D-024·Design and sizing / Air-conditioning design
Air-conditioning cooling load and sizing
Room cooling loads, thermal capacity and dual heating-and-cooling selection.
Cooling capacity is the rate at which an air-conditioning system can remove heat. It is stated as thermal output in kW. It is not the unit’s electrical input and it does not describe the physical size of the equipment.
The required capacity comes from the cooling load of the room or building. Selecting a unit from floor area alone leaves out several important sources of heat gain.
Information used for a cooling load
A room-by-room assessment considers:
- room volume
- glazing area and orientation
- solar gain through windows and the building fabric
- insulation
- occupancy
- ventilation and infiltration
- internal heat gains
Two rooms with the same floor area can therefore need different cooling capacities. Rooms in the same home can also reach their peak cooling loads at different times.
Heating and cooling duties
A reversible mini-split can provide both heating and cooling. The calculations use different design conditions and gains, so a system intended for both duties needs separate heating and cooling results. A unit selected for one duty is not automatically proven for the other.
Under MIS 3005-D, a whole-home air-to-air heating design uses a BS EN 12831-1 heat-load calculation and selects an indoor unit for each habitable room. This is not a general cooling-load calculation. MCS 031 b covers pre-sale heating performance and expressly excludes cooling-mode performance from its scope.
Nominal capacity and operating range
Product literature can list nominal, minimum and maximum capacities. These values are distinct and should not be treated as interchangeable. Capacity range, modulation and output at the relevant outdoor conditions are part of product selection.
Overheating in new homes
Approved Document O applies to new residential buildings in England. Its current guidance expects practicable passive measures to limit unwanted solar gain and remove excess heat to be considered before mechanical cooling is used for compliance.
Approved Document O does not apply to an AC retrofit in an existing home.
Cooling-load calculation inputs
A room-by-room calculation brings together:
- the rooms included and their calculated cooling loads
- the assumptions for glazing, occupancy and internal gains
- the selected unit’s cooling capacity and operating range
- heating design information where the system will also heat
Related entries
Applies to
Heat
Last reviewed
28 Jul 2026