Heat pumps

Heat pump short cycling

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

The heat pump outdoor unit or compressor turns on and then shuts off again after a very short period (e.g., less than 5–10 minutes), only to restart a few minutes later.

What’s normal vs not normal

Normal:

  • The heat pump modulating its output—running at lower speeds for long periods.
  • Cycles that last at least 20–30 minutes in moderate weather.
  • Long pauses when the house is already at the target temperature.

Not normal:

  • Cycles that last only 2–3 minutes.
  • Frequent “stop-start” clicks from the control unit every few minutes.
  • The unit never seems to settle into a steady, long run.

Likely causes (ranked)

  1. Low water flow: The heat pump can’t move heat away fast enough, so it shuts down to protect itself.
  2. Oversized heat pump: The unit is too powerful for the current heat demand.
  3. Thermostat placement: Located in a draft or too close to a heat source.
  4. Incorrect buffer tank setup: Insufficient volume to absorb the heat pump’s minimum output.
  5. Faulty sensors: Incorrect temperature readings causing premature shutdowns.

Quick checks you can do safely

  • Check that all radiator valves (TRVs) are fully open in the main living areas.
  • Ensure the thermostat is not in a draughty spot or in direct sunlight.
  • Check your energy monitor for a “spiky” usage pattern rather than a flat plateau.

When to call an installer

  • If the unit is cycling more than 3–4 times per hour in cold weather.
  • If you notice a sudden increase in electricity usage without a drop in outside temperature.
  • If the unit displays “high pressure” or “low flow” error codes.

Questions to ask your installer

  • “Is the minimum flow rate being met across the system?”
  • “Is the compressor minimum run-time setting configured correctly?”
  • “Do we need a larger buffer tank or to open more radiator circuits?”

Related NetZeroNow tools & guides

FAQs

Why is short cycling bad? It causes excessive wear on the compressor, reduces efficiency (as it never reaches optimal operating temperature), and can lead to uneven heating.

Is it normal in mild weather? It’s more common in mild weather when the heat pump’s minimum output is higher than the house needs, but modern “inverter” heat pumps should still modulate down to avoid it.

Should I use TRVs with a heat pump? Ideally, you should leave most TRVs fully open to ensure maximum flow, using the heat pump’s own controls to manage the temperature.

Can a dirty filter cause this? Yes, if your system has a filter or strainer that is blocked, it can reduce water flow and trigger a shutdown.

Does a buffer tank help? Yes, a buffer tank provides a “reservoir” of water that allows the heat pump to run for longer even if the radiators don’t need all the heat immediately.

Written by NetZeroNow | Last updated on 2026-03-02