Heater displays H5 and will not fire; may occur after long run times.

Safety first: Turn off power and allow the heater to cool before touching any internal components.

• Check water flow: Ensure pump is running at full speed and filter is clean. Low flow can cause localized overheating and trip the high limit.
• Inspect external bypass: If an external AstralPool bypass valve is installed, verify it is correctly balanced so that sufficient water passes through the heater.
• Check for plumbing restrictions: Look for closed valves or partially collapsed hoses on the heater inlet and outlet.
• Reset and test: After the heater cools, restore power and call for heat. If H5 recurs quickly, the high limit sensor or internal water path may be faulty.

Persistent H5 errors often indicate scale or internal blockage; a technician may need to chemically descale or replace the high limit thermostat.