What size heater does a campervan need? 2, 4 or 6 kW, decided by the heat loss
· 8 min read
A 2 kW diesel or gas air heater covers an insulated high-roof van down to about −5 °C. With 25 mm of board on floor, walls and ceiling, 3 m² of glass and normal ventilation, a Ducato L3H2 loses about 1.3 kW at −5 °C outside and 20 °C in. The same van bare loses almost 5 kW and needs the 5–6 kW class. Every heater-sizing answer is one of those two sentences with different numbers, and the numbers come from three terms you can compute in a minute.
This guide does that for eight vans. The heater size calculator does it for all 159 variants, with your insulation, your glass and your climate.
The three terms
Heat leaves a van three ways, and a heater has to replace all of it at steady state:
- Through the shell: Σ (area ÷ R) × ΔT for floor, walls and ceiling. R is the thermal resistance of the finished surface — 0.18 m²K/W for bare steel and its air films, plus insulation thickness ÷ λ. ΔT is inside minus outside temperature.
- Through the glass: area × U × ΔT. Single glass (the whole cab, most sliding windows) is U ≈ 5.8 W/m²K; double-skin acrylic habitation windows about 3.0. A windscreen and two cab doors alone are ~2.5 m².
- With the air: 0.33 × air changes per hour × cabin volume × ΔT — the metric form of the 1.08 × CFM × ΔT term in every American heat-loss sheet. Fixed EN 721 vents and a normally sealed build sit around 0.5 ACH.
Add them up, and the heater class follows: up to 1.7 kW of loss, a 2 kW unit (Eberspächer Airtronic D2, Webasto Air Top 2000, Propex HS2000); up to 3.3 kW, the 4 kW class; above that, 5–6 kW or a better shell. A heater that has to run flat out all night is loud, sooty and short-lived — you want the class above your loss, not on it.
Eight vans at −5 °C
Computed from each van's interior dimensions at the cold preset: −5 °C out, 20 °C in, 3 m² of single glass, 0.5 air changes per hour. “Bare” is unpainted steel; “25 mm” is the planner's default build-up — PIR on the floor, XPS on walls and ceiling. Diesel is for an 8-hour night at 9.9 kWh/L and 85 % efficiency.
| Van | Bare | 25 mm board | Heater class | Diesel / night |
|---|---|---|---|---|
| Volkswagen Transporter L2H1 | 3.2 kW | 0.9 kW | 2 kW | 0.9 L |
| Ford Transit Custom L2H1 | 3.3 kW | 1.0 kW | 2 kW | 0.9 L |
| Fiat Ducato L2H2 | 4.2 kW | 1.1 kW | 2 kW | 1.1 L |
| Mercedes-Benz Sprinter L2H2 | 4.4 kW | 1.2 kW | 2 kW | 1.1 L |
| Ford Transit L3H2 | 4.4 kW | 1.2 kW | 2 kW | 1.1 L |
| Fiat Ducato L3H2 | 4.8 kW | 1.3 kW | 2 kW | 1.2 L |
| Volkswagen Crafter L3H3 | 4.5 kW | 1.2 kW | 2 kW | 1.1 L |
| Fiat Ducato L4H2 | 5.2 kW | 1.3 kW | 2 kW | 1.3 L |
The pattern: insulated, every van in the table is a 2 kW job; bare, every van above a Transporter is 4 kW or worse. The insulation is not a comfort upgrade — it is the difference between the smallest heater sold and the largest.
What a night costs
A heater at steady state delivers exactly the loss. 1.5 kW over 8 hours is 12 kWh: roughly 1.4 L of diesel at 85 % efficiency, or about 1 kg of LPG — a 11 kg bottle is ten cold nights, which is the number to carry into the gas locker plan. From a 12 V battery the same night would be 1,000 Ah, which is why nobody heats a van electrically off-grid and why a 2 kW heater's 10–20 W of fan is the only electrical line that matters.
The glass is a third of the bill
Run the Ducato insulated: shell ~0.8 kW, glass ~0.44 kW, air ~0.06 kW. Once the walls have 25 mm on them, a third of the remaining loss goes through 3 m² of cab glass, and doubling the wall insulation shaves a tenth. The cheapest kilowatt after the first board is an insulated cab divider or fitted screen covers, not a second layer of foam. The calculator flags this whenever glass passes 30 % of the total.
The same glass works the other way in summer: at 250 BTU per square foot and hour of sun-struck window — about 790 W/m² — 3 m² of glass in full sun brings in 2.4 kW, more than most heaters put out. Shade before you size air conditioning.
Diminishing returns
The first 25 mm of XPS takes a wall from R 0.18 to R 0.94 — an 81 % cut in that surface's loss (PIR: 86 %). The second 25 mm takes it to R 1.70 and removes another 8–9 points. Every centimetre of the second layer also comes off the inside of the van twice, which the space-cost guide prices in beds and headroom. For most builds the answer is one good layer everywhere, the ribs filled, the cab dealt with — then the heater class is 2 kW and the argument is over.
Heaters need air, not just fuel
A diesel air heater draws its combustion air from outside and is room-sealed; a gas catalytic or open-flame heater is not, and EN 721 sizes fixed ventilation for it. The 0.5 ACH in the table is what those fixed vents cost you in heat — the ventilation guide explains why you keep them anyway.
Do it with your van
The calculator takes any of the 159 variants or a typed box, a material and thickness per surface, your glass and your climate, and returns watts, BTU/hr, the heater class and the fuel per night. Inside the VanPlot editor the same model reads the shell you actually built — every window and fan cut out, the insulation you specified — and reports the heater class in the Build Sheet next to your power budget. Size yours.
