A heat pump short-cycles when it makes more heat than the system can absorb. Two to three compressor starts per hour in mild weather is normal; six or more means something is wrong. The three usual causes, in order, are an oversized unit, too little water volume in the circuit, and controls or valves shutting off flow before the compressor has settled.
How many times an hour should a heat pump start?
As few as possible. A heat pump reaches its best efficiency several minutes into a run, so every start costs both wear and energy.
| Compressor starts per hour | Verdict |
|---|---|
| 0 – 1 (near-continuous running) | Ideal, and normal at design conditions |
| 2 – 3 | Normal in mild weather |
| 4 – 6 | Investigate volume, controls and minimum modulation |
| More than 6 | Short cycling — fix before the compressor wears |
Most controllers log run time and start count. If yours does not, watch the outdoor unit for an hour on a mild day: that single observation diagnoses more heat pump complaints than any other measurement.
What causes short cycling?
1. The heat pump is oversized. An inverter unit can usually modulate down to about 25–35% of nominal output. A 12 kW machine therefore has a floor near 3–4 kW, while the house in April needs 1.5 kW — so it produces, overshoots, stops, and repeats. Oversizing is the most repeated grievance in heat pump forums and it cannot be corrected afterwards. It is why sizing must come from a room-by-room heat load calculation rather than from floor area or the old boiler's rating. See what size heat pump do I need?
2. There is not enough water in the system. Manufacturers specify a minimum system volume, commonly 10–20 litres per kW, so that the heat produced during the compressor's minimum run has somewhere to go. Modern low-water-content radiators and short pipe runs rarely meet it on their own. The remedy is a volumiser or a buffer tank, sized per litres of buffer per kW — Solibuffer vessels start at 50 L precisely for this minimum-volume duty.
3. The controls are fighting the heat pump. A room thermostat with a wide on/off hysteresis, thermostatic valves closing as rooms warm, or zone valves shutting circuits all shrink the system's ability to absorb heat exactly when the compressor is running. Replace on/off thermostat control with weather compensation plus one open reference room, and leave the radiator valves in that room fully open.
4. The flow rate is too low. A blocked strainer, a pump on a low speed setting, closed lockshields or 8 mm microbore all widen the temperature difference across the condenser until the unit trips on high pressure. Check the measured ΔT: heat pumps are designed around 5 K, not the 20 K a boiler used.
5. Defrost is being mistaken for cycling. Stopping every 40–60 minutes at 0–5 °C in damp air is normal defrost behaviour, not a fault. A four-minute defrost every thirty minutes is not — that points to poor airflow, a fouled evaporator or a sensor problem.
6. Hot water cycling. If the cylinder coil is too small, the heat pump reaches its high-pressure limit before the cylinder is satisfied, stops, and restarts. This is a cylinder problem, not a heat pump problem.
How do I stop short cycling without buying a buffer tank?
Try these first, in order: open every thermostatic valve in the reference room; lower the weather compensation curve so the heat pump works harder for longer at a lower temperature; raise the circulation pump speed to restore design flow; widen the controller's cycle-suppression or minimum-run-time setting if it has one; and clean the magnetic separator and strainer. If starts per hour stay above four after all of that, the system needs volume — a 25–50 L volumiser on the return is often enough, and is cheaper and smaller than a full buffer.
Does it really matter? Yes. Each start briefly starves the compressor of oil, and every restart repeats the low-efficiency settling period. Cycling is a major reason two identical heat pumps in identical houses return seasonal performance factors of 2.4 and 3.6.
Frequently asked questions
How many cycles per hour is normal for a heat pump?
Two to three starts per hour in mild weather, and near-continuous running in cold weather. Four to six warrants investigation, and more than six is short cycling that should be corrected before the compressor accumulates damage.
Is short cycling caused by an oversized heat pump?
It is the most common cause. An inverter unit typically cannot modulate below about a third of its rated output, so a machine sized well above the building's mild-weather load has no choice but to overshoot and stop repeatedly.
Will a buffer tank stop my heat pump cycling?
It stops cycling caused by insufficient system volume, which is one of three main causes. It will not fix an oversized unit or controls that keep closing circuits, so diagnose the cause before adding a vessel.
Is it normal for a heat pump to stop every 40 minutes in winter?
Around 0–5 °C in humid air, yes — that is the defrost cycle clearing ice from the evaporator, and it typically lasts a few minutes. A short defrost repeating every 30 minutes, or defrosts lasting over ten minutes, indicates restricted airflow or a sensor fault.