A propane furnace is a fantastic thing to have in an RV – as long as it’s working! When working properly, you turn up the thermostat and a few seconds later, heat is moving through the 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 this 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!
- 1) How Does an RV Propane Furnace Work?
- 2) What Is a Sail Switch?
- 3) Where Is the Sail Switch Located on an RV Furnace?
- 4) How Do I Know If My RV Furnace Sail Switch is Bad?
- 5) How Do I Test My RV Furnace Sail Switch?
- 6) Can I Clean My RV Furnace Sail Switch?
- 7) Can I Replace My RV Furnace Sail Switch Myself?
- 8) Have You Ever Replaced Your RV Furnace’s Sail Switch?
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, a sail switch, a high-limit switch, an ignitor, and some ductwork. To activate the system, you set the thermostat higher than the RV’s current interior 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 over that surface, it is heated and then distributed throughout the RV via the ducts.
12V DC 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:
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 shut off the propane supply if the burner temperature exceeds safe limits.
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 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:
- This Sail Switch Replaces Sail Switches: 35050, 35052, 35054, 35137, 36133, 37716
- Our sail switch replacement are suitable for various Suburban SF series and SF-20/25/30/35/42, SF-20F/25F/30F/35F/42F
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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.
- Additional Tips - The following incorrect operations may cause the multimeter not to show results: Firstly, the plugs of test leads are not fully...
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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.
- RV furnace and fan unit outside fittings
- Fits DuoTherm and Suburban
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- 【RV Insect Screen 8.5" x 6" x 1.3"】 : Compatible with Atw-ood 6 & 10 Gallon and Sub-urban 6 Gallon water heater vents. Easy to install: It comes...
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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Charles L Haas
Wednesday 19th of February 2025
How come RV furnaces do not have an indoor furnace filter to keep them clean ? I have owned many types of RV over the years and none had a filter !
Mj Proud
Tuesday 4th of February 2025
I have a suburban NT20sq. It will turn on by the thermostat, blower goes on, I can hear a click, then hear the igniter spark, (can see the igniter spark in the view port on this model) but nothing lights. It will re attempt to light every 30 seconds or so. it will go through this cycle for 10 minutes, then turn off.
I'm connected to shore power. I have propane in the tanks. My stove runs. I did replace the main regulator as there was a leak at the crimps on one of the pig tails. However still no furnace.
Any ideas? I am stumped.
Thank you for your assistance,
Mj Proud
TheRVgeeks
Wednesday 5th of February 2025
Hi MJ. Sorry to hear about the trouble you're having... bad time of year for your furnace to be acting up! 🥶 Have you listened outside at the furnace when the thermostat calls for heat to hear if it sounds like propane is flowing or not while the ignitor is clicking? If not, try that and see if you hear any hissing at all. If not, it's POSSIBLE that the trouble is with the circuitry and/or valve on the furnace itself that's not allowing propane to flow.
If you DO hear propane flowing, but it's still not igniting, then it's possible that there's an obstruction preventing the right amount of air to enter the burner chamber, resulting in the wrong mix of propane to oxygen, so it's not igniting (less likely). Or it could be the sail switch... it detects that there's enough air flow when the blower comes on to ensure that the heat and exhaust gases will be blown from the furnace burner chamber, and NOT having enough air flow will prevent the furnace from igniting (again, less likely the culprit since you're hearing the furnace try to ignite... usually, if the sail switch fails and doesn't indicate propare airflow, the furnace never TRIES to ignite). OR... it's possible that even though there's propane flowing to the stove burners, it's not flowing at a high enough rate for the (larger) furnace demand. As a test, you can try bleeding the system using the following steps:
Turn off all propane appliances Turn off the propane supply at the tank(s) Turn on a burner at the stovetop and allow it to flow/burn until it stops (make sure you've got plenty of ventilation inside while doing this... open some windows and turn on a vent fan to prevent propane from building up inside). Then turn it off. Go back out to the tank(s) and SLOWLY turn the propane supply back on. SLOWLY. Allowing it to come on too fast can trigger a leak prevention system that restricts/cuts off the flow. Go back inside and turn on and light a burner on the stove. It may take a little bit for the flow of propane to reach the burner, so be patient. Once lit, let the burner run for a couple of minutes to ensure continuity of the propane in the line(s). Turn it off (and go ahead and close the windows and vent to conserve heat, LOL!). Try turning the thermostat up to get the furnace to fire up. Again, it may take a couple of tries until pure propane has refilled the line(s), so give it a chance. Once the furnace times out, try turning the thermostat off and then back on again to re-trigger.If that fixes it, it was some form of "vapor lock" or "leak prevention" issue at the propane tank and/or regulator (which can happen when you turn the propane on too fast and the sudden surge from the tank can trigger the regulator and/or the safety valves in the tank to cut the flow back/off). If that DOESN'T fix it, then it sounds like it's something at the furnace itself (again, either a circuit failure or valve failure would be our guess).
