BTU Calculator

[pnwpaddler]

Anyone have a calculator that helps them determine what size furnace to spec in a pop top camper? I'm looking at both propane and diesel units that have varying heat outputs. Wondering if any has found a calculator that helps narrow down what's need for the size of the unit, inundation, doors/windows, and climate.

 

[rruff]

I can tell you that mine with no heat sources inside (like people) and a window cracked, uses about 13 W/degree F. That is the temperature difference between inside and outside. 500 W (x 3.4 = 1700 BTU/hr) would maintain ~38 F temperature difference for instance. I measured this with an electric heater, run for 3 days so it was saturated. It's not a poptop, has 1.5-2" thick foam walls, and 3 small windows, 2 of them single pane glass. I did not have them covered with insulation like I normally do if it's cold.

There are so many variables like the amount of wind, radiation to space at night, the amount of infiltration, sun, how many people and dogs, etc. Most heaters made for campers have plenty of output, but it's good to have an idea how much fuel you'll use.

 

[pnwpaddler]

Thanks. Yes, there are so many variations (environmental conditions) that it will be a rough estimate....

 

[rruff]

What is the camper you are speaking of, and what are the conditions?

Whatever you get, I'd recommend investigating how well it works on low settings, and if it can be easily modded. The 5-8KW Chinese diesel heaters can be turned down to ~2KW and still run efficiently, but no lower. And I've heard they don't like to be cycled. If you are looking at that type, a 2KW heater would make more sense I think.

 

[pnwpaddler]

My camper is a soft sided Hower Built - so the roof and lower half have a higher R value than the upper walls. You're comment above is one of my reasons for wanting to figure out the right size; it seems a furnace may provide more comfortable heat if its putting out just the right amount of heat for a long period of time rather than cycle on and off quite a bit.

 

[ramblinChet]

I travel full-time across these United States in a Four Wheel Campers Grandby pop-up truck camper, equipped with a Wallas Nordic DT diesel cooktop/heater (variable output: 3,000–6,400 BTU/h).

To estimate heating needs, you can use this basic formula:

BTU/h required = Volume (cu ft) × Delta T (°F) × K (heat loss factor)

• Calculate the volume: Width × Height × Length in feet
  • In my Grandby, I approximate 6 ft wide × 6 ft high × 10 ft long = 360 cubic feet
  • I use 10 ft for length to account for the cabover overhang, which adds volume despite the base floor being ~8 ft
• Determine Delta T (temperature difference): Desired indoor temperature minus expected lowest outdoor temperature
  • Example: 70°F inside minus 30°F outside = 40°F Delta T
  • Adjust higher (50–60°F) for colder climates or windier conditions
• Apply the heat loss factor (K): This accounts for insulation and air infiltration (BTU/h per cu ft per °F). Typical values:
  • Tent (soft-walled): Poor insulation → K ≈ 0.45
  • Pop-up camper (like the Grandby): Moderate (hard roof/floor/base with canvas sides) → K ≈ 0.30
  • Hardsided camper: Better rigid insulation → K ≈ 0.15

For my pop-up: BTU/h = 360 × 40 × 0.30 = 4,320 BTU/h

In practice, during recent arctic blasts with sustained temps in the teens and twenties (°F), my Wallas Nordic DT performed acceptably on higher settings. It maintained comfortable interior temperatures. Results will vary with wind, sealing, occupants (body heat helps), and exact Delta T, but this setup has proven reliable for my full-time travel.

I hope this is helpful - feel free to share your own experiences!

 

[rruff]

BTU/h required = Volume (cu ft) × Delta T (°F) × K (heat loss factor)

It's surface area, not volume.

 

[ramblinChet]

It's surface area, not volume.

I believe you may be referencing the more common BTU calculation method used for standard homes, which involves calculating the total surface area, multiplying that value by 20, and then adjusting up or down by approximately 10% to account for factors such as excessive sunlight or poor insulation. This is an effective approach for residential structures, but it does not incorporate Delta T (temperature differential) or the K-factor (thermal conductivity coefficient), which are essential for accurately assessing smaller or portable enclosures like campers and tents.

In summary, both methods are valid, though each is optimized for slightly different applications.

If you really want to dig in consider Manual J Residential Load Calculation (8th Edition - Full) [ANSI/ACCA 2 Manual J - 2016]

 

[rruff]

...but it does not incorporate Delta T (temperature differential) or the K-factor (thermal conductivity coefficient)

The thermal conductivity coefficient must be multiplied by surface area and delta T to get the heat (actually power) transferred between inside and out. Volume isn't relevant for conduction, but would come into play if you are figuring so many air changes per hour for infiltration, for instance.