Display shows dP or DP; heater shuts down on water pressure/flow differential fault.

Safety first: Turn off pump and heater, and relieve pressure before opening any water connections.

• Clean strainers: If your AquaCal model has an internal strainer or screen, remove and clean it of debris.
• Check filter and valves: Backwash or clean the main pool filter and ensure valves are fully open to and from the heater.
• Inspect pressure switch tubing: Some models use small tubes to sense pressure. Make sure they are not kinked or clogged.
• Test pressure switch: With a multimeter, verify continuity across the pressure switch when the pump is running. Replace the switch if it does not close under normal flow.