When someone asks “what size space heater do I need?”, they are really asking how much heat output it takes to warm their space efficiently. Getting this right matters. An undersized heater struggles to reach a comfortable temperature, and an oversized one wastes fuel.
In this guide we show you how to estimate the output you need, what affects it, and how it matches the JCB Tools range of diesel space heaters.
Why heater sizing matters
- Too little output: the heater runs constantly, struggles to reach the set temperature and may wear faster.
- Too much output: you waste fuel and overshoot your target temperature.
- The right output: a comfortable space, lower running costs and a longer equipment life.
Step 1: Measure your space
You need three measurements in metres: length, width and height. Multiply them to get the volume:
Volume (m³) = length × width × height
For example, a workshop 10 m long, 8 m wide and 3 m high has a volume of 240 m³.
Step 2: Estimate the output you need
JCB lists the heating coverage of each heater in cubic metres, so the simplest check is to compare your volume with that figure. Across the JCB range, the coverage works out at roughly 40 to 50 watts of heat per cubic metre for a typical building.
For the 240 m³ workshop above, that means around 10 to 12 kW, well within the 20 kW JCB-SH70D.
Step 3: Convert between kW and BTU
Heaters are rated in kilowatts (kW) or British Thermal Units per hour (BTU). To convert:
- 1 kW = 3,412 BTU
- BTU ÷ 3,412 = kW
For example, a 63 kW heater produces about 215,000 BTU.
Step 4: Allow for heat loss
Your building loses heat faster, and needs more output, if it has:
- poor or no insulation
- large doors or bays that are opened often
- high ceilings, where warm air collects above the working area
- a big difference between the outside temperature and the temperature you want inside
If any of these apply, choose a heater with 10 to 20% more output than your estimate, or the next size up.
Step 5: Match your space to a JCB heater
JCB Tools offers three heavy-duty diesel space heaters for workshops, warehouses and industrial buildings:
| Model | Output | Coverage | Best for |
|---|---|---|---|
| JCB-SH70D | 20 kW / 68,243 BTU | Up to 496 m³ | Small to medium workshops and garages |
| JCB-SH140D | 37 kW / 126,250 BTU | Up to 800 m³ | Medium industrial spaces |
| JCB-SH215D | 63 kW / 215,000 BTU | Up to 1,300 m³ | Large warehouses and production halls |
Example: a workshop 20 m long, 10 m wide and 4 m high has a volume of 800 m³. The JCB-SH140D covers up to 800 m³, so it is a match. If the building is poorly insulated or very open, the 63 kW JCB-SH215D gives you headroom.
Step 6: Final checks
- Thermostat: all three JCB heaters have a built-in thermostat, so they only burn the fuel needed to hold your set temperature.
- Heat direction: each model has a fitted flame cone to aim heat at the areas where people work.
- Running costs: check fuel use per hour. The JCB-SH70D uses about 1.9 litres per hour.
- Headroom: a heater that runs below full output most of the time copes better on the coldest days.
- Safety: always follow the ventilation and safety guidance in the user manual.
In short
- Measure your space and work out its volume in m³.
- Compare it with each heater’s coverage, allowing roughly 40 to 50 W per m³.
- Add 10 to 20% for poor insulation, open doors or high ceilings.
- Choose the JCB heater that covers your space with some headroom.
Follow these steps and you will choose a heater that keeps your space warm without overspending on fuel. For a closer look at the range, read our guide to heating workshops and garages with diesel space heaters, or shop JCB space heaters.
Need advice? Call our team on 01646 687 880 or contact us online.
