RV Heater Sizing Calculator

What size furnace do I need for my RV?

You need one that replaces the heat the box loses on the coldest night you'll camp in, a number about your rig rather than a number per foot. A 30 × 8 ft standard-build trailer with 1,012 ft² of walls, roof and floor loses about 10,763 BTU/h holding 68 °F at 30 °F outside, and 19,260 BTU/h at 0 °F. A furnace delivers only 75% of its label at this page's planning figure and needs headroom to recover from a cold soak, so the size to buy is the loss × 1.2 ÷ 0.75: 30,817 BTU/h for that 0 °F night, which rounds up to the 35,000 BTU class. Sized for a 30 °F night instead, the same trailer is fine with 20,000. The common lengths, computed:

8 ft wide, 7 ft sidewalls (8 ft for the 35 and 40), holding 68 °F. "Standard" = stick-and-tin with single-pane windows, effective R-4 and one air change an hour; "four-season" = foam laminate, dual-pane, heated underbelly, effective R-8. Furnace = smallest common input rating with 20% margin at 75% efficiency.

Read the second-to-last column against the plate on your own furnace, because the furnace in the brochure was chosen for the floor plan, not the weather. One sized for freezing nights can be marginal at 0 °F with the slides out, and a four-season build of the same length needs a class smaller for the same night.

How many BTU does it take to heat an RV per square foot?

For the trailer above it's about 45 BTU/h per square foot of floor on a 30 °F night and 80 at 0 °F, two to three times what a code-built house needs (20 BTU/h per ft² or so at its design temperature), because a trailer is a thin box with a huge surface for its floor area, a single-pane window every four feet, and an underbelly open to the wind. A rule of 1,000 BTU per foot of length lands close for standard builds sized for freezing weather (the model says a 30-footer wants 20,000 at 30 °F and 35,000 at 0 °F), but it can't tell a canvas pop-up from a four-season fifth wheel, and that's a factor of 5 in heat loss. What construction alone does to the same 30 ft box at 30 °F: standard (stick-and-tin) 10,763 BTU/h, laminated fiberglass 7,726 BTU/h, four-season package 5,189 BTU/h. Each slide-out on a standard build adds about 1,121 BTU/h at that temperature: three walls and a roof of its own, plus the gasket.

Why does my RV furnace run all the time?

The duty cycle is loss divided by output, and both ends of that fraction move with the weather. The 30 ft trailer's 30,000 BTU furnace (22,500 out) fires 48% of the time at 30 °F and nearly all of it at 0 °F; below that it can't hold 68 °F at all, and the inside temperature settles where loss equals output:

30 × 8 ft standard build, no slides, 30,000 BTU input furnace at 75%. The Propane Duration calculator uses a flat planning duty cycle of (68 − outside) ÷ 100; this table is the rig-specific one.

A furnace that never cycles off is either undersized for the night or heating a rig leakier than its build class assumes. Slides with tired gaskets, an open underbelly, a roof vent left cracked and a single-pane window wall all move a rig toward the "canvas" column. Skirting a parked trailer is the one cheap fix that moves the other way: it takes the wind off the floor and the tanks, and in practice turns a standard build into something close to a laminated one for the floor's share of the loss.

Can a space heater heat an RV?

One heater covers cool weather and two cover freezing weather, and at two you're at the edge of a 30 A service. A 1,500 W heater delivers 5,118 BTU/h, so the 30-footer's 10,763 BTU/h at 30 °F takes 3 heaters; two get you to 10,236, 527 short, which the furnace makes up in a few minutes an hour. How many heaters by rig and temperature:

Standard build, holding 68 °F; heaters rounded up, so "3" may mean two heaters and a furnace that cycles. The 40 ft rig has 8 ft sidewalls.

How many space heaters can I run on 30 amp?

A 30 A pedestal is 3,600 W, and a heater is a continuous load, so the electrical code's 80% rule puts the comfortable ceiling at 2,880 W. Two heaters at 3,000 W hold on a mild evening and trip on a cold one when the pedestal breaker is already warm, and with the water heater on its electric element, one heater is the limit. The combinations, checked the same way the shore power calculator checks the air conditioner:

"Fits, hot" = under the breaker but over the 80% continuous rule. 50 A is two 6,000 W legs; three heaters on one leg is still a problem the panel decides.

Two things the tables can't see: the heater's own cord and outlet. A 1,500 W heater pulls 12.5 A through a 15 A receptacle for hours, which is fine for a good outlet and a plug straight into the wall, and a fire risk for a power strip or a worn outlet that runs warm. Run heaters on separate circuits, never through a strip, and put the water heater on propane the night you run two.

Is an RV heat pump better than the furnace?

Above its 40 °F lockout a heat pump is the cheaper heat: the furnace's job for the price of a 1,500 W heater and none of the propane. A rooftop heat pump is the air conditioner run backward, about 12,000 BTU/h of heat at 47 °F for 1,500 W. Its output falls as the outdoor coil gets colder, and at about 40 °F the unit locks itself out to keep the coil from icing, which is exactly the night you wanted it. Against the 30 ft trailer's loss, the same envelope the AC sizing calculator runs in July:

Typical rooftop heat pump (12,000 BTU/h at 47 °F, fading linearly toward 20 °F, 40 °F lockout); check your unit's data plate. Standard-build 30-footer, no slides.

So the practical setup is both: heat pump for the shoulder-season evening on a pedestal, furnace armed underneath it for the hours after midnight. The calculator shows the heat pump line whenever your rig has one saved, and says "furnace" plainly when the temperature you entered is below its lockout.

What lowers the number?

The temperature gap moves it most, then the skin. Dropping the set point from 68 to 62 °F on a 30 °F night cuts the loss by 16%, and it costs a blanket. Skirting and an enclosed underbelly attack the floor, which is 24% of the skin on the worked trailer. Reflective window covers turn R-1 glass into something like R-3 across the leakiest 15% of the wall. Closing off a slide room you're not using shrinks both the area and the air volume. And a leak found with a candle on a windy night (the slide gasket, the fridge vent, the cable hatch) is often the cheapest 1,000 BTU/h on the list. Run the tool with the build class one step better and the slides at zero to see the ceiling on those fixes for your rig; the propane calculator turns the resulting duty cycle into days per cylinder, and the Suburban 2025 catalog lists its SF furnaces from 20,000 to 40,000 BTU/h if the answer is a bigger one.