What To Do If Your RV Furnace Fan Runs, But There’s No Heat Coming Out

Have you ever been in a chilly RV, flipped on your furnace to make the RV nice and toasty, and all you got was a steady flow of cold air? If you’ve experienced what happens when an RV furnace fan runs but there’s no heat then you understand why we’re writing this post. When temperatures drop, the last thing you want to face is a cold night in your RV. So we’re offering some troubleshooting tips for this very situation, should you find yourself in a chilly fix.

Why is Your RV Furnace Blowing Cold Air?

We’ll be focusing on the most typical RV furnace, the propane furnace. Hydronic heat (HydroHot, AquaHot, Oasis) is a whole different breed, and we’re not covering those here. But if you’ve got a typical propane RV furnace, pay close attention so that you’ll be prepared in advance should your RV furnace fan run, but there’s no heat being generated.

First, an important safety note: Bear in mind that your RV furnace uses both propane and electricity, both of which can potentially pose a danger. If you’re not comfortable dealing with propane or electricity, call in a professional for anything other than the most basic of these troubleshooting ideas.

Fuel Source – The #1 Reason For No Heat From Your Furnace

Let’s start with the most obvious reason why an RV furnace fan may run with no heat: the fuel source. It’s the first thing to check, and while it may seem obvious, it’s an easy one to overlook, so we’d be remiss not to mention it here.

How to Test If Your Propane Supply is Working

The easiest way to test your fuel source is to see if your other propane appliances are working. If your water heater and propane stove are working, then you can move on to other troubleshooting options. First, try lighting your propane stove. If it doesn’t ignite and your water heater doesn’t fire up, then move on to checking the following items:

• Propane tank level (How full is your propane tank?) If you don’t have a gauge, try this.
• Propane tank valve (Is it open or closed?)
• Onboard propane regulator (Has it failed, or is it clogged?) This site explains what a propane regulator is, how to troubleshoot it, and how to replace it.

PRO TIP: If you’ve recently refilled your propane tank(s), it’s possible that a bubble of air has formed in the line to the furnace. After ensuring that all your other propane appliances are still working (i.e., there’s propane available), be sure to allow your furnace enough time to clear the line. Set your thermostat to furnace mode and wait for it to try igniting. Turn the thermostat off, wait a bit, and turn it back on to re-trigger the attempt. It may take more attempts than you’d expect, so be patient! 😉

Sail Switch Failure (or Low Battery Voltage)

It’s not uncommon for an RV furnace fan to run but produce no heat due to an issue with the sail switch. Among the possible problems is low battery voltage. So, check your RV’s house batteries to make sure they’re not weak. Here’s why:

An RV furnace is different from a residential furnace. In a residential furnace, the blower doesn’t start until the plenum warms up. However, an RV furnace starts blowing as soon as it’s turned on. That’s why an RV furnace always blows cold air at first.

An RV furnace uses a single motor to spin two blowers simultaneously:

1. The hot air blower that circulates the heated air to the vents in the RV
2. A second blower that moves air through the combustion area inside the furnace itself

If your batteries are too weak to sufficiently power the blower motor (or if the motor were to fail right as the burner was being lit), the furnace would become a serious fire hazard. This is why it has a safety feature known as a sail switch. The sail switch detects whether the blower fan has come on and is blowing enough air through the furnace before igniting the burner.

Here’s an example of a sail switch:

This one happens to be for an Atwood/Dometic RV furnace, and we’re showing it to you so you have an idea of what a sail switch looks like. You can search for the appropriate Suburban RV furnace parts for your RV here, and for Atwood/Dometic RV furnace parts here.

A sail switch is air-sensitive: it’s located in the duct and overrides the gas valve of the burner. So if the airflow isn’t sufficient, the sail switch won’t be activated. And if the sail switch isn’t activated, the burner won’t light (or if it’s already lit when this occurs, it will shut off). 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.

However, if you’ve got plenty of propane and your batteries are strong, it’s possible that the sail switch itself is either stuck (from a buildup of dust, debris, rust, or insect nests) or defective. Either way, the furnace won’t ignite if the control board never sees it opening and closing.

You can remove and clean the sail switch. You can also take a bit of precaution and install insect screens on the exterior of your rig at the furnace exhaust ports. Our rig takes this type:

Other RVs may require different types of screens, but in general, these are good to use to prevent insects from making nests in places like your furnace. Here’s a product that comes with screens for several RV vents:

How to Test Sail Switch and Low Battery Voltage

• Test the condition of your house batteries using a good multimeter.
• Check the outside and inside vents for dust, leaves and other debris, insect nests, etc. You can vacuum or hand-clean vents. Remove the inside vent and gently clean around the burner (any obstruction around the burner area can result in incomplete combustion).
• You can read about how to test the sail switch on a Suburban furnace here, and how to troubleshoot the sail switch on an Atwood/Dometic furnace here.

You can test the sail switch with a multimeter set to ohms. You’d need to close 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. With the circuit closed (and if your sail switch is good), you’ll have a reading of between 0.0 – 0.2 ohms.

But there’s an easier way to test whether the sail switch is the reason your RV furnace fan is running, but there’s no heat.

Sit out at the exhaust port, have someone turn on the furnace, and listen carefully. The first thing you should hear is the fan starting. After about 15 seconds, you’ll hear a click (the gas solenoids clicking) and then a rapid-paced ticking (the ignitor striking an arc over the burner head, trying to ignite the furnace). If you don’t smell LP gas and the furnace hasn’t ignited, you’ve ruled out the sail switch as the problem for several reasons.

