Quick answer: Measure length × width × ceiling height in feet to get cubic feet, then pick the smallest Rex Nordic model whose rating exceeds it: AH-210i (44,000 BTU/h) up to ~31,800 ft³ · AH-310i (51,000 BTU/h) up to ~53,000 ft³ · AH-810i (75,000 BTU/h) up to ~70,600 ft³.
Step 1 — your space in cubic feet
Example: a 40 × 60 ft shop with 16 ft ceilings = 38,400 ft³.
Step 2 — match a model
| Model | Output | Coverage (up to) | Typical buildings |
|---|---|---|---|
| AH-210i | 44,000 BTU/h (13 kW) | ~31,800 ft³ (900 m³) | 2–3 car garages, hobby shops, farm outbuildings |
| AH-310i | 51,000 BTU/h (15 kW) | ~53,000 ft³ (1,500 m³) | 40×60 shops, pole barns, service bays |
| AH-810i | 75,000 BTU/h (22 kW) | ~70,600 ft³ (2,000 m³) | Warehouses, halls, large enclosed job sites |
Step 3 — adjust for reality
Ratings assume a reasonably sealed building. Go one size up for poor insulation, frequently open doors, or design temperatures below ~0 °F. Infrared also lets you spot-heat a work zone inside a bigger hall — size to the zone, not the building.
Worked examples
30×40×12 garage = 14,400 ft³ → AH-210i. · 40×64×14 pole barn = 35,840 ft³ → AH-310i. · 60×80×14 warehouse bay = 67,200 ft³ → AH-810i.
Why infrared beats forced-air in tall buildings
Forced-air stratifies at the ceiling; radiant infrared warms the slab, the steel and you — comfort at working height first, faster recovery after the door opens, less fuel doing it.
What it costs to run
Full-output fuel use: 0.25 / 0.30 / 0.50–0.58 gal/h (210i / 310i / 810i) — $0.90–$2.00 per hour at $3.50/gal, and the thermostat cycles the burner once you're at temperature. Electricity: ~80 W ≈ one cent per hour.
Still unsure?
Call and we'll size it in two minutes: USA +1 (855) 578-9090 · Canada +1 (855) 508-9090 — or see all heaters.
Related: Infrared vs. torpedo vs. propane · USA & Canada FAQ · Ordering & shipping FAQ