Heater displays HS and shuts down; may restart after cooling.

Safety first: Turn off power and allow the heater to cool completely before opening panels.

• Check water flow: Ensure pump is running, valves are open, and filter is clean. Low flow can cause HS trips.
• Bleed air: If the system was recently serviced, bleed air from the filter and high points in the plumbing to prevent hot spots.
• Inspect high limit sensor: Locate the high limit/temperature sensor on the header. Check wiring and connectors for corrosion or damage.
• Look for scaling: If the heater is older or used with hard water, the heat exchanger may be scaled internally, causing overheating. This requires professional descaling or exchanger replacement.
• If HS recurs frequently: Replace the high limit sensor first. If the problem continues, have a technician evaluate the heat exchanger and flow rates.