Short cycling is a heat pump compressor starting and stopping in rapid succession — run times under about 10 minutes, or more than four to six compressor starts per hour — instead of long, steady runs. It is caused mainly by oversizing and too little system water volume, and it raises electricity use while accelerating compressor wear.
Why does a heat pump keep starting and stopping?
A heat pump short cycles when the heat it produces has nowhere to go: the compressor starts, satisfies the small connected load within minutes, hits its high-temperature cut-off and stops — then restarts as soon as the water cools. The two dominant causes are oversizing and insufficient system water volume, and they usually appear together.
Even an inverter compressor has a modulation floor, typically around 30% of rated output. A 12 kW unit therefore cannot produce less than roughly 3.6 kW continuously — yet in mild weather a well-insulated European home may need only 1–2 kW. Below the floor, the unit has no option but to stop and start. Zone valves and thermostatic radiator valves make it worse by closing emitters and shrinking both the load and the water volume the heat pump can push heat into. The full symptom-by-symptom diagnosis is covered in Why does my heat pump keep switching on and off?.
How much damage does short cycling actually do?
Each start is the most stressful event in a compressor's life: inrush current several times the running current, momentary oil starvation before lubricant circulates, and thermal shock across motor windings. Compressor longevity correlates with start counts as much as with running hours, which is why manufacturers design controls to minimise starts.
Efficiency suffers too. A compressor spends its first minutes after start-up building the refrigerant pressure differential before delivering useful heat, so a system that runs in 5-minute bursts repeats that unproductive phase dozens of times a day. The result shows up as a lower seasonal COP and a higher electricity bill for the same delivered heat — one reason field-measured performance so often trails the SCOP on the datasheet.
How do I know if my heat pump is short cycling?
Check the controller or app statistics: modern units log compressor starts and run hours. As a rule of thumb, sustained operation at more than four to six starts per hour, or average run times under about 10 minutes in cold weather, indicates short cycling. A flow-temperature graph that looks like a sawtooth — sharp rise, cut-off, fast fall, restart — tells the same story.
| Symptom | Likely cause | First fix |
|---|---|---|
| Cycling only in mild weather | Load below modulation floor | Weather compensation, wider hysteresis |
| Cycling in all weather | Unit oversized for the building | Check sizing against a room-by-room heat load |
| Cycling when zones close | Too little open system volume | Open emitters, add volume |
| Cycling during hot water runs | Undersized cylinder coil | Review coil surface area |
How do you stop a heat pump short cycling?
Start with the free fixes: enable weather compensation so flow temperature tracks the real load, widen the control hysteresis, and keep as many emitters open as possible — a heat pump prefers to run long and low.
If cycling persists, add water volume. A buffer tank sized at roughly 20–50 litres per kW of heat pump output gives the compressor a thermal flywheel to charge, while a simpler in-line volumiser can add volume where no hydraulic separation is needed; Solimpeks builds its Solibuffer range from 50 to 1,000 litres for exactly this duty. If the unit cycles even with generous volume and open zones, the machine itself is too large — work through Is my heat pump oversized? before replacing hardware, because no accessory fully rescues a badly oversized unit.
Frequently asked questions
How many compressor starts per hour is normal for a heat pump?
A well-matched inverter heat pump in steady heating should run continuously or cycle only a few times per hour. Sustained operation above four to six starts per hour is generally treated as short cycling and worth investigating.
Does short cycling increase my electricity bill?
Yes. Every start repeats an unproductive pressure-building phase before useful heat flows, so the same delivered heat costs more electricity. Frequent cycling is a common reason measured seasonal performance falls short of the datasheet SCOP.
Will a buffer tank always fix short cycling?
No. A buffer sized at 20–50 litres per kW fixes cycling caused by low system volume or closing zones. If the heat pump is simply oversized for the building's heat load, it will still cycle in mild weather — sizing has to be corrected at the unit, not the pipework.
Is short cycling worse in winter or in mild weather?
Usually mild weather. In spring and autumn the building load drops below the compressor's minimum modulated output, forcing stop-start operation. In deep winter the load is large enough to keep the compressor running continuously.