Furnace troubleshooting

[davidmurphy02]

I've been through several problems with the furnace in our 2019 5.0TA and in the process have learned a lot about how the furnace works, troubleshooting steps, and found some good resource information so I thought I'd share that here for others who may have furnace problems down the road.

Sometime in 2018 Escape started using a furnace with a large exterior door that allows easy access to most of the components. Prior to that many repairs required removing the furnace from inside the trailer, so if you have a later model trailer with the outside door consider yourself lucky.

If your furnace doesn't work - runs briefly and then stops - the first thing to do is to check the flashing light on the circuit board. That means removing the four screws holding the exterior door in place. The circuit board will be visible and will have an LED flashing - either repeating one, two or three flashes or staying on constantly. Nearby on the black plastic blower motor housing there will be a sticker with the trouble codes that tells you what the different flashes mean, and that will give you an indication of what the problem is.

When the furnace is activated by the thermostat, first the blower turns on. The circuit board gets a signal from both the sail switch and high limit switch that all is well, this takes about 15 seconds. Then it opens the gas valve and activates the igniter to light the burner. If the burner doesn't light within 5 seconds it stops and the blower will continue to run to blow out any unburned propane. 15 seconds later it will try the ignition process again. It will make three attempts at ignition, and if all three fail it will shut down.

The easiest way to quickly identify the nature of a furnace problem is to turn on the furnace and then run outside and listen at the exhaust vent. If there is a sail switch problem the blower will run for 30 seconds or so then stop and you won't hear the igniter fire. If the sail switch is OK, you'll hear a rapid clicking noise as the igniter fires. When the burner lights it makes a noticiable whooshing sound and you'll feel hot air coming out the exhaust. If the burner doesn't light, it will make two more attempts at ignition when you'll hear the clicking sound of the igniter.

If your blower only runs 30 seconds or so and the igniter never clicks and fires, you probably have a sail switch problem. The sail switch has a spring loaded arm with a small flat metal "sail". When the blower is running, it moves the sail switch and closes a contact point which tells the circuit board that the air flow is OK. Without that signal the furnace will not attempt to ignite. People have often reported finding hair, fuzz, and other debris in the sail switch that may have prevented it from closing and that by removing it and blowing it off and cleaning it, it began working again. However, it's common when that happens to have that recur and you can wind up cleaning the sail switch frequently. My opinion, based only on my own experience, is that the sail switch is a poorly manufactured component that is subject to a high failure rate. After cleaning mine several times I replaced it with a new one which has worked flawlessly without intervention for almost two years now. So my recommendation is to purchase and carry a spare sail switch. If your sail switch malfunctions while on the road, it's worth removing and cleaning it, often that will get it working again for a while.

In the photo below the screwdriver is pointing at the sail switch. It mounts on a black plastic block and the long part of the switch goes inside the blower motor housing in the airflow. Above the sail switch is the circuit board - you can see the LED sticking up near the lower right corner, this is what will flash repeatedly with an error code if you have a problem. And directly below the sail switch is the sticker that tells you what the error codes indicate.

[davidmurphy02]

There is a high limit switch in the same circuit as the sail switch. Both of these are safety devices - the sail switch makes sure there is good airflow when the burner is burning, and the high limit switch is designed to shut down the furnace if for any reason the internal temperature gets too hot. The high limit switch is something that very rarely fails, but is also a possibility if you are getting an air flow error and the sail switch is OK. It's a round plate about the size of a nickel with two wire connectors and is mounted on the wall of the furnace directly under the gas valve / burner. It is normally closed and passes voltage through, and above a certain temperature (in this case 190 degrees) it opens and breaks the circuit. If you suspect a high limit switch problem and the burner has not been running, a quick way to test it would be to disconnect the two wires and connect them together. If the furnace works after that, your high limit switch is bad. In the photo below the high limit switch is black with two wires and is above the exhaust pipe and below the gas valve assembly on the back wall.

There are a couple of other things worth checking when your furnace won't ignite or run. If your batteries are low (assuming you're not on shore power), the low voltage may be preventing it from working, or may be causing the blower motor to run too slowly to activate the sail switch. If you have been having any issues with sputtering flames on your other propane appliances it's worth having your propane pressure checked. You can have this done at a repair shop or purchase an inexpensive manometer, usually $50 or less, that will allow you to check it yourself - there are threads on the forum about using a manometer and it is not difficult.

