy to replace later.

Many thanks,
- bob

cbr46

Flowery Branch, GA

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Posted: 11/14/19 03:04pm Link  |  Quote  |  Print  |  Notify Moderator

Job done. Those little critters were looking for anywhere to hide.

The original nests . . . .
[image]

Dumped them out. This was a few weeks ago. Thought that was the only problem . . . . NOPE!
[image]

These came out of the supply tube today.
[image]

And a closer look . . . .
[image]

Now we're all hot and toasty again.

Thanks for pointing me in the right direction!

Best,
- bob

cbr46

Flowery Branch, GA

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Posted: 11/14/19 03:10pm Link  |  Quote  |  Print  |  Notify Moderator

Some people put screens over the outside ports. Manual says not to.

What say you and what size screen?

If nothing else I'll cap the ports over the summer. Tricky part will be not turning it on . . . .

Best,
- bob

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