Why so many aircon installs are underpowered
Published 24 Jul 2026 · sizing methodology: CIBSE Guide A (environmental design) · checked 24 Jul 2026
Most disappointing air conditioning was not broken at install. It was sized by habit: the installer quoted the unit they fitted last week — a 2.5kW for a bedroom, a 3.5kW for a living room — and nobody measured how much heat your particular room actually gains. On the August evening you bought it for, the unit runs flat out and the room never quite gets there.
What actually decides how much cooling a room needs?
Not floor area. Heat gain. The load on a room is the sum of what comes through the glass, the walls and the roof, plus what the occupants and their equipment give off — and the spread between similar-sized rooms is enormous. The biggest single variable is glazing: unshaded glass in direct sun admits several hundred watts per square metre, so a wide west-facing window can add more load than doubling the floor area. Then orientation (west beats south for evening misery, because the sun is low and the day’s heat has accumulated), then what is above you — a top-floor room under an uninsulated roof carries a load a first-floor room never sees. Two flats with identical floor plans can need double the capacity of each other (est.).
London’s stock makes this worse at both ends. Victorian solid-wall terraces store the day’s heat and release it into the night; new-build flats with floor-to-ceiling west glazing overheat so reliably that the Building Regulations grew a whole section about it.
Approved Document O (overheating), Building Regulations 2010 · applies to new residential buildings · checked 24 Jul 2026
How can you tell if a quote was calculated or copied?
By what the installer asked. A calculated quote requires questions: which way the windows face, how big they are, what floor you are on, what is above the ceiling, how many people use the room and when. If the survey never touched those — if the number arrived from the floor area and a glance — it is a copy of the last job, not a design for yours.
Ask directly: “What did you calculate the peak heat gain for this room to be, in watts, and what did you assume for the glazing?” A good installer answers without blinking. The wrong answer is a BTU figure recited from the box.
Undersized, oversized — both are real failures
Undersized is the common one and the crueller: the unit runs continuously at full power on the evenings that justified the purchase, loud and losing. Oversized fails differently — it hits temperature fast, switches off before it has dehumidified, and leaves the room cold-yet-clammy while cycling itself to an early death. The fix for both is the same boring discipline: size from the calculated gain, not the habit.
Sizing has one more consequence flat owners should know: an honest calculation sometimes shows that one properly sized unit does the work of the two you assumed — which matters when the permitted-development route allows exactly one outdoor unit per block.