Thursday, February 2, 2023


RV Fact or Fiction? A propane furnace emits lots of moisture ANSWER

Today’s Question:

RV Fact or Fiction? To keep RV windows from sweating, use an electric space heater instead of the RV propane furnace as propane heaters emit lots of moisture.
*Paraphrased/condensed from this source.


Fiction. RV propane furnaces do not add moisture to the interior of an RV.

It is true that a considerable amount of moisture (1.64 pounds of water per gallon of propane) is generated during the combustion of propane. However, since the combustion in an RV furnace takes place in a sealed combustion chamber vented to the outside, none of that moisture enters the interior of the RV.*

Fact or Fiction - furnace combustion chamber
RV furnace – Showing sealed combustion chamber. photo

“At the root level, a forced air furnace draws in fresh air from outside the vehicle, mixes it with propane and burns it in a sealed combustion chamber.” Per the late Gary Bunzer, the RV Doctor.

The hot air you feel emitted from the heat ducts/registers is air that was drawn in from the interior of your RV, circulated around the exterior of the combustion chamber and pushed out through the heat ducts/registers. This is no different than the air being drawn into the back of a portable electric heater, heated by electric elements, and blown out the front.

Click here if you would like to learn more concerning propane heat and moisture problems in the interior of the RV like condensation forming on windows.

Fact of fiction - condensation
RV furnaces do not add condensation to the interior of the RV. photo

So why do many RVers believe that using an electric space heater rather than the propane furnace in their RV minimizes condensation?

Here are my observations

We have all experienced condensation forming quickly on the inside of single-pane RV windows after turning on the RV furnace on a cool, rainy day. However, if you turn on a portable electric heater instead of the RV furnace, the condensation doesn’t immediately form.

Here’s why

One kilowatt hour (kwh) of electricity is equal to 3,412 BTU/hour. Therefore a 1,500-watt (1.5 kwh) electric space heater, a favorite of RVers, will produce 5,118 BTUs in an hour (3,412 BTU/hour X 1.5 kwh = 5,118 BTUs). Compare that to a 30,000 BTU propane RV furnace operating around 70% efficiency.** In one hour that 30,000 BTU furnace will blow approximately 21,000 BTUs of heat into your RV. That is more than four times the amount of heat (BTUs) as the 1,500-watt electric space heater. Since condensation forms when warm, moist air comes in contact with a cold surface, like a window, it only stands to reason the faster the air is heated in your RV the faster the condensation appears. This leads people to believe in the myth that electric heaters are better than an RV gas furnace regarding controlling condensation.

*If your furnace is adding moisture to the interior of your RV, you have a compromised combustion chamber that needs to be repaired immediately. It is not only allowing moisture into the interior but potentially deadly carbon monoxide as well. Not to mention it is also a fire hazard as flames could escape the confines of the combustion chamber.

**RV propane furnaces are nowhere near as efficient as the newer gas furnaces found in most residences. Roughly 30 percent or more of the heat produced during the combustion process fails to be transferred through the combustion chamber into the interior of RV. The uncaptured heat is then exhausted outside through the exterior furnace vent of your RV.

Now, some questions for you:

  • Is there a recurring half-truth you keep seeing online that you would like to see addressed?
  • Were you taught something by other RVers that turned out to be bad advice?
  • Have you recently read something that left you wondering, “Is that true?”
  • Do you know something to be true, but none of your RVing friends believe you?

Please share your comments using the comment box below and we will do our best to provide the facts in a future entry.



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1 month ago

To be addressed in a future article: Fact or Fiction: A towed vehicle can weigh up to the rated Towing Capacity of a towing vehicle.

For all who want to answer it in these Comments; I know that Cargo Capacity is a much-more important number than the Towing Capacity. But, so many folks don’t know that, I feel it would be helpful for Dave to address in his article.

1 month ago

Fact or fiction: covering one or two furnace vents will damage your rv propane furnace.

1 month ago

If you had read the spec plate on a propane furnace you would realize that they are 80% efficient and know that only 20% of the heat is lost.

Steve Hericks
1 month ago

Taken further;
1 gallon of propane weighing 4.23lbs
7.47 lbs of water/gallon of propane x 1 gallon of propane/4.23lbs
1.768lbs (28.3oz.) of water is produced for each pound of propane combusted.

