Furnace Short-Cycling

[agiannetti]

2018 Furnace - Atwood/Dometic model AFSD20141 is short-cycling.

I tried the following everything:
Jumped the thermostat
jumped the high temp limit switch
jumped the sail switch
removed discharge vents to assure no blockages
checked return air for obstructions
made sure battery fully charged
(I have NOT yet checked the gas pressure)

I turned the main gas valve to almost closed to lower gas pressure (as evidenced by reduced stove flame) - that corrected the furnace for short-cycling, but only for one day.

MAJOR CLUE TO THE ISSUE: Without fail, when I remove the exterior panel and vent tube, it works fine. The INSTANT I start to replace the panel, the burner shuts off. Is it possible/probable, when I start to put the cover on, the change in airflow disrupts the flame under the electrode and causes the burner to shut off? Would the absence of a strong flame immediately shut off the burner?

[BFL13]

agiannetti wrote:
Dometic's response to the purpose of the piece inside the vent tube:
"There is no direct purpose for the piece, it just keeps the unit from water back flow issues."

It ran ok without that piece, and now you know it is safe to go without it.

"Water back flow issues" might be when you wash your RV and stick the hose into the furnace vent? So don't do that. 🙂

[wilson612000]

if the problem comes back hook a meter to the power going to the fan or as close as you can low voltage due to a bad ground or weak power to the fan will cause the fan to slow down and shut things down. I had one I had to run a new wire fused to the unit to get it to keep running

[agiannetti]

Dometic's response to the purpose of the piece inside the vent tube:
"There is no direct purpose for the piece, it just keeps the unit from water back flow issues."

[agiannetti]

I pulled out the orifice. It looked clean and clear, no noticeable debris. I blew out the inside of the valve with an air compressor. Put it back together and it worked fine (just like it had every other time I've messed with it, only for the problem to return). I tested it for 2 consecutive days and it's still working OK. I have not received an answer from Dometic as to the purpose of the piece inside the exhaust pipe vent. Doug's understanding that it is to prevent water intrusion makes sense.

[LVJJJ]

Junk the Atwood, buy a Suburban. I help repair all the park model owners around me in a gated RV park, ALWAYS have lots of problems with Atwood, plus they sound like a jet engine when igniting. The one park model I've NEVER had trouble with has a Suburban furnace.

[dougrainer]

BLF13, The 2nd link refers to water intrusion. Atwood/Dometic has a Recall retrofit kit to move the Control module up off the floor of the furnace to prevent corrosion and shorting out of the Control board. Also, I have never asked, but my understanding of the slope metal on some furnace exhaust vents and what is called a restrictor(round type washer) on other is to prevent water intrusion. It keeps rain from sliding down the tube and getting in the combustion chamber. Doug

[dougrainer]

I just remembered-----Dometic/Atwood has a recall/TSB for defective Sail Switch and mount on some models. Not specifically for this problem, but it may be that the sail switch is closing and opening causing the short cycling. Since you have talked to Dometic, send them the model and serial if you have not done so, and see if this model is under that TSB. Doug

[BFL13]

EDIT--Have you new propane since owning the rig? If not could bad fuel be the cause?
-----------
This diagram seems to show a number of vents to choose, but is incomplete on info. Must be a better link for the particular furnace model?

https://pdxrvwholesale.com/products/atwood-hydroflame-furnace-model-afsd20141-tune-up-kit

This is about a different furnace, but might be relevant. Was mention of the gas valve being out of place, and also since air seems to be involved:

"I'm told the inverted louvers are intentionally designed that way to prevent the furnace from sucking in hot 'exhaust' (i.e. CO) air from the chrome 'chimney' below into the furnace's fresh air. This could extinguish the furnace flame."

(Mostly nothing to do with OP's otherwise)

https://www.forestriverforums.com/forums/f113/new-atwood-furnaces-afmd-series-79088.html

[agiannetti]

No. No, will do tonight.

[wnjj]

agiannetti wrote:

dougrainer wrote:
The difference in the restrictor may make a difference. BUT, Has the furnace operated correctly prior to this problem? If so, then it should have malfunctioned from the beginning if the restrictor is wrong. Doug

Good point. I bought the RV used two months ago, did a quick test of the furnace at that time, but not long enough to notice short cycling.

Was there any evidence the previous owner replaced the exhaust, possibly with the wrong one? Have you looked at the sticker on the back side?

[agiannetti]

dougrainer wrote:
The difference in the restrictor may make a difference. BUT, Has the furnace operated correctly prior to this problem? If so, then it should have malfunctioned from the beginning if the restrictor is wrong. Doug

Good point. I bought the RV used two months ago, did a quick test of the furnace at that time, but not long enough to notice short cycling.

[dougrainer]

The difference in the restrictor may make a difference. BUT, Has the furnace operated correctly prior to this problem? If so, then it should have malfunctioned from the beginning if the restrictor is wrong. Doug

[agiannetti]

Just received interesting information from Dometic (see below). There are TWO, almost identical vent tubes. However, their restrictors are different sizes (the one I have is more restrictive than the other). Perhaps I have the wrong one. I have asked about the purpose of the restrictors, waiting for response.

[agiannetti]

A1RVTraveler wrote:
I do not see where you indicated you removed the small LP orifice from the end of the gas valve and cleaned it.

No I haven't. I have pulled the furnace and inspected the burner, but didn't inspect the orifice. I will inspect the orifice, that is my last hope.