EQUIPMENT
The heat pump is not warming the water
A heat pump works more slowly than most people expect — about 0.3 to 0.5 °C an hour — so if the water is not warming at all, check the flow, the set temperature and the airflow around the unit, in that order.
A heat pump does not boil water; it moves heat out of the air into the pool a little at a time. A typical domestic pool gains about 0.3–0.5 °C an hour, so going from 26 to 30 °C takes 8–12 hours, not half an hour. Switching it on in the morning and testing the water at lunchtime may not be a fault at all.
Check the pool pump is actually running. A flow switch holds the heat pump off whenever there is no water moving through it.
Check the filter timer is not set shorter than the heating window — the heater stops when the pump does.
Look at the set point. It must be at least 1 °C above the current water temperature.
Check the unit is in heating mode, not cooling and not auto.
Put a hand in front of a return while the unit runs. If the water coming out is slightly warmer than the pool, it is working and simply needs time.
Look around the unit. It needs at least 50 cm of clear space on every side for the air to move.
Clear any leaves, cover or planting away from the coil at the back. A heat pump that cannot breathe loses most of its output.
Switch the breaker off, then rinse the coil fins at the back gently with a hose, top to bottom.
Check the air temperature. Below about 15 °C the output falls noticeably — a cool early morning heats far more slowly than the afternoon.
If you have a cover or slats, close the pool overnight. An uncovered pool gives back through the night nearly as much heat as the pump put in all day.
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