[davidmurphy02]

If, while listening at the furnace exhaust, you can hear the igniter clicking rapidly but the burner does not ignite the problem is likely a bad gas valve. If this is happening you probably got three flashes on the circuit board indicating an ignition problem. This is not a repair that everyone would want to attempt, but for DIY inclined folks it's not really too bad. The gas valve, igniter and burner are one unit that come out together. The gas line comes in from above and has enough flexibility to push it up and off once the fitting attaching it to the gas valve has been loosened. There are a couple of wire connections to remove, and then just three screws fasten it in place on the furnace wall. You'll need a long bit extension to access those screws. I also found it helpful to remove the exhaust tube below the gas valve in order to have a little more room to maneuver the valve assembly around when removing and installing it. There is a small brass orifice on the end of the gas valve - check this for any lint or other obstructions. If you are replacing the gas valve the new one will not come with an orifice, you can transfer the one from your old valve to the new one. I've included photos below showing the gas valve / burner / igniter assembly and a shot of the furnace compartment with that assembly removed.

If you do decide to delve into replacing your gas valve, here is a YouTube video from an RV tech that I found very helpful:

https://www.youtube.com/watch?v=elpPdk_MSbU

[davidmurphy02]

The current furnace installed over the last several years is the Dometic DFSAD12131. Older models may have an A rather than a D for the first letter, indicating that they were made when Atwood was still a separate company before being bought and consolidated into Dometic. If your furnace is older the model number and serial number should be on a tag on the blower motor housing or possibly another location depending on the model. The photo below explains what the various letters and numbers in the model number mean and will tell you about the specifications of your furnace.

In my research I came across an Atwood Furnace Training Manual designed to help service techs diagnose and repair furnaces. It's long as it includes several different models of furnace, but it has a wealth of information about exactly how the furnace works, troubleshooting, parts diagrams, etc. http://techsupport.pdxrvwholesale.co...anual-2015.pdf

[davidmurphy02]

One additional note - some people have thought that another possible restriction of airflow that could prevent the blower from operating properly is the duct and louvers that direct the warm air inside the trailer. These furnaces are fairly low powered - an advantage in that they use less electricity to operate the blower - but in some circumstances if the air flow is restricted may not have the power to blow strong enough to operate properly. Some owners have replaced the flexible corrugated vent, which is curved and offset in some cases, with a smooth metal vent that allows easier air flow. I tried this in our furnace and while it did not fix my problem it does seem to blow air noticeably stronger into the trailer. If you are having air flow problems and a new sail switch does not correct them, this may be worth a try.

[Tin]

Nice work David, your valve is fine. One of your solenoid has failed, happens to gas clothes dryers all the time, just replace them there cheap.

[davidmurphy02]

Originally Posted by Tin

Nice work David, your valve is fine. One of your solenoid has failed, happens to gas clothes dryers all the time, just replace them there cheap.

Thanks Tin. So many things are not repairable these days - I knew that it was possible for the solenoids to fail but didn't find them as a separate part and since the whole valve wasn't too expensive ($69) just replaced the whole thing. I don't have enough of working knowledge of valves and solenoids to be able to see what was going on with it.

There are two solenoids on the valve. Do you know if they both do the same thing and are redundant, or do they have different functions?

[Perry Butler]

This thread should be a sticky!

Thanks David!

Perry

[Tin]

Originally Posted by davidmurphy02

There are two solenoids on the valve. Do you know if they both do the same thing and are redundant, or do they have different functions?

Each solenoid activates a valve that opens allowing gas to pass through.
usually a main and secondary valve. When one fails best to replace both.

[EscapeBoulder]

I’ve got some different symptoms on my 2018 19’ers furnace: The fan comes on, and the burner fires for maybe 10 - 30 seconds (I didn’t time it). Then the burner shuts off, fan still running. It cycles like this way maybe a half dozen times, then completely shuts down, fan and all. The LED light came on maybe once while I was watching it, and only for the briefest second. Then is off the whole time. And the LED being off isn’t listed on the sticker. Might this be the board being bad?

