What Is an RV Furnace Sail Switch?

A propane furnace is a fantastic thing to have in an RV – as long as it’s working. When it’s working properly, you turn up the thermostat and a few seconds later, heat is moving through the heating ducts and warming your RV’s living space, just like the furnace in any home. But, between the turning up of the thermostat and the heat flowing through the RV a series of events occurs that not only creates heat for your rig but also protects you from any harm that could come from the combination of a flame and some propane. This includes two safety switches – the sail switch, and the high-limit switch. In today’s post, we’re taking a closer look at the RV furnace sail switch – what it does, how it works, how you’ll know if it needs cleaning or replacement, and how to do it.

Let’s go!

How Does an RV Propane Furnace Work?

An RV propane furnace is a forced-hot-air system that requires a thermostat, a control board, a relay, a blower motor, sail switch, high-level switch, ignitor, and some ductwork.

To activate the system, you set the thermostat higher than the RV’s current temperature, which activates the blower motor. A propane burner generates heat, and air drawn in from the interior of the RV is blown past a heat exchanging surface. As it passes by that surface it becomes heated and is then distributed throughout the RV through the ducts.

12V electricity is required to turn on the furnace and to operate the fans.

The exhaust gases created by the combustion process are vented to the outside of the RV through vents that look something like this:

What Is a Sail Switch?

The sail switch is the first of two safety switches in the system. It exists to prevent the furnace from even trying to ignite if the blower fan can’t run at the proper speed. This can happen under certain circumstances, including if your RV’s house battery bank is discharged too low to run the fan at full speed.

The sail switch senses when the blower motor’s fan is moving air at the proper speed. If the switch does NOT sense proper fan speed, the furnace won’t fire. And that’s a good thing!

Your sail switch is an important part of your RV propane furnace because it tells you that something isn’t right, AND it doesn’t allow the flow of propane (required for the furnace to light) under circumstances that aren’t optimal. If the sail switch doesn’t sense that the blower fan is running at the proper speed, the propane valve won’t open.

It’s important to know that your RV furnace has to confirm two things before it can ignite. The first is that the blower fan is running at the proper speed, and this proof is made by the sail switch.

The second thing your furnace has to determine in order to ignite is that its high-temperature limit switch, (sometimes referred to as the “furnace plenum thermostat”), hasn’t been tripped. This safety switch will cut the propane supply if the burner temp exceeds safe levels.

Essentially, 12V electricity will not make it to the control circuit to ignite the furnace unless both of these switches respond appropriately when the furnace is turned on.

Where Is the Sail Switch Located on an RV Furnace?

The sail switch is located in the duct where it overrides the gas valve of the burner. If you have an access panel for your RV’s furnace giving you access from the outside wall of the RV, the sail switch is easiest to access and remove for cleaning or replacement.

The following wiring diagram gives you an overview of the location of the sail switch and all other components of an RV furnace, as well as the path the circuit takes to drive the heat. This diagram happens to be for an Atwood/Dometic furnace, but placement should be similar on a Suburban furnace.

This is what a sail switch looks like – as you can see, the sail switches for Atwood, Suburban, and Dometic RV furnaces are all very similar:

If you need to replace the sail switch in your RV furnace, be sure to consult your owner’s manual to make sure you’re ordering the appropriate switch for your furnace.

For additional information on how to troubleshoot your RV furnace if it isn’t putting out any heat, please see our post, “My RV Furnace Fan Runs But There’s No Heat! Now, What?”

How Do I Know If My RV Furnace Sail Switch is Bad?

If you turn up your thermostat and you hear the furnace turn on, go through its initial 15-second startup phase, and then either stop running altogether or continue blowing only cold air, you’ll have confirmed that it has attempted to light but hasn’t been successful in getting past the sail switch.

This can happen if your RV’s house batteries are too weak to spin the blower motor fast enough to create sufficient airflow for the sail switch to activate. This causes the furnace to blow cold air.

There’s also a system of fault lights on the control board (Ignition Control Diagnostics) to assist with troubleshooting. Your owner’s manual will give you the fault codes for your particular furnace, but in general, if the blower comes on and about 30 seconds later turns off on its own, and you see a single flash, it’s very likely that the sail switch is either dirty or needs to be replaced.

How Do I Test My RV Furnace Sail Switch?

You can test the sail switch with a multimeter set to ohms.

You’ll want to close/trigger the sail switch by hand, and then attach the red and black leads of your multimeter to each of the two connections on the switch. The multimeter will read “OL” (open line) while the sail switch is open. Close it by hand and then take your reading. If your sail switch is good, then with the circuit closed you’ll have a reading of between 0.0 – 0.2 ohms.

Remember that the sail switch is essentially an opening in the electrical circuit. In order to allow current to pass through it, the switch needs to close to complete the circuit. So, that’s what you’re doing when you close the circuit by hand in order to test it.

Can I Clean My RV Furnace Sail Switch?

Yes, you can. Dust, animal hair, bug nests, or other debris can collect on a sail switch making it incapable of doing its job. If you find you have a dirty sail switch, cleaning it may be all you need to get your furnace working again. However, it’s not a bad idea to keep an extra sail switch on hand in case yours needs to be replaced, particularly if you use your RV in very cold weather.

Can I Replace My RV Furnace Sail Switch Myself?

Yes, you can! Once you’ve located and removed the sail switch, it’s not difficult to replace it. As always, we suggest taking a photo before removing any wires so that you’ve got something to fall back on if you need it.

Though not related to the function of the sail switch, you may also want to consider the easy installation of screens on the furnace vents on the outside of your RV to prevent things like mud daubers from building nests, etc., and dust, dirt, and debris from collecting. They can really make a world of difference.

Our vents take these covers, but you can also buy a set of a variety of screen covers for your rig.

Have You Ever Replaced Your RV Furnace’s Sail Switch?

So tell us – have you ever found yourself with a failed RV furnace sail switch that required either cleaning or replacement? We’d love to hear about your experience.

And again – if you’re in a situation where your furnace is running but isn’t providing any heat, check out this post where we cover a variety of troubleshooting issues for RV furnaces.

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