The core idea
A thermostat is the simplest feedback controller there is: measure, compare with the setpoint, switch. But an on/off (two-position) controller can never hold exactly at setpoint — it needs a deadband (hysteresis). Cooling switches off at 22.0 °C and will not switch on again until 23.0 °C: a 1 K differential. Without that gap the controller would chatter — switching on, overshooting, off, on again within seconds.
The deadband is not sloppiness; it protects the machine. A compressor start is the hardest thing it ever does: the motor draws inrush current ~5–7× its running current, and it must accelerate against a pressure difference before refrigerant pressures have equalised. Every start heats the windings and stresses the valves, so the number of starts per hour — not merely the running hours — sets compressor life. The duty cycle tells you how hard the unit works on average: D = Q_load / Q_capacity — a 12 kW unit on a 6 kW load runs about half of every hour. An oversized unit reaches its high D-efficiency easily but pays in cycles: short bursts, many starts.
There is a three-way trade-off: a wide deadband means fewer cycles but a wider temperature swing; a narrow one holds tight but hammers the compressor. The engineering fix is modulation — an inverter-driven compressor that ramps its speed to match the load and runs continuously at low output instead of bang-bang at full. Controls people call two-position control the crude case and proportional/modulating control the refined one. A small anti-recycle timer (3–5 min minimum off-time) sits on virtually every modern board to guarantee pressures equalise before a restart.
Real-world example
Listen to a kitchen fridge: click, hum for ten minutes, click, silence, repeat every quarter-hour or so. Each click is the deadband working. Now scale up: a fixed-speed 5 kW split unit in a small, well-insulated flat on a mild day faces a 1.5 kW load — duty cycle 30%. It can only run at 100%, so it delivers in 3-minute bursts, eight to ten starts an hour, with the room temperature sawtoothing around the setpoint and long gaps in which the coil warms and moisture control lapses. The inverter mini-split next door faces the same load by running continuously at 30% speed: flat temperature, far fewer starts, better dehumidification, quieter. Industrial plants take the same idea to scale — supermarket compressor racks stage several fixed-speed compressors in and out so no single machine short-cycles.
Common pitfall
Narrowing the thermostat differential to "hold the temperature tighter" — 0.2 K instead of 1 K, or defeating the anti-short-cycle delay. The controller obligingly starts chattering: the contactor's contacts pit and weld, the motor bakes on inrush current, and the compressor dies young. The deadband is a designed-in mercy for the machine, not an error to be tuned away. (Related myth: setting the dial far below the target does not cool faster — cooling rate is set by capacity, not by how deep the setpoint is.)