Why Is My AC Not Cooling? The 10-Minute Diagnosis (Old Houses Included)

A blocked filter, a condenser jammed into a lightwell, or a portable venting through a gappy sash account for more lost cooling in British homes than compressor faults ever will. Refrigerant loss is real, but suspect it last: a system that has genuinely lost gas has a leak, so topping it up without finding that leak is money poured away.

What follows is the order an engineer works in — measure first, free checks next, expensive ones last — plus the case the manual ignores: a 2.5 kW split in a solid-walled Victorian room with 3.2-metre ceilings and lath-and-plaster overhead.

Everything below depends on one number. Run the system in cooling at its lowest setpoint and highest fan for 15 minutes, then hold a probe thermometer in the return air at the intake grille, and again in the air leaving the louvres. The difference is your diagnosis.

• •8-11°C split: the refrigeration circuit is fine. The problem is the room, not the machine.
• •4-7°C: airflow is restricted or the charge is low. Work through the causes below.
• •Under 3°C: the compressor is not running, is short-cycling, or the charge is badly down.
• •Lights on but no air moving: indoor fan motor, run capacitor, or a comms fault between the units.
• •Ice on the coil or pipework: run fan-only for an hour to thaw it. Running it iced wrecks compressors.
• •Then step outside: air off the outdoor unit should be unmistakably hot and moving hard. Weak and lukewarm means the fan or compressor is failing.

• •Cool air but never reaches setpoint — undersized, or solar and solid-wall heat gain is outrunning it.
• •Fine last summer, weak this year — dirty filters and a furred condenser, in that order.
• •Water down the wall or the ceiling below — blocked condensate drain or a failed pump.
• •Humming outdoor unit, static fan — seized motor or dead capacitor. Kill it at the isolator.

On a British callout list, roughly this order holds — and the first three are free to put right.

• •1. Filthy filters. Two clogged mesh filters cut airflow 30-40%, drop the split to 5°C and ice the coil. In pollen season wash them fortnightly, using the method in our guide to cleaning an air conditioner.
• •2. Restricted or recirculating condenser: fins packed with seed fluff, or a unit boxed into a side return re-breathing its own 45°C exhaust. Expect to lose a fifth of capacity.
• •3. Wrong mode. Dry and fan both feel like weak cooling, as does 24°C in a room gaining heat faster than the unit removes it.
• •4. Undersized equipment — the commonest design error in period homes, and no repair fixes it.
• •5. Blocked condensate drain, which on many units trips a float switch and stops cooling.
• •6. Failed capacitor, fan motor or contactor: a real fault, but cheap at £150-£400 fitted.
• •7. Low charge from a leak, usually at a flare joint — look for oily residue there.
• •8. Compressor failure: rare, and usually the financial end of the line on a domestic split.

A 215mm solid brick wall stores several times the heat of an insulated cavity wall. Over a three-day heatwave that mass charges up, and by 9pm it radiates back at 26-27°C while the unit fights it: day one it holds 22°C, day three it runs flat out at 25°C. The building has become a storage heater — one of the common problems in 100-year-old houses modern kit is never specified for.

• •Sash windows leak continuously at the meeting rail and parting bead. Brush-pile seals cost £60-£120 a window fitted, and the same air leak testing that finds winter draughts finds them in July.
• •Single glazing admits 600-700 W per square metre of unshaded south-facing glass at midday in July — a quarter of a small split's output through one window. External shutters beat internal curtains, which absorb the heat indoors.
• •Ceilings of 2.9-3.4m mean sizing on volume, not floor area: 35-40 W per cubic metre for solid walls and single glazing. A 4m x 5m room at 3.2m is 64 m³, so 2.3 kW minimum — yet 2.0 kW is what usually gets fitted. Check yours against our method for sizing an air conditioner.
• •Lath and plaster will not hold a 9-14 kg indoor unit. Its bracket must land on studs or added noggins; fixings creeping in friable plaster cause the drumming mistaken for a bearing fault.
• •Lightwells and side returns are almost always too tight: allow 300mm behind the outdoor unit and a clear metre in front.
• •Victorian drainage struggles with condensate — 0.5-2 litres an hour in humid weather. Teed into an old hopper head or a shared gully, one blockage sends it into the ceiling below.
• •Pre-1970s wiring meets a portable pulling 1 kW for hours. On rewireable fuses and cloth-covered cable with no RCD that is a real load, and a dedicated supply is squarely electrical work you should never DIY.

Work down the list and stop when the split recovers. Isolate at the fused spur before removing any panel.

• •Thaw any ice on fan-only for 60 minutes. Never chip or heat it.
• •Wash both indoor filters in lukewarm water, dry fully, refit. Ten minutes, free.
• •Brush the outdoor coil in line with the fins and cut planting back a metre. No pressure washer — it folds the fins flat.
• •Flush the condensate line and confirm it discharges to a gully, not a blocked hopper.
• •Check the mode is Cool, the setpoint 5°C below room temperature, and no eco setting is capping the compressor.
• •Kill the gains: blinds down on the sunny side by 10am, doors shut, and never run a single-hose portable in an open hallway — it drags hot air in as fast as it cools.
• •Reset at the isolator for five minutes to clear a latched code. If it returns, believe it.

Refrigerant is the hard boundary. Under GB F-gas rules it may only be handled by a certified engineer, and deliberately venting it is an offence — top-up cans sold online are neither lawful nor sensible on a domestic split. Budget £90-£150 for a diagnostic visit, £250-£600 for leak detection and repair, £150-£350 to recharge.

Electrics come second: a dedicated circuit is notifiable work in England and Wales, and with a pre-1970s consumer unit the board usually goes first — price it against typical electrician rates. Planning is third; an outdoor unit on a listed building or in a conservation area needs consent before, not after.

• •It trips the RCD, MCB or fuse more than once, or smells of burning or hot plastic.
• •The coil ices again within a day of a thaw and fresh filters, or oily residue shows at the flare joints — refrigerant leaving with the compressor oil.
• •Water is staining a lath-and-plaster ceiling below the unit. Saturated plaster fails suddenly, and stopping the leak beats rebuilding the ceiling.