Heater shuts down when pump speed is reduced on a variable-speed pump.

Safety first: Turn off power to the heater before adjusting internal components. Pump speed changes can usually be made from the pump controller.

Step-by-step checks:

• Increase pump speed: Raise the variable-speed pump RPM during heater operation until the heater runs reliably. Note the minimum RPM that maintains flow.
• Clean filter and baskets: Reduce system resistance by cleaning the filter, skimmer baskets, and pump strainer.
• Adjust bypass: If a bypass is installed, partially close it to force more flow through the heater at lower pump speeds.
• Coordinate controls: Program the pump schedule so that heater operation only occurs at speeds that satisfy the Pahlen flow requirements.

When to stop: If the heater still shuts down at reasonable flow rates, have a technician verify the flow switch operation and system hydraulics.