The three most common sizing failures: oversized (short cycling, poor efficiency), undersized (system never reaches target temperature at −10 °C outdoor), or correctly sized for one zone instead of several (workers in the colder end of the hall start bringing in their own electric heaters). Step by step below.
Step 1: Volume and insulation coefficient
Hall volume (V) in m³ is length × width × average ceiling height. For mixed-height geometries (partly 8 m, partly 12 m) take the area-weighted mean.
Heat-loss coefficient (k) is fixed by the building's insulation class:
- Poorly insulated older industrial hall (uninsulated steel cladding, prefab roof without insulation): k = 0.8 – 1.0 W/m³·K
- Standard industrial hall (sandwich panel 50–80 mm, insulated roof or steel-PUR sandwich): k = 0.5 – 0.7 W/m³·K
- Well-insulated modern hall (sandwich 100–150 mm, or masonry with 250 mm insulation): k = 0.3 – 0.4 W/m³·K
For radiant heating (infrared, gas radiant plaques) you may reduce k by 25 % relative to the values above, because radiant equipment does not need to heat the entire volume to the same temperature.
Step 2: Temperature differential
ΔT = target indoor temperature − design outdoor temperature. For Slovenia, the design outdoor value is −13 °C (zone A, most of the country; zones B and C per SIST EN 12831).
Typical indoor targets by activity:
- Warehouse without permanent workstations: 8–12 °C (condensation prevention, goods protection)
- Production with sedentary work: 18–20 °C
- Production with physical work: 14–16 °C
- Sports and exhibition halls: 15–18 °C
- Vehicle service halls: 10–14 °C
Step 3: Core formula
Q (W) = V × k × ΔT
Example: warehouse 50 × 20 × 8 m = 8 000 m³, sandwich panel (k = 0.6), target 12 °C at −13 °C outdoor (ΔT = 25 K).
Q = 8 000 × 0.6 × 25 = 120 000 W = 120 kW
That is the heat loss for unit-heater (convective) systems. For radiant heating subtract 25 %: 120 × 0.75 = 90 kW.
Step 4: Air exchange and add-ons
Doors that open during operation, mechanical ventilation, and air leakage add 10–30 % to the base loss. Rule of thumb:
- Doors that do not open during operation: add +10 %
- Sectional doors that open 5–10 times per day: add +15 %
- Doors held fully open (warehouse with constant forklift traffic): add +25 to +30 %, or install an air curtain over the door
Final installed power is base loss plus the air-exchange add-on. For 120 kW base + 15 % doors = 138 kW, rounded to the nearest module size (Solaronics Minigaz or AC series for two-zone control).
Tool
We provide a simplified calculator for quick orientation. For a project specification (referencing SIST EN 12831, calculated per Slovenian design rules) you need a qualified HVAC engineer. Request a quote and we will send a project template with measurement points and operational regime steps.
Before sizing, also review ErP class and seasonal efficiency (ηₛ), at identical nominal power a conventional unit burns 8–14 % more gas than a condensing one.
Common errors in initial sizing
Three recurring errors you can prevent with 10 minutes of thought before the quote:
- k-value for masonry used on a sandwich panel. Old 60 mm PUR sandwich panels are often misclassified as insulated masonry (k = 0.3). Real k = 0.55 to 0.65. Verify panel thickness and installation year.
- Gross floor volume instead of working height. A hall with a 12 m ceiling and a 4 m working zone has no 12-metre heat demand, for radiant heating, compute working height + 1 m. Keeps power within budget, avoids short-cycling.
- Ignoring seasonal operating regimes. Plan profiles: night setback of 6 K saves 18–25 % of annual consumption; weekends + holidays in logistics add another 8–12 %. Without these profiles, installed power is 15 % oversized.