Forum Discussion

cbr46's avatar
cbr46
Explorer
Nov 13, 2019

Atwood Furnace Problems

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
Class A
  • dougrainer's avatar
    dougrainer
    Nomad
    Nov 15, 2019
    cbr46 wrote:
    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


    Use the Screens that are sold for your model furnace. The disclaimer in the manual is a standard CYA. They are worried that the INTAKE could pull in outside debris that then clogs the screen and restricts the fresh air intake, causing a very rich black soot problem. As long as you monitor the screens for this type problem, use them. I always tell my customers that use the screens---IF you have a warranty period malfunction, just remove them before you bring the unit in. As to screens for refer doors, DO NOT USE THEM. They do 2 things bad
    1. They restrict the fresh air intake required to adequately cool the refer by almost 50%.
    2. Depending on how well you install and how the rain water runs down the sidewall, they can direct gallons of water into the back of the refer floor and then inside the RV. I have seen this 3 times in the past 10 years. Customer comes in complaining about lots of water flowing under the refer cabinet in heavy rain and flooding the interior kitchen area. It was those screens installed on the outer refer doors. the small lip catches the water and directs it inside. Doug
  • cbr46's avatar
    cbr46
    Explorer
    Nov 14, 2019
    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
  • cbr46's avatar
    cbr46
    Explorer
    Nov 14, 2019
    Job done. Those little critters were looking for anywhere to hide.

    The original nests . . . .


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


    These came out of the supply tube today.


    And a closer look . . . .


    Now we're all hot and toasty again.

    Thanks for pointing me in the right direction!

    Best,
    - bob
  • cbr46's avatar
    cbr46
    Explorer
    Nov 13, 2019
    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
  • A1RVTraveler's avatar
    A1RVTraveler
    Explorer
    Nov 13, 2019
    Chris Bryant wrote:
    Remove the orifice and clean that short manifold.


    This is what you need to do to solve your problem.
  • dougrainer's avatar
    dougrainer
    Nomad
    Nov 13, 2019
    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
  • opnspaces's avatar
    opnspaces
    Navigator II
    Nov 13, 2019
    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.
  • cbr46's avatar
    cbr46
    Explorer
    Nov 13, 2019
    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's avatar
    opnspaces
    Navigator II
    Nov 13, 2019
    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?

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