Your RV Furnace Doesn’t Cause Condensation

Combustion of hydrocarbons certainly does, but not from your furnace. Your furnace doesn’t cause condensation, or even raise humidity in your RV. This post explains why, and what else might.

Chemistry of Combustion

When you burn hydrocarbons (propane, natural gas, wood, etc.), you’re breaking chemical bonds between hydrogen and carbon, and forming bonds with oxygen. In the case of complete, stoichiometric combustion, you end up with carbon dioxide ([math]CO_2[/math]) and water ([math]H_2 O[/math]).

Here’s the reaction from proper burning of propane, as you should have with a typical RV furnace:

Of course, combustion isn’t always perfect. The air mixture might not be quite right, there might be other chemical components of the fuel you’re burning or the atmosphere you’re burning it in. If temperatures are too high, we get reactions between nitrogen and oxygen in air, resulting in nitrous oxides. Sulfur reacting with oxygen gives us various oxides of sulfur. We’ve know the results of those, in the form of smog and acid rain.

We can also get incomplete combustion, where one of the resulting molecules is carbon monoxide. Unlike CO2, carbon monoxide is particularly poisonous, and in high enough concentrations, can be lethal. That’s why you should have a CO detector on board, in case there’s a leak of combustion gases and improper combustion creating a hazard on board. You won’t smell, taste, or see carbon monoxide.

The H2O on the right side of the equation is water, in vapor form. Yes, it will condense. But it’ll condense outside your RV, not inside, when working properly.

Furnace Combustion Gases are Vented Outside

So we have a chemical reaction producing water vapor. That increases humidity and leads to condensation, right? No. Let’s look at how the furnace works to see why.

Key components in a typical RV furnace

In the image above, I’ve highlighted a few key components. There’s a single blower motor, turning blower wheels in two separate chambers. Air for combustion is drawn in the upper tube on the right by the blower on the right. It’s forced into a burner, where it mixes with propane and ignites.

As the mixture burns, it is blown through a serpentine tube in the far part of the image, eventually looping back to the bottom right corner, where it exits the side of your RV.

At the same time, the blower for room air is moving air across the outside of the tubes containing combustion gases. But the combustion gases and the room air never mix. The water vapor and carbon dioxide created go outside. The oxygen that mixes with propane to burn comes from outside.

What Propane Fixtures DO add Moisture Inside?

Any appliance that heats with a combustion reaction, and uses the room for the oxygen it needs. In short, if it isn’t drawing in outside air, and exhausting combustion gases outside, its use should be limited in time and with proper ventilation.

Ever seen those warnings about not trying to use the stove or oven to keep a room warm? Recall the chemical reaction we talked about. When a stovetop burner is lit, it’s slowly consuming oxygen in the room. Most of the time, a burner isn’t on long enough or at high enough power to overcome the air exchange that occurs with normal RV use. Every time you open a door or window, when the wind blows air in around slide out seals, or when you operate a vent, you’re trading some of that oxygen-depleted air for fresh outside air.

But if you have a habit of slow cooking on the stovetop, consider a portable electric cooker (preferably an induction burner over a traditional hot plate). If you’re using the oven, keep the door closed. When the door is open, the temperature controls will call for more heat to make up for what’s lost, running the burner more than necessary. The heat goes into the room, along with moisture, and oxygen leaves.

Catalytic Heaters

Catalytic heaters are popular with RVers, as they don’t require electricity to operate, don’t have an open flame, and are relatively portable. However, they also don’t source air for combustion from outside. The moisture produced by the combustion process is still left in the room, along with carbon dioxide.

Over time, with a reasonably well-sealed RV, your catalytic heater will deplete the supply of oxygen available for you to breathe. It’s unlikely it’ll ever get to the level of you passing out, but certainly can get low enough for you to feel tired or notice lightheadedness or headaches.

Always make sure if you’re operating one of these inside that you open a window a bit, to make sure you have adequate oxygen available.

Other Appliances that DO NOT Add Moisture Inside

Electric Fireplaces

This should be obvious. Those fake flames aren’t the result of combustion. They also don’t produce any more heat than any other standard 1500W space heater–that’s all they are, just with a different appearance.

Clothes Dryers

If you’re lucky enough to have a propane clothes dryer on-board, like the Grey Ghost does, know that it’s safe to use. While it doesn’t source its combustion air from inside, it exhausts its combustion products and the moist air from heating your clothes, out the vent.

That means that it creates a very slight negative pressure in your RV, drawing outside air in to replace the air it blows out. It won’t deplete the oxygen inside, or add moisture.

Water Heater

Like your furnace, an RV water heater takes on outside air, burns it with propane, and exhausts it back outside. Here it doesn’t take a blower though. The natural draft created by the heated air is enough to keep the air moving through the center of the water tank.

Of course, if you actually use hot water, however, you’ll of course add moisture to the inside atmosphere. Just like every warm shower you’ve ever taken.


In short, if it’s using fuel (like propane) and isn’t exhausting outside, the appliance is both adding moisture to the RV and depleting its oxygen supply. Use should be limited in time, and a supply of fresh air should be available.

If it’s exhausting outside, like your furnace or water heater does, it has no bearing on the inside moisture level. In a future post, we’ll talk about how to control moisture levels inside, and how to prevent condensation (and mold!) from forming.

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