If your shed or warehouse is freezing in winter, buying “a big heater” can feel like the right move. But if the heater is too small, it will run flat out and never really warm the space. If it’s too big, you can end up wasting fuel and money.
This guide walks you through, step by step, how to work out what size infrared diesel heater you actually need – in plain English, without complex maths.
We’ll Cover
How to measure your space | What difference insulation and draughts make | Simple rules of thumb to size a heater | Real-world examples for sheds and warehouses | When to ask a specialist for a proper calculation
Why Getting the Size Right Really Matters
Running Costs
Too much power and you’re burning excess fuel; too little and the heater runs constantly.
Comfort
Under-sized heaters leave staff and animals cold, even if they’re on all day.
Equipment Life
Heaters that are always running flat out may wear out faster.
Safety
Using lots of small plug-in heaters to “top up” heat can be riskier and harder to manage.
Industrial and commercial heater suppliers generally start by looking at room volume, temperature rise and building type to estimate the kW needed. The good news is you can do a simpler version yourself.
Step 1
Measure Your Shed or Warehouse
First, you need to know the size of the space.
Measure the length (m)
Measure the width (m)
Measure the average height (m) – if the roof is pitched, use the average between the eaves and the ridge.
You now have:
Floor area = length × width (in m²)
Volume = length × width × height (in m³)
Both are useful. Many infrared heater guides work in watts per m² for simpler spaces, while some industrial calculators work in watts per m³.
Write these numbers down – they’re the starting point for everything else.
Step 2
Check Your Insulation and Draughts
Two sheds of the same size can need very different heater sizes, depending on how well they hold heat.
Ask yourself:
Are the walls and roof insulated, or is it bare metal or single-skin construction?
Are there gaps and draughts around doors and eaves?
How often are big roller doors opened?
Are the windows single or double-glazed (if you have any)?
Many heater sizing guides use an “insulation factor” or watts per m³ based on how good the building is: better insulation = fewer watts needed.
Very roughly:
Well-insulated modern building – needs less heat per m² or m³
Older, single-skin shed or warehouse – needs a lot more
If your building is basically a big metal box with gaps, expect to size on the higher side.
Step 3
Decide How Warm You Want It
Next, think about the temperature rise you need:
How cold does it get in winter? (e.g. –5°C)
What temperature do you want inside? (e.g. +10°C or +15°C)
The difference between these two is your temperature rise (ΔT). Many industrial heater calculators assume a rise of around 20°C for UK workshops and warehouses.
You don’t always need “T-shirt warm” in a shed. Sometimes you just need it:
Warm enough so staff don’t need to work in thick coats and gloves
Warm enough to protect products, livestock or equipment
If you only need a gentle background warmth, you might get away with a smaller heater or running a bigger heater at lower power.
Step 4
Simple Rules of Thumb for Heater Size
There are many ways to size heaters, and different suppliers use different formulas. For infrared heaters in commercial spaces, some guides suggest 50–80 watts per m² in reasonably insulated buildings, increasing for poorer insulation.
However, with infrared diesel heaters you’re usually looking at kW output rather than panel wattage, so most people use:
Volume-based formulas (using m³ and an insulation factor)
Or tested coverage ranges from the heater manufacturer (e.g. “suits 150–250 m² in a typical shed”).
For a rough starting point (not a guaranteed spec), you can think in terms of:
10–20 kW – small workshops and sheds
20–30 kW – medium sheds and smaller warehouses
30–50+ kW – large warehouses, big livestock sheds and tall industrial spaces
The exact size you need depends heavily on your building and climate, so always treat this as a guide, not a promise.
Step 5
Working Examples
Let’s look at some simple examples using floor area and building type. These are illustrations only – not formal designs.
Example 1 – Small Workshop
Size: 8 m × 6 m, average height 3 m
Floor area: 48 m²
Use: vehicle bay / small workshop
Building: older, not brilliant insulation
For a small but not tiny workshop like this, many people look at heaters in the 10–15 kW range, especially if it’s used every day in winter.
An infrared diesel heater around that output can usually keep this kind of space comfortable when correctly positioned, without the need for a huge power supply.
Example 2 – Medium Farm Shed
Size: 15 m × 10 m, height 4 m
Floor area: 150 m²
Use: machinery shed and general farm work
Building: single-skin with some draughts
This is a bigger, more open space. A heater in roughly the 20–30 kW range is often a better fit, giving you enough power to warm people and equipment in the main work zones.
