Quick answer: measure your space in cubic metres (length × width × height) and pick the smallest Airrex model whose rating covers it. Airrex AH-500i (11 / 15 kW) heats spaces up to 1 500 m³. Airrex AH-900i (19 / 24 kW) heats spaces up to 2 500 m³. For larger halls, use several heaters or heat only the working zone – infrared heat can be aimed exactly where people work.

Step 1 – how big is the space in cubic metres?

Multiply floor length × width × ceiling height. Example: a 20 × 12 m workshop with a 4 m ceiling is 960 m³. A 30 × 25 m hall with a 3 m ceiling is 2 250 m³.

Step 2 – match the space to a model

Both models are diesel-fired infrared heaters with a Blue Flame burner, selectable output (Dual Power), automatic thermostat, built-in WiFi with the Rex Nordic app (optional 4G), and run on renewable diesel HVO100 (EN 15940) or standard diesel (EN 590). Electricity use is only 50–60 W in operation, so a normal 230 V socket is enough.

Step 3 – adjust for real conditions

• Poor insulation, open doors, uninsulated steel or tent structures: choose the larger model or run on HIGH output.
• Very cold climate (well below 0 °C outdoors): go one size up, or use two units in a large hall.
• Only part of the hall is used: size for the working zone, not the whole building. Infrared warms people, machines and floors directly, so a heater placed near the work area is often enough.
• Well-insulated building: once walls and floor are warm, the thermostat keeps the heater running only in short cycles and fuel use drops well below the full-output figures.

Worked examples

• Car repair garage 15 × 10 × 4 m = 600 m³ → AH-500i on LOW output is usually enough.
• Workshop 20 × 12 × 4 m = 960 m³ → AH-500i.
• Warehouse 30 × 25 × 3 m = 2 250 m³ → AH-900i.
• Industrial hall 40 × 40 × 6 m = 9 600 m³ → several AH-900i units placed around the hall, or one unit per working zone. Ask us for a sizing recommendation.

Why infrared instead of a fan heater in a tall building?

A fan heater (torpedo heater) blows hot air that rises to the ceiling and escapes every time a door opens. An infrared heater radiates heat into floors, walls, machines and people at working height. The structures store the heat and give it back, so the temperature recovers quickly after a door is opened and less fuel is burned. Airrex heaters have no fan: no dust circulation, no draught, and only 47–48 dB of noise. Read more: fan heater or infrared heater?

What does it cost to run?

At full output the AH-500i uses 1,05–1,42 litres of diesel per hour and the AH-900i 1,81–2,3 litres per hour. In practice the thermostat cycles the burner once the building is warm. Example from our documentation: an insulated 500 m³ industrial hall kept at +15 °C with 0 °C outside needs only about 2,5 litres of diesel per day for maintenance heating. See diesel heater fuel consumption and running cost.

Safety and ventilation

Airrex heaters must never be used in rooms where people sleep. Like every fuel-fired heater they need supply air; an exhaust pipe can be fitted to lead combustion gases outside. Read how much supply air a large-space heater needs and large space heaters and exhaust management.

Warranty, service and spare parts

Every Airrex heater has a 2-year warranty, extended to 5 years in total when you register the heater. An annual service with the original service kit keeps the extended warranty valid. Rex Nordic keeps 100 % of spare parts in stock and runs its own service centre; delivery within the EU takes 1–5 days from our warehouses in Finland and Germany.