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HighSpeedAlumin's avatar
Apr 10, 2016

Furnace problems

My furnace has never worked since I bought it new. so far we have spent over $750 to get it working and it has been reimbursed by the manufacturer, but I doubt they will offer to spend much more. We have had it serviced by three different technicians and I want to try to get it fixed so I can heat the holding tanks...every time I call Atwood they say to "do this" without hearing all the details and so their just guessing. So here goes.

It's an Atwood AFMD35 heater without the exterior door.
The fan works normal and you can hear ignition but the flame goes out 2-50 seconds after ignition every time. It will relight after a few seconds and the process repeats.
The ignition cycle never times out. It will continue to light and flame out indefinitely.
Circuit board has been replaced.
The propane line works well, as I have a buddy heater downline of the furnace propane supply and it works fine. (I don't use the buddy heater when trying to get the furnace lit.)
When the furnace was bench tested, it worked fine. When it was reinstalled, he put aluminum tape around the ducting insert into the furnace (at Atwoods suggestion) and it would stay lit on the second or third attempt. But has never been reliable at sea level altitudes. And won't start at all at 7000ft.

I suspect a intake/exhaust ducting problem. I just don't know where to start. It just seems like it can never get enough air. Any ideas?
  • It's not the regulator, we already had that replaced and now the buddy heater works downline of the furnace.

    I read another article on the sail switch. I need to go and reread it. Would that work if it was on a bench and then not work if it was in the rv for some reason? Like If the exhaust ducting was not aligned properly?
  • "I suspect a intake/exhaust ducting problem. I just don't know where to start. It just seems like it can never get enough air. Any ideas?

    If this is the problem, it is most likely affecting the operation of the sail switch. There is an airflow sensor in the ducting path that closes when the blower is operating. If the blower stops, or the switch is not closed, the flame will go out. This is an obvious and necessary safety feature. It is called a sail switch because there is a "sail" or flat piece that catches the wind and actuates the switch. If you are correct, there is just barely enough breeze to maintain the switch. You can verify your theory by finding the switch and temporarily jumping it out. If the flame stays on, you have found the problem that the techs missed. Now you have to figure out how to fix it.

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