Control indicates 'High Limit Open' or 'High Temperature Limit'; burner will not fire until the limit resets.

Safety first: Turn off power. Water and components may be very hot; allow the system to cool before touching piping or sensors.

• Check system pressure and flow: Verify that system isolation valves are open and that the circulator pump for the water heater loop is running when there is a call for heat. Listen for pump operation and feel for vibration.
• Inspect high limit control: Locate the high limit aquastat or sensor on the tank or heat exchanger. Some models have a manual reset button; press it firmly if present. If it immediately trips again, there is an overheating or flow problem.
• Bleed air from system: Air-bound piping can stop flow and cause rapid temperature rise. Use air vents or purge valves to remove trapped air from the water heater loop.
• Check for scale or blockage: If the unit is older or in a hard-water area, internal scaling can restrict flow. This typically requires professional descaling of the heat exchanger.
• Do not bypass safety: Never jumper or bypass a high limit switch. If it continues to trip, have the system inspected by a qualified technician.