How do I calculate what size air conditioning unit I need?

Calculate the total cooling load by adding fabric heat gains (through walls/roof), solar gains (through windows), occupant heat (90W per person), and equipment heat. Add a 20% safety margin and select a unit with at least that capacity. A typical UK bedroom needs 2-3 kW, a living room 3-5 kW, and an office 5-10 kW depending on size and glazing.

What outdoor design temperature should I use for the UK?

CIBSE recommends using a summer design temperature of 30-32°C for most of England and Wales, based on 1% design conditions (exceeded for only 1% of summer hours). London and the South East may use 33°C. Scotland typically uses 27-29°C. These represent near-worst-case conditions for sizing purposes.

How much cooling capacity does a south-facing room need?

South-facing rooms receive the most solar radiation in summer, with peak solar gains of approximately 200 W/m² through single glazing. This can add 1-3 kW of cooling load depending on window size. Solar control glazing or blinds can reduce this by 50-70%, significantly reducing the required AC unit size.

What is the maximum pipe run for a split AC unit?

Most residential split AC systems allow pipe runs of 15-25 metres between indoor and outdoor units, with a maximum height difference of 10-15 metres. Longer pipe runs reduce efficiency and may require larger refrigerant charges. Check the specific manufacturer data — premium units like Daikin and Mitsubishi often allow up to 30m.

Do I need planning permission for air conditioning in the UK?

For domestic properties, outdoor AC units usually fall under permitted development if they meet noise limits and are not on a wall facing a highway. Listed buildings and conservation areas require planning permission. Commercial installations typically need planning permission and must comply with local noise regulations. F-gas regulations also apply to refrigerant-containing units.

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Air Conditioning Calculator — Room Cooling Load

Calculate the cooling capacity needed for any room. Accounts for solar gain, occupants, equipment, and fabric heat gains per CIBSE Guide A methodology.

Room Length

Room Width

Ceiling Height

Room Type

Outdoor Design Temp

CIBSE summer design: 30-32°C for England

Desired Indoor Temp

Glazing Area

Total window area on primary facade

Primary Window Facing

South/west facing = highest solar gain

How We Calculate This

This calculator estimates the total room cooling load using a simplified version of the CIBSE Guide A methodology. It sums all heat gains that the air conditioning unit must remove to maintain the desired indoor temperature.

Heat gain components

Fabric gain: Heat conducted through walls and roof based on U-values and temperature difference
Solar gain: Radiation through glazing — varies by orientation (south-facing = highest)
Occupant gain: 90W sensible heat per person (CIBSE Guide A)
Equipment gain: Computers, lighting, appliances

Unit sizing

The total cooling load is multiplied by 1.2 (20% safety factor) to determine the recommended unit capacity. This accounts for duct losses, part-load inefficiency, and future increases in heat load.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.