Air conditioning not cooling

What it usually means

Air conditioning fails in two quite different ways and they need distinguishing before anything else. Either the unit is producing cold air and there is not enough of it, or the unit is not producing cold air at all. The test takes a minute and needs no tools worth mentioning: run it on cooling at its lowest setting for ten minutes, then compare the temperature of the air being drawn into the indoor unit with the air leaving it. A healthy system will show something in the order of eight to twelve degrees of difference. Anything close to zero means it is not cooling. A good difference with a room that never gets comfortable means it is cooling and losing.

Take the second case first, because it is more often misdiagnosed. A split system is sized for a particular heat gain, and rooms change: a conservatory added, a glass door replaced, a south-facing blind removed, a home office with three computers and a router that used to be a spare bedroom. Add a hot week where the outdoor unit cannot shed heat into forty degree air and the same equipment that coped for years stops coping. Nothing has broken. The same applies to doors and windows left open, and to a unit fighting a room whose curtains admit direct sun all afternoon.

The first case has a short list, and airflow is at the top of it. An indoor filter clogged with dust does not merely reduce output; it starves the coil of air until the coil gets cold enough to ice over, at which point airflow drops further and the unit either blows nothing or protects itself and stops. Ice on the indoor coil almost always means restricted airflow, and the filters in a wall-mounted unit lift out and rinse under a tap in a couple of minutes. The outdoor unit has the equivalent problem in reverse: it has to reject heat, so leaves packed into the coil, a shrub grown around it, or a decorative screen too close will all cut performance.

Then there is refrigerant. This is where the honesty matters, because "it needs a re-gas" is the most oversold phrase in the trade. A sealed circuit does not consume refrigerant. If it is short, it has leaked, and refilling it without finding the leak means paying twice — once now and once next summer. A system genuinely short of refrigerant reports it, and the published fault is a shortage rather than a service item.

Two settings account for a good number of calls. The mode has to be cooling, not fan, dry or auto, and on most remotes the icons are small and easy to change by accident. And the outdoor unit usually has its own isolating switch, fitted outside near the unit, which gets switched off for painting or gardening or during winter and never switched back on — an indoor unit blowing room-temperature air with a silent outdoor unit is very often exactly that.