An office manager in Holborn described the problem perfectly: “The AC makes a noise like it’s starting up, runs for about two minutes, then clicks off. Five minutes later it does the same thing. It’s been doing it all day and the office is getting warmer.” That’s short cycling — the compressor starts, runs briefly, shuts down on a safety, waits for the lockout timer to expire, and tries again. It never runs long enough to actually cool the space.
Short cycling is one of the more destructive faults an AC system can develop. Every compressor start draws significantly more current than steady-state running. Frequent starts and stops create electrical stress, mechanical wear, and heat buildup in the motor windings. A compressor that short-cycles for weeks will fail months or years before one that runs in normal cycles.
What Causes Short Cycling
Refrigerant Undercharge
Low refrigerant causes the suction pressure to drop below the low-pressure safety switch threshold. The switch opens, the compressor stops. Pressure equalises, the switch closes, the compressor starts again — and the cycle repeats. This is the most common cause we see in systems older than 3-4 years.
Thermostat Problems
A thermostat in direct sunlight, above a heat source, or in a draught can send erratic signals to the system. It reads the temperature as too warm, calls for cooling, then immediately reads a drop and switches off. This is particularly common with poorly positioned wireless room sensors.
Oversized System
A system that’s too powerful for the space cools it so quickly that the thermostat is satisfied before the compressor has run long enough to establish stable operation. The compressor cycles off, the temperature rises slightly, and it starts again. This is a design problem — the system needs to be correctly sized for the heat load, not oversized “just in case.”
Blocked Airflow
Clogged filters or a blocked evaporator coil restrict airflow across the indoor unit. The evaporator gets too cold, ice forms, and the system trips on low-pressure or anti-freeze protection. Clean filters and coils are the first thing we check.
Electrical Issues
Failing contactors, weak capacitors, and loose electrical connections can cause the compressor to start and immediately trip. The electrical supply itself might be the issue — voltage drops or phase imbalance on three-phase systems cause protective relays to open.
What Not to Do
Don’t keep resetting the system hoping it’ll sort itself out. Every short cycle stresses the compressor. If it’s done it three times in a row, turn it off and call an engineer. Also don’t lower the thermostat setting thinking it’ll “force” the system to run longer — it won’t help and may make things worse.
Short cycling always has a cause and it’s always fixable. Call 020 3974 1419 and we’ll diagnose it properly.