If you can't get the RV in to service, you can try getting a mobile service tech to come to you. There's actually a new service that a friend of ours (Heath Padgett) just started up called RV Help (https://www.rvhelp.com) that's aggregating access to mobile RV service techs, which makes it a lot easier to find one when you're traveling, etc. So they may have a listing in/near your current location.
Let us know how you make out!
Gary George
Monday 30th of December 2024
Loud noise coming from RV heater-blower. Blowing fan and sounds of igniter trying to light, may have been running for hours. Propane was turned off and cabin temp off-turned propane on and set cabin thermostat to warm, furnace ignited and sound of igniter clicking stopped. However once cabin temp reached sound of igniter trying to light again but no combustion, just kept clicking with blower blowing cold air. Finally I pulled a red connector and that stopped the running. Dirty sail switch? Will check next. Summary; clicking of igniter and blower will not turn off. Sure would appreciate your experience and knowledge commenting on my dilemma. Thank you, Gary George
TheRVgeeks
Monday 30th of December 2024
Hmmm... this is head-scrather, Gary. From the fact that the furnace started up when you shut everything down and restarted, we wouldn't think the problem was the sail switch... though it's possible (so cleaning it couldn't hurt). But the fact that the furnace keeps trying to light even after the thermostat says it's reached temperature would seem to indicate something wrong with the main circuit board (the furnace's brains). Clearly, it's lost the order of things from start to finish (blower on, wait for sail switch signal, start ignitor, open gas flow, thermocouple reports ignition and steady burn, ignitor goes off, burner & blower continue while thermostat continues call for heat, once temp is reached propane goes off, then blower continues for set time to cool off burner chamber... rinse and repeat).
It's also possible that the fault is the thermocouple that detects successful flame? But that seems like a longer shot... since if it was indicating flame when there wasn't any, we'd think the furnace would never ignite (no clicking at all, since it thinks it's already burning)... while if it NEVER indicated successful ignition, the clicking wouldn't stop and the furnace would probably cycle on and off while it was waiting for signal that the flame was holding.
So... again, we think it's pointing to the circuit board. But, since we HATE just throwing money at parts without a conclusive diagnosis, we'd say it might be time to call in a professional (mobile tech?) to look it over for you. They may have additional troubleshooting ideas that would help them isolate the root cause.
Sorry we're not more help!
Eddie Smith
Friday 18th of October 2024
We have a 2008 Montana RV and our furnace has stopped working . WE are connected to shore power and have charged our 12 volt battery but when the furnace starts the fan runs correctly for about 5-10 seconds then slows down and the 12 volt lights in our RV all begin to flash off and on and after a while the furnace shuts off with out ever producing any heat or blowing air. We removed the furnace earlier this year and cleaned it and it worked fine this spring but now we are having this problem.I have used other fully charged 12 volt batteries but had the same results.Got any ideas? Thanks
TheRVgeeks
Friday 18th of October 2024
Sorry to hear about the trouble, Eddie. We haven't heard of this exact problem before, but (without seeing and testing ourselves) it sounds to us like one of two things: either (1) there's a short somewhere in the power lines to the furnace, causing an excessive draw... could even be a short inside the motor itself (if you're able to set your thermostat to furnace, set the temperature below current room temp so it doesn't come on, and then set the fan from "Auto" to "Lo" or "Hi" to get it to run without the furnace firing up, you could see if it keeps going for more than the 5-10 seconds... which would help eliminate the blower motor as the culprit) or it could be (2) your converter/charger is failing and can't supply sufficient 12V power when there's a large 12V draw (you could check that by unplugging from shore power so the converter/charger is off and the furnace just runs directly off the battery).
Hope one of those two leads you toward a solution.
Patty Canady
Sunday 14th of January 2024
Domestic thermostat inside temperature reads 32 degrees on heat but thermostat readers 71 or higher. False inside reading so heater doesn’t cut off
Kevin
Wednesday 24th of April 2024
@Patty Canady, it’s probably on Celsius. To change from Celsius to Fahrenheit hold both arrow buttons at the same time.
TheRVgeeks
Monday 15th of January 2024
Hmmm, sorry to hear about that Patty. Dometic has made a couple of different thermostats over the years, but if you can identify the model... Google for "Dometic XXX Thermostat Reset" to find the reset procedure, which might solve the problem.
If that doesn't do it, then it's possible that the temp sensor has failed. If you have a remote sensor (usually on a wall or up underneath a cabinet... behind a small plastic dome with holes in it), that may need to be replaced. If not, the whole thermostat may need to be replaced (there's a sensor in there, too).