An infrared radiant heater follows the inverse-square law: radiant intensity on the floor falls with the square of distance from the source. The difference between "heats" and "comfortable" is in W/m² at the surface, not in total kW of nominal power.
Effective intensity at the work surface
A person doing static work perceives comfort at 140–180 W/m² of radiant intensity at working-surface level (1.2 m above floor). Below 100 W/m² is felt as "heats but not enough". Above 220 W/m² is felt as "hot head, cold feet".
Typical coverage by mounting height
Solaronics SRII-25 (25 kW nominal, 30° beam angle):
| Mounting height | Effective floor area | W/m² on surface |
|---|---|---|
| 4 m | 56 m² | 178 |
| 5 m | 72 m² | 162 |
| 6 m | 95 m² | 145 |
| 7 m | 120 m² | 128 (edge of comfort) |
| 8 m | 145 m² | 108 (too low for static work) |
For ceilings above 8 m, choose a higher-output unit (SRII-40) or a narrow-beam radiant (15°), used for spot-heating workstations, not area coverage.
Spacing between radiant units, geometry
Rule: spacing between two units = 1.4 × mounting height. At 6 m height, spacing is 8.4 m; at 8 m, 11.2 m. Smaller spacing means overlap (>200 W/m² in the centre), larger spacing means dead zones (<100 W/m²).
For two parallel rows: row-to-row distance = 1.1 × mounting height. The two beam paths must overlap, not leave a gap.
Safety minimum clearances
Every Solaronics radiant data sheet states:
- Minimum distance to combustibles below: typically 1.8 m for SRII range
- Minimum lateral clearance to structure: 0.8 m
- Minimum clearance above (to ceiling): 0.3 m (for upward heat plume)
For wooden or composite ceilings (PUR sandwich), the required distance doubles. For non-combustible composite ceilings (metal-clad sandwich) the data-sheet value holds.
Worked example: 1 000 m² hall
Hall 50 × 20 m, ceiling 6 m, static work throughout. Target: ensure ≥ 140 W/m² over the entire working surface.
- Units: SRII-25, suspended at 6 m, each covers 95 m²
- Required count: 1 000 / 95 = 10.5 → 11 units
- Layout: 4 rows × 3 units (4 × 12 = 48 m of effective length vs 50 m hall, 4 % redundancy)
- Within-row spacing: 1.4 × 6 = 8.4 m
- Between-row spacing: 1.1 × 6 = 6.6 m (rows 1–2: 6.6 m; 2–3: 6.6 m; 3–4: 6.6 m + 0.8 m to sidewall, total 21.4 m for a 20 m wide hall → drop to 6.4 m between rows)
Total nominal power: 11 × 25 = 275 kW. With radiant regime and good insulation this covers ΔT = 30 K (−12 °C outdoor, +18 °C indoor) with 15 % margin.
Common placement errors
Suspended too high: "The ceiling is tall, so we mount close to the ceiling." Consequence: floor intensity too weak, the space never warms up. Correct: drop the unit to 6–7 m (with chains or drop rods) even if the ceiling is 10 m.
Above a workbench in direct line with someone's head: standing 0.5 m below a 4 m suspended radiant, the head intensity is >300 W/m². Never place directly above a permanent workstation, offset by 1.2–1.5 m.
Facing a glazed façade: 30 % of radiation passes through glass (it does not reflect back). On exterior walls, orient the radiant toward the interior, not toward the façade.
Related: heating-power calculation for an industrial hall gives the total required power; this article tells you how many radiant units and in what layout.