In some cases, one larger infrared diesel heater is chosen; in others, you might use two smaller units focused on the areas where people actually work, rather than trying to heat the entire shed evenly.
Example 3 – Large Warehouse
Size: 30 m × 20 m, average height 6 m
Floor area: 600 m²
Use: storage and picking
Building: mixed insulation, big loading doors
This is a large space with height and likely heat loss through doors. Here you’re more likely to look at high-output heaters (e.g. 40–60 kW units) or a combination of heaters aimed at specific zones (packing area, loading bay, etc.).
In a case like this, it’s usually worth speaking to a specialist supplier or using their calculator or design service, as there are more variables to consider.
How Infrared Diesel Heaters Behave in Big Spaces
Infrared heaters work differently from traditional “hot air” blowers:
They emit infrared radiation, which warms people, animals and objects first, then the air.
This makes them especially useful in tall or draughty spaces, where hot air would otherwise rise and escape quickly.
With infrared diesel heaters, the diesel provides the heat and a small amount of electricity runs the controls and fan, so you don’t need a big electrical supply.
In sheds and warehouses, this often means you can:
Use a lower kW rating than you’d need with pure warm-air heating for the same comfort, or
Get better comfort in the work zone with similar kW, because you’re heating what matters (people and work areas), not just the roof space.
Other Factors That Affect Heater Size
1. Layout and Work Zones
Do you need to heat:
The whole building, or
Just the areas where people spend time (bench, workstations, packing lines)?
Focusing heat on the work area can reduce the heater size needed compared with trying to heat the full volume evenly.
2. How Often the Doors Open
If you open big doors all the time:
Warm air escapes quickly
With infrared, objects and people stay warmer for longer than the air alone, but you still lose some heat
Buildings with constantly open doors may need:
A bigger heater, or
Two heaters aimed at different zones to recover heat faster.
3. Power Supply and Fuel
Infrared diesel heaters are handy because they:
Run on diesel / red diesel / biodiesel
Only need a standard 230 V socket with very low electricity draw
This means:
You’re not limited by the amperage of your electric supply in the same way as with large electric heaters
You can focus mainly on the kW output and heat coverage, as long as you can safely store and handle fuel
4. Future Changes
Are you planning to:
Extend the building?
Add or remove insulation?
Change how you use the space?
If you know things will grow, it may be worth thinking one step ahead (e.g. choosing a model with enough power for planned changes, even if initially run at lower settings).
Common Sizing Mistakes to Avoid
Here are some pitfalls to watch for:
Choosing purely on price
The cheapest heater may be far too small. If it can’t keep up on the coldest days, it becomes wasted money.
Ignoring ceiling height
A tall building has a much larger volume and can need more heat, even if the floor area doesn’t look huge.
Not thinking about insulation
A well-insulated modern warehouse may need far less kW than an old, draughty shed of the same size.
Trying to heat everything equally
It’s often more sensible (and cheaper) to heat where people work, not every cubic metre of the building.
Mixing lots of small heaters
A few ad-hoc electric or gas heaters can quickly add up in running costs and complexity. One correctly sized infrared diesel heater is usually easier and more efficient to manage.
When to Ask for Expert Help
There’s a point where a simple rule-of-thumb isn’t enough – especially if your building is:
Very large (e.g. 500 m²+)
Very high (6 m+ roof height)
Highly draughty or has special requirements (food production, livestock, public safety rules)
In those cases it’s wise to:
Use a radiant or space heater calculator from a reputable supplier, or
Speak to a company that regularly sizes infrared and industrial heaters for sheds and warehouses
They can take into account:
Your exact dimensions
Insulation level and construction type
Typical outdoor temperatures
Warmth needed for staff, animals or products
and then recommend a specific kW rating or model range.
Bringing It All Together
To size an infrared diesel heater for your shed or warehouse, you don’t need to be an engineer. You just need to:
Measure your space – length, width, height
Think about insulation and draughts – good, average or poor
Decide how warm you want it – gentle background heat or close to room temperature
Use simple rules of thumb and manufacturer coverage ranges to shortlist a suitable kW range
Avoid common mistakes, and ask for expert help when the building is large or complicated
Get those basics right and you’ll be far closer to choosing a heater that:
Actually keeps your shed or warehouse comfortable
Doesn’t cost a fortune to run
Fits the power and fuel you already have on site
If you’re stuck between two sizes, it’s usually better to ask for advice before you buy, rather than guessing and living with a heater that isn’t quite up to the job.
Share this post: