Cooling Load Calculator — Total Heat Gains

A cooling load calculation sums all heat gains the cooling system must remove: solar gain through windows (often the largest component), internal gains from people (about 70-90W sensible each), lighting, and equipment, plus fabric gains through walls and roof. CIBSE Guide A provides the methodology used in UK building services design.

Per CIBSE Guide A, a sedentary person gives off roughly 90W of sensible heat at a 20°C room, falling to about 80W at 22°C and 70W at 24°C as more of the body’s heat shifts to latent (the total stays near 140-150W). People doing moderate physical work give off 140-210W sensible. This calculator scales the sensible figure with your indoor design temperature, so at the default 24°C it uses about 70W per person.

The g-value (solar factor) is the fraction of solar radiation that passes through glazing. Single glazing has a g-value of about 0.85, double clear glazing 0.72, low-E double glazing 0.50, and solar control glass as low as 0.25. Lower g-values mean less solar heat gain but also less natural light.

A typical UK office has a cooling load density of 60-100 W/m². Heavily glazed offices facing west, south or east (which take the worst low-angle summer and early-autumn sun) can exceed 150 W/m². Server rooms may reach 500-2000 W/m². These figures help validate detailed calculations and provide quick estimates for budget purposes.

Yes, in a full CIBSE calculation, infiltration and ventilation loads should be included. Air entering the building at outdoor temperature must be cooled to indoor temperature. This calculator focuses on the primary gains; add approximately 10-15% for infiltration in naturally ventilated buildings.