Dear Dave,
My daughter just purchased this fifth wheel. When we turn the heat on it runs for a while then triggers the airflow/limit fault. I don’t know what to look for as the cause. I was thinking a dirty filter, but I can’t find one. —Ricky, 2020 Coachmen Brookstone
Dear Ricky,
According to the spec’s I can find online, the Brookstone has a 40,000 btu automatic furnace but I cannot find the make. The two most popular are Suburban and Atwood/Dometic, and both work similarly.
I have posted several articles in the past on RV furnaces. However, since colder weather is coming it is a good time to revisit the furnace, especially since you are actually getting a fault code. I assume this is displaying as blinking lights on the circuit board?
How the furnace works
The thermostat is set to a desired temperature. When the ambient temperature inside the rig reaches that, the thermostat “closes.” That sends 12-volt power to the control module at the furnace. The fan motor starts, which draws outside air in the vent and circulates that air through the burner assembly to purge any lingering fumes.
The fan also draws inside air through the cold air return. This air is blown over the burner chamber past what is called a sail switch. This switch must be lifted high enough to verify there is enough airflow, at which time it also provides a “closed” electrical signal sent to the module board. That opens the LP valve and provides a spark to ignite the LP/air mixture and start the furnace.
If the furnace does light initially, as you indicated, it most likely is an issue with the sail switch. The sail switch can be affected by several issues. First, is the unit plugged in to shoreline power or are you dry camping when this happens? The furnace is powered by the house batteries. If you are dry camping, the batteries could be sulfated and have an initial surface charge that would start the furnace, but then the voltage drops fast and the sail switch drops, shutting off the gas. Try plugging the unit into shoreline power which would provide at least 13.2 volts from the converter or other charger to the batteries so there is constant power.
Check the heater vents and hoses
Check all the vents inside the rig to ensure that nothing is covering a vent or the hoses under the bed, sofa, or cabinets are not kinked. This would cause restricted airflow which might allow and then not allow the sail switch to raise to a closed condition. It might even cause an overheating condition, since the hot air cannot flow to the vents and the temp or limit switch shuts it off.
If the above conditions do not fix the issue, then you will need to check the sail switch. It is typically located behind the squirrel cage. I would first inspect and clean it as dust, hair, and other items could prevent it from getting high enough. It could also be bent just enough to cause an issue or simply be weak. Once you have access to the sail switch, it’s an easy replacement.
In rare occasions, the airflow is restricted coming out of the furnace to the plenum due to a design flaw or improper installation with a misalignment of the plenum* or other obstruction. In this case, you would need to contact customer service at Coachmen to assist in getting it fixed, depending on your DIY skills. [*From Merriam-Webster: plenum: an air-filled space in a structure, especially one that receives air from a blower for distribution (as in a ventilation system).]
You might also enjoy this from Dave
Why does my RV’s furnace only ignite when I lift the sail switch?
Dear Dave,
I have determined that my RV’s furnace blower airflow is not sufficient to activate the sail switch. The batteries are new and charged, but it will only ignite when the sail switch is depressed manually. There appears to be sufficient air blowing out of vents. I can’t see any blockages in lines or hoses, and there does not appear to be much dust on impeller blades. I am stuck and would appreciate any guidance. Thanks in advance. —Anthony, 2016 Bigfoot 25FB
Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
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Thanks for the refresher Dave! Another issue we found is the wires were corroded on the sail switch. After properly cleaning them and using a good electronic cleaner/spray, it worked fine. A suggestion. I understand what most things are on your pics. However, if you could use a red pointer or circle the item in question, it would be greatly received and appreciated. Thanks for being here on RVT to keep us old guy’s on the road!
Thank you, Dave!
Looking for a filter? You’ve got to be kidding. They cost money.
my neighbor said his furnace was getting noisy and it needed a filter changed. I told him there was none. He didn’t believe me and I had to remove the return air grill to prove it
maybe the amish should stick to something they understand like farming. That one picture of the missing plug (vent from furnace) a LOT or heat is going out but not where it is needed. And for extra cost they could use insulated ducting. Keep heat in the pipe till it reaches the output grill.