Open doors admit warm, humid air. A simplified estimate uses Q = V × n × density × enthalpy difference / 86,400.
In short: the uncertain input is air changes; it depends on opening size, time, traffic, curtains and building pressure.
Use a recognised method for final design. Reduce load with short openings, good seals, closers and suitable barriers.
The formula
Q_infiltration = V × n × ρ × Δh / 86 400
- V = internal volume (m³)
- n = air changes per day (from the table below)
- ρ = air density (~1.2 kg/m³)
- Δh = enthalpy difference between outside and inside air (kJ/kg)
- divide by 86 400 (seconds per day) to express the result in kilowatts (kW) — multiply by 1 000 for watts (Δh is in kJ/kg)
Air changes per day
Air changes per day fall as the room gets bigger (the open door is a smaller fraction of the volume) and rise with traffic:
| Room volume | Heavy use (changes/day) | Normal use (changes/day) |
|---|---|---|
| 5 m³ | 44 | 17 |
| 10 m³ | 32 | 12 |
| 25 m³ | 20 | 7.5 |
| 50 m³ | 14 | 5.3 |
| 100 m³ | 10 | 3.8 |
Reducing infiltration
Reducing infiltration is the cheapest load you can engineer out. Strip curtains, air curtains, fast-acting or self-closing doors, and disciplined door procedure all cut n directly. For freezers, this also reduces frost build-up on the evaporator.
See Door types and specifications for the hardware that lowers n.
Related articles
- Heat Load Calculation: The Four Components
- Transmission load explained
- Product load explained
- From total load to design load
- Door types and specifications
For a checked calculation, use the Frostbox size calculator or contact [email protected].
