Heater cycles on and off rapidly, with short run times and frequent restarts.

How to Fix / Troubleshooting

Safety first: Isolate power before adjusting internal controls.

Check sensor placement: Ensure water temperature sensors are in well-mixed water, not in dead legs or near inlets/outlets.
Adjust pump speed: If using a variable speed pump, set a stable speed that maintains consistent flow during heating.
Review controller settings: Increase the temperature differential (if adjustable) to reduce cycling frequency.
Use pool cover: A cover can stabilise temperature and reduce rapid swings.

If short cycling continues, consult a pool heating specialist to review system design and control strategy.