How to Size an RV Furnace: BTU Calculator Guide for Every Rig
Buying a new RV furnace without knowing the right BTU output is like buying a water heater without checking the tank size — you might get lucky, but you're more likely to end up cold on a winter trip. Getting the BTU sizing right is one of the most important decisions you'll make when replacing your furnace, and it's easier than most people think.
This guide walks you through exactly how to size an RV furnace by BTU rating. Whether you're in a small travel trailer or a 40-foot fifth wheel, you'll know exactly what you need before you buy.
Why BTU Sizing Matters for RV Furnaces
BTU stands for British Thermal Unit — it's the standard measure of heating output. A furnace with too few BTUs won't keep your rig warm when temperatures drop. A furnace with too many BTUs cycles on and off too fast, wastes propane, and can cause uneven heating throughout your RV.
Most RV furnaces range from 16,000 BTU to 40,000 BTU. The sweet spot for your rig depends on three things:
- Square footage of your living space
- How well-insulated your RV is
- The climate you camp in most
Getting all three right means comfortable heat, lower propane bills, and longer furnace life.
RV Furnace BTU Chart by RV Type and Length
Use this chart as your starting point. These are general guidelines — adjust up if you camp in extreme cold or have poor insulation.
| RV Type | RV Length | Recommended BTU Output |
|---|---|---|
| Pop-up / Folding Camper | Under 16 ft | 16,000 BTU |
| Small Travel Trailer | 16–20 ft | 20,000–25,000 BTU |
| Mid-Size Travel Trailer | 21–28 ft | 25,000–30,000 BTU |
| Large Travel Trailer | 29–35 ft | 30,000–35,000 BTU |
| Fifth Wheel | 28–36 ft | 30,000–40,000 BTU |
| Large Fifth Wheel / Toy Hauler | 36 ft+ | 35,000–40,000 BTU |
| Class B Van / Class C (small) | Under 25 ft | 16,000–20,000 BTU |
| Class C (mid-size) | 25–32 ft | 25,000–30,000 BTU |
| Class A Gas | 32–38 ft | 30,000–40,000 BTU |
| Class A Diesel Pusher | 38 ft+ | 35,000–40,000 BTU |
Note: Many larger Class A coaches use dual-zone heating systems with two furnaces. If your RV came with two furnaces from the factory, replace each with a unit of the same BTU rating.
How to Calculate Your Furnace BTU Needs
If you want to go beyond the chart and get a more precise number, here's a simple formula used by RV technicians:
BTU Needed = RV Square Footage × 20 (mild climate) to 30 (cold climate)
Example:
- Travel trailer with 280 sq ft of living space
- You camp in cold weather (below 20°F regularly)
- 280 × 30 = 8,400 BTU per hour minimum
Wait — that seems low compared to the chart numbers. That's because the chart above already builds in overhead for startup cycles, heat loss, and ducting inefficiency. Real-world RV furnaces run at around 60–70% efficiency due to duct runs and air mixing, so the chart recommendations already account for that buffer.
The takeaway: trust the chart for most situations, and use the formula to double-check if you're on the fence between two BTU sizes.
Adjusting for Climate and Insulation
Two factors can push your BTU needs up or down significantly:
Cold-Climate Camping (Below 20°F)
If you regularly camp in freezing temperatures — think mountain camping, winter storage trips, or full-timing through northern winters — size up by one tier. An RV listed at 25,000 BTU in the chart should probably be replaced with a 30,000 BTU unit.
Older RVs with Poor Insulation
RVs built before 2010 typically have less insulation in walls, floors, and ceilings than newer models. If your rig is older and drafty, add at least 5,000 BTU to whatever the chart recommends.
Mild-Weather Camping Only
If you only camp in spring and fall, rarely dipping below 35°F, you can size down slightly from the chart. A mid-size travel trailer might do fine with a 20,000 BTU unit instead of 25,000.
Our Top Furnace Recommendations by Size
Here are the most popular furnaces we carry at The RV Surplus, organized by BTU output:
16,000–20,000 BTU — Best for small travel trailers, pop-ups, and Class B vans
Suburban NT-20SEQ (20,000 BTU) and Dometic DFSD20111 (18,000 BTU) are the go-to options here.
25,000–30,000 BTU — The most popular range for travel trailers and mid-size Class C coaches Suburban SF-25FQ and Dometic DFMD30131 are strong sellers. Both are direct-duct models and easy to replace in standard openings.
35,000–40,000 BTU — For fifth wheels, large Class A gas coaches, and toy haulers
The Suburban SF-35VHQ and Dometic DFMD35121 are the top choices in this range. These units move serious air volume and handle large floor plans well.
Pro tip: Always check the duct size and mounting dimensions of your old unit before ordering. BTU output is critical, but so is fitment. Most standard RV furnaces use a 4-inch duct outlet — but confirm before buying.
What to Look For Beyond BTU
Once you've nailed down the BTU range, here are a few other specs worth checking:
- Ignition type: Most modern furnaces use direct spark ignition (DSI). Avoid older models that require manual lighting.
- Blower speed: Higher CFM (cubic feet per minute) means better heat distribution through longer duct runs.
- Propane consumption: Measured in BTU/hour. Larger furnaces consume more propane per hour but run for shorter cycles, so overall usage can be similar.
- Warranty: Suburban and Dometic both offer warranties — check the terms before buying.
Frequently Asked Questions
What BTU furnace do I need for a 30-foot travel trailer?
Most 30-foot travel trailers fall in the 30,000 BTU range. If you camp in colder conditions or have an older, less-insulated rig, go with 35,000 BTU.
Can I put a higher BTU furnace in my RV than the original?
Yes, within reason. Going up one tier (e.g., from 25,000 to 30,000 BTU) is generally fine as long as the unit physically fits and your propane supply can handle it. Going up two tiers may cause short-cycling and uneven heating.
Is a 16,000 BTU furnace enough for winter camping?
It depends on your RV size and temperatures. For rigs under 20 feet in mild cold (above 20°F), yes. For anything larger or colder, you'll likely need more.
Do I need to match the exact BTU of my original furnace?
Not exactly, but stay within one size tier. Most technicians recommend replacing with the same BTU rating when possible for easiest installation and predictable performance.
Does furnace BTU affect propane usage?
Yes. Higher BTU furnaces consume more propane per hour of runtime. However, because they heat the space faster and cycle off sooner, the total propane consumption over a night can be similar to a lower BTU unit running longer.
Ready to Find Your Furnace?
Now that you know what BTU range you need, finding the right model is straightforward. At The RV Surplus, we carry Suburban and Dometic furnaces at discount prices with free shipping to the continental U.S.
Shop RV Furnaces at The RV Surplus →
Not sure if your current model is still available? Check our RV Furnace Crossover Chart to find a direct replacement. And if you're still troubleshooting your existing unit, our RV Furnace Troubleshooting Guide walks you through every common failure.
Questions about sizing? Call or text us at 574-218-0549 — we're happy to help you find the right fit.