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What Is an RV Furnace Sail Switch?

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:

The furnace vents on the exterior of our Newmar Mountain Aire.

These are the Suburban furnace vents on the exterior of our Newmar Mountain Aire. If you have an Atwood or Dometic RV furnace, your vents make look a bit different.

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.

The wiring schematic for an Atwood/Dometic RV furnace. (Diagram and photo credit: Atwood/Dometic)

The diagram offers an overview of the 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. (Diagram and photo credits: Atwood/Dometic)

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:

Atwood 36680 Sail Switch
  • BUILT FOR RV - Sail switch specifically designed for use with RV's and Campers.
  • FULLY GUARANTEED - Includes manufacturer warranty.
Suburban 232261 Sail Switch
  • Product type : AUTO ACCESSORY
  • package height : 11.0 cm

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.

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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.

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JCJ M-300 Mud Dauber Screen for RV Furnace Outside Fitting
  • RV furnace and fan unit outside fittings
  • Fits DuoTherm and Suburban
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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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Steve Lamb

Tuesday 22nd of February 2022

Ok this will be easy for you, because I think these may be dumb questions: I have two zones in my RV, front and back each with it's own thermostat and temperature sensors. Does that mean I have two furnaces? Also you said the furnace uses the batteries to run, but if you are plugged into shore power are the batteries essentially taken out of the equation? Thanks fellas ~ Steve

TheRVgeeks

Tuesday 22nd of February 2022

Hey Steve! Great question. A propane furnace will have a very distinctive rectangular vent outlet on the outside of the RV. If you have two of those, you have two furnaces. If you're not sure you're looking at the furnace outlet, try turning on the furnace, and you'll hear it blowing quite loudly when you stand next to it outside your rig. If you have another identical outlet anywhere else on the RV, that's a second furnace.

Another "tell" is whether both thermostats have a heat setting. If one can only be set to cool... no furnace. We're guessing that two thermostats make it more likely than not that you have two, although systems vary. Our 2002 Bounder Diesel had two air conditioners, two 20,000 BTU propane furnaces and two thermostats. Basically, two completely separate HVAC systems, not connected to each other at all.

Our current rig has two heat-pump/air-conditioner units, but only a single (42,000 BTU) propane furnace, and a single thermostat that controls ALL of it. It's able to control/split the front and rear of the RV into two zones by using remote temperature sensors... one in the living room, and one in the bedroom.

Things can vary a lot depending on how the system is set up. Even though you have two thermostats, it's possible that you have only one furnace... IF (and only if) you have TWO air conditioners. Of course if you only have one A/C unit and one furnace, there's no way you'd have two thermostats. But since you have two thermostats, they could be used to control the two air conditioners, even if there's only one furnace.

The furnaces always use power for the electronics and fans, but when you're hooked up to shore power, that keeps the batteries full anyway, so that power is basically replaced as it's used.

Eberhard

Sunday 20th of February 2022

Related to the sail switch...... We were on a long road trip in November when temps were getting low and our furnace was not working. I tried resetting it, but nothing happened. I noticed that the blower was not turning on. After several attempts of trying to get it started, frustration set it. I had the outside furnace cover off and dialed up the thermostat. Knowing that the normal sequence is that the blower starts, I went outside and manually spun the blower with my finger. The blower started and the remaining sequence happened as expected. The heater worked again and has been since that one risky finger reach....... The motor was somehow between magnetic fields and unable to spin up.

TheRVgeeks

Monday 21st of February 2022

Oh wow, Eberhard. Good troubleshooting... and glad you got it fixed. Certainly sounds like the blower motor was seized somehow (maybe even a bit rusted in place if it hadn't been used much/recently?). Hope it keeps on blowin' hot for you when you need it!

Oochie1

Sunday 20th of February 2022

Our first step was to check the coach battery to ensure it was not too weak. OK. Then we removed the outside access panel for the furnace and pressed the reset switch. No go. Then we removed the sail switch and cleaned it with canned air. No go. So we replaced the sail switch. Success! Make sure you get the part number off the defective switch and model number of the furnace before you order a replacement. I didn't have a multi-meter at the time to test the switch. Now I have one!

TheRVgeeks

Monday 21st of February 2022

Glad to hear you got your furnace fixed! Always a good idea to check the part number if you can before ordering a replacement. And enjoy your new multimeter! 😉

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