Heater displays HS and will not fire or shuts down shortly after starting; unit may feel very hot near the outlet manifold.

Safety First: Turn off power and allow the heater to cool completely before servicing. Hot surfaces can cause burns.

• Check water flow: Verify the pump is running at adequate speed and that the filter is clean. Low flow can cause localized overheating.
• Inspect valves and bypass: Make sure inlet/outlet valves are fully open and any external bypass is set per Hayward specifications.
• Reset high limit (if manual): Some models have a manual reset high-limit switch on the header. Press the reset button if present and accessible.
• Inspect for scale: If the heater is older or used with hard water, scale may restrict the heat exchanger. Look for signs of boiling or popping noises. Descaling is typically a professional job.
• Test high-limit sensor: Check wiring to the high-limit sensor. If HS persists with good flow and clean filter, the sensor may be faulty and should be replaced with a Hayward OEM part.