First, the furnace wouldn’t try to ignite if the sail switch was the issue. And second, if you don’t smell LP gas, the problem is very likely to be the gas valve.

The following diagram happens to be a wiring schematic of an Atwood/Dometic RV furnace, but the Suburban furnaces are similar, though perhaps with different component placement. Here you can see the sail switch, the gas valve, and other components, as well as the path the circuit takes.

Again, you can search for Suburban RV furnace parts here, and Atwood/Dometic RV furnace parts here.

Gas Valve

Here’s how the gas valve (shown on the left of the schematic above) works: when the furnace control board calls for it, the two wires in the gas valve are energized with 12 volts, opening the solenoid inside the gas valve and allowing propane to flow through the valve to the burner orifice.

How to Test the Gas Valve in Your RV Propane Furnace

To test the gas valve, you’d need to remove it from the furnace. Many of our readers may not wish to undertake this task, but we’ll outline a few steps for testing the gas valve for those who are interested.

This is what the gas valve for our Suburban SF42 Ducted Propane Furnace looks like:

The orifice itself can be clogged by insects or dust. The orifice can be unscrewed from the gas valve for inspection and cleaning. You can also check to make sure the gas line is clean by spraying canned air into the gas valve. If you don’t feel it coming out the other end, then you have a clogged gas valve.

If the orifice is clean and the gas valve is clear, then one or both of the solenoids in the gas valve could be bad. A multimeter can be used to test the solenoids. Each solenoid should test at 40 ohms of resistance (here’s a video showing how). Test each wire separately against ground. If you don’t find close to 40 ohms at each solenoid wire, then you have a bad solenoid in your gas valve.

You may not want to remove and test your furnace’s gas valve, but it’s good to know how to troubleshoot it easily. So, to recap, if your RV furnace’s fan turns on and you hear the click of the solenoids and then the rapid ticking sound, but you do NOT smell liquid propane gas, the issue is with the gas valve.

Ignitor Failure

Going back to the same test we just discussed, if you’re seated at the exhaust port troubleshooting the furnace and you hear the fan turn on, the solenoid click, the rapid ticking (or NO ticking), and you DO smell LP gas coming out of the exhaust but the furnace hasn’t ignited, then the problem could be the ignitor.

Here’s a Suburban ignitor:

An Atwood/Dometic ignitor will have a similar appearance. We’re posting this specific part to give you a visual understanding of what the ignitor looks like – NOT to suggest that this is THE ignitor you should be buying for your RV furnace. As always, you should check the specifications for the make & model of your RV furnace before purchasing a part.

How to Test for an Ignitor Failure

First, make sure your LP gas is disconnected. You can remove the ignitor from the combustion chamber while it remains connected to the furnace. Try starting the furnace. You should see a spark jumping between the ignitor tips as the furnace tries to start (BEWARE: don’t touch any wires or metal in the vicinity of the ignitor when it’s being energized… the shock can be QUITE painful).

Inspect the ceramic insulation around the ignitor tips carefully, looking for any cracks. If any are present, it may be the sign of a faulty ignitor. Furnace manufacturers specify gaps between the ignitor tips. Therefore, you’ll want to check the specs and make sure the tips of your ignitor are in line with the specifications.

Also, look at the gas chamber to make sure there are no holes that would allow LP gas to escape before ignition.

If you don’t see a spark jumping between the ignitor tips as the furnace tries to start, and you don’t hear the rapid ticking sound we discussed above (the ignitor trying to light the furnace), then the ignition module of the control board may be bad.

Faulty Control Board

The control board, sometimes referred to as a circuit board, is the electronic brain of the furnace. When you’re considering the control board as the source of your problem, you’re looking to make sure that power is getting to the control board and is also traveling THROUGH the control board to the other components of the RV’s furnace. (Be sure to check the parts list for your RV furnace before ordering a new control board.)

How to Test for a Faulty Control Board in Your RV’s Propane Furnace

First, inspect the control board looking for any visible damage, like any burned or discolored areas anywhere on the board.

You can test the control board using your multimeter. If you want to make sure power is getting to the control board, set your multimeter to DC volts and connect one lead to the ground and the other lead where the main power connects to the control board. Your reading should be at least 12V.

To make sure that power is making it through the circuit and back to the control board, you want to check the contacts on the control board where power returns from the sail switch and high-level switch. With the thermostat ON, take your reading, which should be 12V or higher.

If you want to check the control board relay to find out if power is getting past the relay to the blower motor, attach one of your multimeter’s leads to the contact after the relay leading to the blower motor, and attach the other lead to a ground. With the thermostat ON, again your reading should be 12V.

Conclusion

We hope this post is helpful to you if your RV furnace fan runs but no heat is being generated. It’s always wise to test your furnace before you head out on a trip where you might encounter some cold weather, of course, but sometimes systems fail at the most inopportune times (like when you’re dry camping in the remotest spot possible… on a holiday weekend… during a snow storm).

For this reason, when camping in areas where cold weather is possible ☃️, it’s also wise to carry backup heat in the way of extra warm blankets, an electric blanket or bed warmer, a small ceramic backup heater (if you have a way to power it), or an indoor-safe portable propane heater like a Portable Buddy or Big Buddy heater… just in case!

And if you want to learn all about other heating options, check out our post Furnaces, Heat Pumps & Space Heaters. Oh My! The Pros & Cons of RV Heat or learn more about What Is an RV Heat Pump?

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