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?

its your propane regulator is bad replace it this problem will go away

"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.

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?

Each appliance has its own copper supply line which connects to a manifold under and outside the RV. Look for a low point in the copper line to the furnace that could be an oil trap blocking gas flow to the furnace.If it is an oil trap it may/will be intermittent depending on a changing viscosity with temperature changes.

Have you completely bypassed the thermostat?

Furnace problems | Good Sam Community

13 Replies

larry_cad
Explorer II
Apr 13, 2016
Wow! Thanks for sharing that information. I wonder how many others there are out there with the same problem.
Old-Biscuit
Explorer III
Apr 13, 2016
wa8yxm wrote:
Toddupton wrote:
Have you completely bypassed the thermostat?

His post indicates that is NOT the problem

To the O/P. I am assuming it tries to fire 3 times then shuts down. If so check the curcuit board for a LED that should be flashing 3 times

I do not now the particulars of YOUR furnace but if you are in S.E. Ga I'm 3 miles off I-95 and can take a look at it.. I had a problem like that with mine (Mine was the circuit board) and learned far more than I wanted to about RV furnaces tribble shooting..

Here is a bit of info:
There are basically two types.
One has a single "probe" that is both the flame sensor and one of the ignition points.. IF it is lighting the ignition part is working

The other has two probes, one ignition (often the gap is too wide) and one Flame Sensor.

If you have the first, Circuit board problems are a major feature of that design..... but there is a cure.

Tad late to the party..

OP posted EARLY this AM what issue was and that it was FIXED :S
wa8yxm
Explorer III
Apr 13, 2016
Toddupton wrote:
Have you completely bypassed the thermostat?

His post indicates that is NOT the problem

To the O/P. I am assuming it tries to fire 3 times then shuts down. If so check the curcuit board for a LED that should be flashing 3 times

I do not now the particulars of YOUR furnace but if you are in S.E. Ga I'm 3 miles off I-95 and can take a look at it.. I had a problem like that with mine (Mine was the circuit board) and learned far more than I wanted to about RV furnaces tribble shooting..

Here is a bit of info:
There are basically two types.
One has a single "probe" that is both the flame sensor and one of the ignition points.. IF it is lighting the ignition part is working

The other has two probes, one ignition (often the gap is too wide) and one Flame Sensor.

If you have the first, Circuit board problems are a major feature of that design..... but there is a cure.
HighSpeedAlumin
Explorer
Apr 13, 2016
And after posting my last post, I was informed that he just used a rig system to realign the Thermocouple into the flame. So there you have it, Old Biscuit nailed it. Thanks guys!
HighSpeedAlumin
Explorer
Apr 13, 2016
A quick update. Thanks to all that posted, And especially Newman for the phone help.

Apparently Atwood is having a lot of teething problems on their new heater design. Mine was a gas valve problem. I had three different RV professionals look at my RV, called and talked to Atwood 3 different times and the RV techs called as well and every time Atwood was called, we got a different answer!

I removed the heater myself and took it to Camping world where the technician had just gone to Heater refresher school. He had it working in 5 minutes. The hardest part was connecting the wires to supply power. All he did was bend the gas valve to provide proper gas flow to the unit.
Mandalay_Parr
Explorer
Apr 10, 2016
Checking the sail switch by jumping is a good idea.
Also look for air flow restrictions.
Make sure good 12 volt supply so motor runs fast.
Might get a good electrical person to help you.
I think there is an over temp switch too.
larry_cad
Explorer II
Apr 10, 2016
HighSpeedAluminum wrote:
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?

In a word, "yes". A restricted air flow path will cause the airflow switch to either not work at all, or be intermittent. It is even possible that the sail switch is not installed correctly (at the factory) or may even be bent. Take a close look at this picture. You will see the switch, and the flat piece of metal that is the sail. It is designed to catch the airflow. If it is bent or not installed correctly, it will not work properly.
Old-Biscuit
Explorer III
Apr 10, 2016
Spark electrode provides 'Flame Proving'

Circuit board is loosing that 'flame proving' signal so it drops DC to gas valve
But t-stat set point hasn't been meet so furnace refires

Spark electrode is not being continually engulfed by main flame....hence loss of 'flame proving' signal.
Need to check spark electrode position oer burner slots, spark gap 1/8", good clean/tight grd connection and no 'tracking' to grd thru ceramic insulator

Sail switch is working or main flame/ignition would never occur
Toddupton
Explorer
Apr 10, 2016
Have you completely bypassed the thermostat?
Harvard
Explorer
Apr 10, 2016
Each appliance has its own copper supply line which connects to a manifold under and outside the RV. Look for a low point in the copper line to the furnace that could be an oil trap blocking gas flow to the furnace.

If it is an oil trap it may/will be intermittent depending on a changing viscosity with temperature changes.