And I did change out the sail switch with a spare I have.

Thank in advance from cool to cold Grand Canyon Nat’l Park in coolish April.

[davidmurphy02]

Originally Posted by EscapeBoulder

I’ve got some different symptoms on my 2018 19’ers furnace: The fan comes on, and the burner fires for maybe 10 - 30 seconds (I didn’t time it). Then the burner shuts off, fan still running. It cycles like this way maybe a half dozen times, then completely shuts down, fan and all. The LED light came on maybe once while I was watching it, and only for the briefest second. Then is off the whole time. And the LED being off isn’t listed on the sticker. Might this be the board being bad?

And I did change out the sail switch with a spare I have.

Thank in advance from cool to cold Grand Canyon Nat’l Park in coolish April.

Sounds like a bad control board is the most likely culprit. Get the Dinosaur Fan 50+ board if you can, higher quality than the OEM board.

[Viajante]

Originally Posted by EscapeBoulder

I’ve got some different symptoms on my 2018 19’ers furnace: The fan comes on, and the burner fires for maybe 10 - 30 seconds (I didn’t time it). Then the burner shuts off, fan still running. It cycles like this way maybe a half dozen times, then completely shuts down, fan and all. The LED light came on maybe once while I was watching it, and only for the briefest second. Then is off the whole time. And the LED being off isn’t listed on the sticker. Might this be the board being bad?

And I did change out the sail switch with a spare I have.

Thank in advance from cool to cold Grand Canyon Nat’l Park in coolish April.

You might inspect the flame sensor for corrosion. It sounds like it is lighting ok but doesn’t sense that it is lit and shuts down.

[davidmurphy02]

Originally Posted by Viajante

You might inspect the flame sensor for corrosion. It sounds like it is lighting ok but doesn’t sense that it is lit and shuts down.

The flame sensor on these furnaces is also the igniter, it does both functions. If the burner is lighting then the igniter/flame sensor is probably OK. Could be slightly out of adjustment but you have to disconnect the gas line and remove the whole burner assembly to get at it. If it were me I'd still try replacing the board first, that is a much easier job, only a few wires to disconnect.

[Viajante]

Originally Posted by davidmurphy02

The flame sensor on these furnaces is also the igniter, it does both functions. If the burner is lighting then the igniter/flame sensor is probably OK. Could be slightly out of adjustment but you have to disconnect the gas line and remove the whole burner assembly to get at it. If it were me I'd still try replacing the board first, that is a much easier job, only a few wires to disconnect.

I suggest this because my water heater had the same type of problem—lighting but not staying lit. Based on advice from this forum I cleaned the igniter/sensor and it solved the problem. The water heater burner was easily accessible from the outside, unlike the furnace.

[davidmurphy02]

Originally Posted by Viajante

I suggest this because my water heater had the same type of problem—lighting but not staying lit. Based on advice from this forum I cleaned the igniter/sensor and it solved the problem. The water heater burner was easily accessible from the outside, unlike the furnace.

That makes sense. The water heater burner is a lot like the fridge burner, small and more susceptible to clogging up. And easy to access and clean. The furnace burner is massive by comparison, less likely to go wrong but as we all know pretty much anything can malfunction.

[EscapeBoulder]

Thank you for the info, gentlemen. I ordered a replacement board. I’ll report the outcome for people who follow this great thread.

[EscapeBoulder]

BTW... This is a great video if you want to understand the basics of how our RV furnaces work. It also discusses a specific problem with the burner, but really covers the basics for those of us who are interested.

https://www.youtube.com/watch?v=8sZdA8wnlrw

[EscapeBoulder]

Epilogue: Yes. In my case it was a board gone bad. Who knows why, but Reace pointed out in his sail switch video that the board occasionally also goes kaput.

[bdornbush]

When I tried to test my furnace before a recent trip, the fuse blew after about 3 seconds or so. I called a mobile repairman, and he diagnosed it as a bad blower motor. He was right. We ordered a replacement motor and he installed it yesterday. The furnace now works again.

I recall that there were a few times where the fan motor seemed to slow down and speed up and I speculate that it was starting to fail.

[sameo416]

Thanks David for the excellent overview. Just fixed our first sail switch issue after waking up in a 10C RV.