Steve Hericks
1 month ago

You normally do a good job but in this case, you used a web reference without verifying it and it is (substantially) wrong (‘1.64lbs water per gallon of propane’). It is far worse than that;

The simple numbers I typically use (which are easy to remember) which include a combustion efficiency of 85% is that 16oz of propane produces 26 oz of water.

Ignoring combustion efficiency and running the calculations using the website’s data;

100,000BTU of (100% efficiently) combusted propane produces .98 gallons of water
1 gallon of propane contains 91,502 BTU
1 gallon of water weighs 8.34 lbs
(91,502BTU/gallon propane) / (100,000BTU/.98 gallons water) x (8.34lbs/gallon of water)
7.47 pounds of water are produced per gallon of propane combusted.

1 month ago
Reply to  Steve Hericks

And if the system is working correctly ALL of that water is going outside, not into the RV so the actual number is interesting but of no value to the question of the reason for condensation from propane furnace.

Steve Hericks
1 month ago
Reply to  xctraveler

You are correct in so far as furnaces are concerned. Combustion byproducts are irrelevant as long as they don’t end up inside. My unspoken issue is; The high fuel consumption of furnaces has led a startling number of RV users to turn to unvented ‘buddy heaters’ that exhaust all combustion products inside the rv. The attitude that ‘just leaving a window slightly open’ will solve this problem is really the point. It won’t and that is the ‘urban legend’s that I think needs to be dispelled.

1 month ago

This is RV related due to increasing clouds ..especially under jet paths, for example .. Seattle to Las Vegas, where many camps and dispersed areas are located.
The contrails seen during certain temperature spans tend to form platforms for more clouds to form. Especially Cirrus (high, thin, sometimes feathery clouds).

Gregory Illes
1 month ago

A catalytic or ceramic propane heater (think Wave or Mr Heater) “exhaust” their burning gases (H2O, CO2) inside the coach. These heaters will indeed create condensation. A bit of ventilation helps.

Steve H
1 month ago

An RV furnace works like the “Recirculate” button on the dash of your truck or car. It just warms or cools the air already in the interior of the vehicle. So, any moisture produced by the breathing of the vehicle’s passengers is added to that recirculated air. That’s why the heaviest frost occurs inside a vehicle’s windshield and driver’s side window if only the driver occupies the vehicle. In winter RVing, the heaviest frost occurs on windows adjacent to beds, not due to the propane furnace, but to the breathing of the sleeping occupants. If a vent or window is left slightly open, fresh air can mix with interior air and may reduce frost buildup (depends on outdoor humidity!).

Larry L
1 month ago

RV Fact or Fiction: If your RV’s axles have grease zerk fittings, the only maintenance that needs to be done is a few squirts of grease once a year. No need to repack your wheel bearings.

1 month ago
Reply to  Larry L


Michael Roach
1 month ago

I found the question to be confusing as the wording stated both a propane furnace and propane heater. I know a furnace is contained and vented to the outside but a heater like the Big Buddy will create a lot of condensation. Just sayin’

1 month ago

Fact or fiction: when using a water filter, it should be installed between the spigot and the hose.

Bob p
1 month ago
Reply to  Doug

Depends on whether you want filtered water running through the hose or not.

Jesse Crouse
1 month ago
Reply to  Bob p

Depends on whether the source of the water can be confirmed as potable for human consumption. Many campgrounds have questionable water as well if the previous user at your site got water that was contaminated and used that hose on your sites water connection. As a Master Plumber a lot of my work is to this issue of safe water.

Jim Johnson
1 month ago
Reply to  Doug

Of greater importance – be consistent. A potentially ‘contaminated’ hose always used in front of a filter isn’t going to admit any more contamination at a new site than it did at the old site. Personally, my setup has the filter at the tap. We frequently setup in cold weather. I do so because it is easier to cover/insulate both the tap and the filter and heat both gently with a loop of our heated water hose.

Seann Fox
1 month ago

Strange… I stopped using the RV furnace 10+ years ago, opting for a catalytic heater, with no moisture buildup.

Betty Studzinski
1 month ago

WDH hitches should not be used on a unibody vehicle. Fact or fiction?

Bob p
1 month ago


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