Thursday, November 30, 2023


Maintain your RV furnace and keep warm!

By Russ and Tiña De Maris

Having swung through Nevada’s high desert region a few days ago, we were struck by how quickly the seasons change. Stepping out the door one morning, there was a decided nip in the air. Yep, fall is decidedly here! If you’re planning any RV forays before spring arrives, it’s time to do some furnace maintenance. Don’t get stuck on a trip with no heat!

RV furnace outside vent. R&T De Maris photo.

In this discussion, we’ll talk about forced-air furnaces. A quick overview of how these critters work will help. As the name implies, the furnace relies on fresh air – lots of it. Outside your rig you’ll likely see a furnace vent with two jobs: Pulling outside air into the furnace, and dumping exhaust out. Air is drawn in, mixed with LP gas, burned in a combustion chamber (sealed to keep nasty fumes from getting into your living area), and air is blown over that chamber to heat up the rig. Typically a blower motor shoves heated air through ducts, although small RVs may just have heated air blown directly out of the furnace front.

It’s a fairly simple system, and when it works, it’s great. But without regular maintenance, things can go out of whack, leaving you shivering. Happily, maintenance is fairly straight forward. It breaks out into three basic areas, as follows:

Keep it clean
Safety here: Before starting out on this section, you’ll need to turn off the thermostat and shut the valve(s) to your RV propane supply. When cleaning dust, it’s wise to wear a dust mask and eye protection.

Dirt is an enemy to efficient furnace operation. Too much dirt simply shuts the whole system down. Most furnaces employ “squirrel cage” fans to both pull in fresh air to fuel the furnace flame, and to force the heated air through your rig. Dirt and lint can throw a monkey wrench in the works. They can get into the cage bearings, making them harder to turn, and if enough dust builds up on the squirrel cage, it can make it so heavy it no longer operates as it should.

A typical furnace sail switch: Air flow must hit and move the sail to allow the furnace to start and operate.

A safety feature built into forced-air furnaces is called a sail switch. As the fan turns, the air it produces flows past this switch, and as it is named, pushes on a fin, or sail. The sail rotates on a shaft (providing enough air pushes on it) “telling” the control system that it is now safe to start up the furnace and heating process. But if the blower wheel (squirrel cage) is weighted down with dirt or its bearings bound up with gunk, it won’t spin fast enough to produce the required amount of air to push the sail switch. Result: The controls never get the go-ahead to start up, and you’re left without heat.

What to do? Get the dirt out! You’ll need to access the furnace, which calls for pulling a furnace cover, maybe even an access panel. The object is to expose the squirrel cages and other inside portions of the furnace. With your vacuum cleaner and a source of pressurized air (“canned air” works fine), suck and blast to get the dirt removed from the works. A damp rag is also useful.

Cleaning also extends to ducts. Here you’ll need to remove the heat registers (vents) and carefully vacuum into the duct as far as possible. Flexible plastic ducts are sensitive to damage, so go easy! And air flow is critical coming into (and out of) the coach. Check out the outside furnace vent. Bugs seem to love to build nests inside them, and road dirt can accumulate. It’s critical to make sure the vents are clean so plenty of air can move in and out.

Since furnaces depend on electricity for signaling, there are a couple of areas to look at. First, the thermostat. Simple “bimetallic” thermostats have accessible electric contact points. Commercial electric contact cleaner (like this one from Amazon) sprays on and takes the oxides off the contacts. At the very least, use a pencil eraser to brighten up the contacts. If you can access the furnace circuit control board, remove the cover, unplug any connectors, and shoot the contacts with cleaner. Be sure to allow them to dry before plugging the connector back in, exactly following the directions of the contact cleaner manufacturer.

Keep it juiced
Just as air is critical to furnace operation, so is the flow of electricity. A primary cause of forced-air furnace issues is that of battery voltage – or lack thereof. When we were “green” RVers, with our first rig, a tiny truck camper, we learned this the hard way on our first night out. Our camper had no “house” battery, but relied on the truck’s starting battery. After running the furnace for a few hours, we suddenly had no heat. We’d sucked the truck battery down. (We also ended up begging for a jump start!) The bigger the furnace, the greater the demand for power. If you’re a boondocker, you may find that forced-air furnaces and your approach to RVing aren’t that compatible. Hence the call for catalytic or “blue flame” heaters that demand little or no electricity.

In any event, if your battery voltage falls off to less than 10.5 volts (as seen by the furnace) you can be assured your furnace will go out on strike. If your batteries are “on their last legs” or not fully charged, you’re begging for a chilly night. The lower the voltage, the slower the squirrel cage turns and, soon, that sail switch won’t rotate and the whole system goes down.

Keep it breathing
A free-flow of air to your furnace is vital. We’ve already talked about ensuring that your outside vent is clear of obstructions. But inside, too, you’ll need to keep a clear path. Air from inside the coach needs to be drawn into the furnace area and blown over the combustion box in order for hot air to get back through the system and out your registers.

Furnace air returns should be kept clear of obstructions. R&T De Maris photo.

It seems that by our natures, RVers tend to be a little “squirrelly” and we pack every available inch of space with stuff. Don’t be tempted to store stuff in the furnace compartment, or to block the air return to your furnace. Yes, neat freaks, it’s important to keep the dust cleaned off your furnace’s internal parts, so why not “add on” a filter like that furnace back home? Adding on a non-specified filter simply blocks the flow of air required for your furnace to operate. Don’t stick filters anywhere in the system.

Furnace maintenance is critical, but generally not out of the reach of the average do-it-yourselfer. Keep it clean. Keep an eye on your battery power supply. And make sure you keep a free flow of air from both outside and inside, and your furnace should keep you warm as winter approaches.


Russ and Tiña De Maris
Russ and Tiña De Maris
Russ and Tiña went from childhood tent camping to RVing in the 1980s when the ground got too hard. They've been tutored in the ways of RVing (and RV repair) by a series of rigs, from truck campers, to a fifth-wheel, and several travel trailers. In addition to writing scores of articles on RVing topics, they've also taught college classes for folks new to RVing. They authored the book, RV Boondocking Basics.



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TLT (@guest_53426)
4 years ago

Good article, understanding how the furnace works is a huge help.

Janet (@guest_53395)
4 years ago

I was told that pet fur will cause the sail switch to fail, so I put a filter in the return vent area. I haven’t had a problem, yet. I replace the filter once a year, before starting up the furnace.

Sheila (@guest_53130)
4 years ago

Thank you so much! It’s easy,simple maintenance! I’ll be vacuuming the furnace in the morning, that is, if it’s not snowing!,

Tommy Molnar (@guest_53093)
4 years ago

Oddly enough, we’ve never done any of this. Our last trailer (which we had for 16 years) chugged along just fine. Our 2012 trailer is also just chugging along just fine. I know I’ll catch some grief for this, but I’m just passing along my experience.

Bill (@guest_53019)
4 years ago

I’d also like to add: My new 5th wheel furnace vents are attached to the furnace with flexible hoses…….most of these hoses were not effectively sealed at the attachment points. These leaks reduced the heat that would reach the living space.

Sharon B (@guest_52961)
4 years ago

I’d like to see a YouTube on how to maintain my furnace

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