Air heat loads
Room cooling load from internal and solar gains (simple)
- where
- sensible cooling load (kW)
- number of occupants
- sensible gain per person (W)
- lighting load (W/m²)
- floor area (m²)
- small power load (W/m²)
- sunlit glazing area (m²)
- glazing g-value (ratio)
- peak solar irradiance on the glazing (W/m²)
- latent cooling load (kW)
- latent gain per person (W)
- total cooling load (kW)
- load per m² of floor (W/m²)
Explain this equation
Gives a quick peak cooling load for a room by adding heat from people, lights, equipment and sunshine through windows. Use it to rough-size an indoor unit or check a quotation.
N q_s and N q_l are the sensible and latent heat given off by occupants. L_p A and E_p A are lighting and equipment loads spread over the floor. A_g g I is sunshine through the glass: g is the fraction of the sun's energy the glazing lets in, I the sunshine striking it. Q_tot is the sum and q_a is the load per square metre.
Every watt of electricity used in the room ends up as heat, and the sun's heat through glass adds directly. Dividing by 1000 converts watts to kW.
Sense check: a 50 m² office with six people typically comes out around 50 to 80 W/m². Sunny west-facing glass with a high g-value can double that.
Watch out: this ignores heat stored in the structure, conduction through walls and fresh-air load, so it is a first estimate only.
A quick peak cooling load for a room from people, lights, equipment and sun through the glass. Use it to rough-size a split or VRF indoor unit before a full load calculation.
Enter your values to see the result.
Good to know
Per-person defaults are ASHRAE’s moderately active office work (75 W sensible, 55 W latent); CIBSE Guide A gives similar tables. Lighting and small power should come from the actual installation or CIBSE Guide A; 8 and 15 W/m² are only example office figures. Peak irradiance on a vertical UK façade is typically several hundred W/m² (south and west worst). Choose an indoor unit on its sensible capacity at the design conditions, not just the nominal total.
Common mistake
Adding fabric gains, people and sun at their individual peaks and treating that as simultaneous, or forgetting that the g-value of clear double glazing (about 0.6–0.75) is far higher than that of solar-control glass.
Limits
Steady-state sum with no thermal storage, no fabric conduction, no ventilation or infiltration load and no diversity. For design use the CIBSE Guide A admittance method or dynamic simulation.
Sources
- ASHRAE Handbook — Fundamentals (2017), chapter 18 Nonresidential cooling and heating load calculations, Table 1 heat gain from occupants (moderately active office work: 75 W sensible, 55 W latent); simplified air equations qs = 1.23 Q Δt and ql = 3010 Q ΔW (Q in L/s)
- CIBSE Guide A: Environmental design (2015), chapters 5 and 6 (thermal design, internal heat gains from people, lighting and equipment)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.