Display shows “HI” and the unit will not run; water is already warm or hot.

Safety first: Turn off power before removing any panels or sensors.

• Check setpoint: Verify the thermostat set temperature on the Heat Siphon controller. If the pool water is already at or above this temperature, the HI indication is normal and the unit will not heat.
• Measure actual water temperature: Use a reliable pool thermometer in the return stream. Compare to the displayed temperature.
• Inspect water temperature sensor: Locate the sensor in the water manifold. Check for scale buildup or poor contact with water flow. Clean gently if fouled.
• Reset and test: Power cycle the unit. If HI appears even when water is clearly cool, the sensor may be out of calibration or shorted and should be replaced.