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Old 07-16-2016, 06:29 PM   #11
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Hmmm, hubby seems to think that the RV tech mentioned a lock-out (old age memory for both of us). I did a search and found this article ( which mentions a lock-out.

"Lockout & Reset
Newer furnaces may go into "lock-out" if a potential safety problem is detected. Lock-out simply means that the Control and/or ignition circuits shut down and do not allow furnace operation. Lock-out can be reset by turning the thermostat switch off and changing the Set Temperature to a low enough value so there is no demand for heat and waiting 10-15 seconds. Then turn the thermostat on and set the temperature to the desired level and the furnace should attempt to start again."

Sorry we can't remember exactly what he did ....



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Old 07-16-2016, 07:29 PM   #12
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For those of you who may encounter a furnace problem in the future, here's a good basic description from one of my favorite RV tech sites, of the components and issues that can cause it to stop working.

The furnace is designed to operate at voltages between 10.5 to 13.5 VDC. Low voltage will not run the blower motor at the proper speed to commence the ignition sequence.

Return air is the air that flows in to replace the heated air that the blower pushes out through the ducting. This air is pulled in by the furnace through louvered openings in the side of the furnace cabinet.

The exhaust vent (outside) must be clear of all obstructions for proper furnace operation. Inspect the vents for insect or bird nests or other debris.

Time delay relay
This relay performs two separate jobs - one to handle the relatively high current needed to run the blower motor - and two, to allow the blower to run for 45 to 90 seconds after the thermostat is satisfied. This allows excess heat in the chamber to dissipate before the blower stops.

The relay is normally open and should always have power from the circuit breaker. Only when power from the thermostat is present does the relay close after a 20 second delay. Power then flows to the blower motor.

Next in the ignition sequence is the blower motor. It drives two squirrel cage fans to provide separate air flow for the combustion process and for distributing the heated air to the coach. A heat exchanger is used to separate the heat from the burning gas while preventing exhaust gases from entering the living space.

Sail Switch
The sail switch is an on/off device. (normally in the off position) It gets it's name from the "sail" or paddle that is attached to the switch mechanism. As the blower comes up to speed, it blows air onto the sail with enough force to push the switch closed, thus allowing electrical current to flow to the next component in line.

It's job is to determine if there is adequate air flow for proper combustion to take place. If the battery voltage is low or the fan does not come up to 75 per cent of it's design speed, the sail switch will not close. Possible causes of this malfunction are: low battery, restricted return air inflow, restricted or inadequate outlet vents, restricted combustion air inlet or exhaust, or a faulty sail switch.

Limit Switch
The limit switch is a simple temperature controlled switch. It's function is to monitor the combustion chamber heat level. If the temperature of the combustion chamber exceeds the preset limit, the switch will open and disrupt the flow of current to the circuit board, in effect, shutting down the main burner. Once the chamber temperature cools sufficiently, the limit switch resets. This initializes the ignition sequence and starts another cycle.

Circuit board
The circuit board will not receive power until the sail switch is closed by adequate air flow from the blower. The power must also flow through the limit switch. When the circuit board is triggered by this current, it delays ignition for about 15 seconds to allow the blower to purge the combustion chamber of any unburned gases. The circuit board then sends high voltage pulses to the electrode assembly, providing a series of sparks to ignite the gas/air mixture. At the same time, the circuit board sends power to open the gas valves allowing the fuel to flow to the burner.

Flame Sensor
The circuit board monitors the burn cycle through a sensor that detects the presence of the flame. If the sensor does not detect a satisfactory flame within about 10 seconds, the board then shuts off the gas valves and discontinues the ignition spark. All electrical connections should be clean and tight. Misfire problems can often be solved by simply cleaning and inspecting every electrical connection, especially the sensor/electrode wire. The sensor sends a microamp reading to the circuit board when the flame is burning. Any impedance to this tiny amperage flow will cause the board to shut things down.

Depending on the board design, it will try for ignition up to three times. Beyond that point, it goes into lockout mode, will not retry for ignition until reset and the blower will continue to run.

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Old 07-16-2016, 11:04 PM   #13
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Originally Posted by rbryan4 View Post
Depending on the board design, it will try for ignition up to three times. Beyond that point, it goes into lockout mode, will not retry for ignition until reset and the blower will continue to run.
Yup, nothing like waking up at 2AM on a mountainside ski resort during an ice storm with the furnace blowing cold air and concluding you HAVE to drive down the mountain to a motel. AND put on the chains.............
Charlie Y

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Old 07-17-2016, 11:58 AM   #14
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We have the Atwood 7900. The sail switch failed after a couple of years. The furnace actually has internal diagnostics when it fails. If you remove the cover, there is a flashing LED after a failure to start. From the manual:

Internal Circuit Board Failure: Steady on, no flashing
Limit switch/airflow problems: 1 flash with 3-second pause
Flame Sense Fault: 2 flashes with 3 second pause
Ignition Lockout Fault: 3 flashes with 3 second pause

When our sail switch failed, the furnace diagnostic was 1 flash (with 3 second pause). It's a (relatively) simple fix.
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Old 07-29-2016, 06:07 PM   #15
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After waiting for an extra day for the sail switch to come in we were already on the road and had to go back to Bandon to pick it up. Thank goodness Humbug Mt. S.P. is not that far away.

Today (in the California Redwoods...and another campground away) we finally had time for Craig to address the malfunctioning furnace.

I am happy to report that after wrestling with the unit itself the sail switch was replaced in a matter of minutes. More wrestling to get the unit reseated property et voila'....we have heat again!

Thank you all for your helpful replies to our distress signal and to Reese for his troubleshooting over the phone.

Josie and Craig

"Not all those who wander are lost." (J.R.R. Tolkein from Lord of the Rings..."Strider's Poem")
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Old 04-11-2018, 03:04 PM   #16
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We have had our 2014 19' for one year now and love it except for one problem. The Atwood furnace fails to light. It failed on our first trip, and since that failure, it has worked only three times, and only for a short period each time, always after a service person checked it out for us. One RV place in Oregon put the circuit board on his test machine and pronounced it ok. Another place started diagnosing voltage and found inconsistencies. Like when pulling 12 volt fuse labeled furnace, the blower motor still runs. one of the LED lamps flickers. The indicator for Grey water tank and black water tank are reversed. I pulled the furnace out today, and removed the sail switch. The switch test fine with an ohm meter. the sail arm is clean and it moves. I have read hours of posts about this problem and at a point of frustration. I'm wondering how much more money do I sink into this
Atwood product. Looking at Propex.....
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Old 04-11-2018, 03:58 PM   #17
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Scot, the high limit (sensor) is on the same circuit.
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Old 04-11-2018, 09:49 PM   #18
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I'm not sure what that means......?
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Old 04-11-2018, 10:45 PM   #19
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Originally Posted by BCnomad View Post
Scot, the high limit (sensor) is on the same circuit.
Originally Posted by Scotty...B View Post
I'm not sure what that means......?

He means there is a high limit switch inline with the sail switch. See schematic on page 4. If the furnace is still on the bench you could jumper it to see if it resolves the issue.
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Old 04-11-2018, 10:51 PM   #20
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Just dont leave the jumper if it does fire up of course. Did that with a gas spa heater 20 years ago and almost had a fire.

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