Boiler shuts down and displays HL or High Limit; burner will not fire until reset.

Safety first: Overheating can damage the boiler and piping. Turn off power at the service switch and allow the boiler to cool. Do not reset repeatedly without finding the cause.

• Check system pressure and air: Verify the system pressure on the gauge (typically 12–20 psi for residential hydronic systems when cold). If pressure is low, there may be a leak or air in the system. Do not add water unless you understand the fill procedure; call a technician if unsure.
• Verify all valves are open: Confirm the isolation valves on the boiler supply and return, and any zone valves, are fully open. A closed valve can cause no flow and rapid overheating.
• Check the circulator pump: With power restored, call for heat and listen/feel the Laars system pump (e.g., Grundfos or Taco circulator). If it does not run, check that the pump switch is on and that the pump is not seized. A seized pump usually requires replacement.
• Reset the high limit: Many Laars boilers have an automatic reset high limit sensor on the supply pipe or heat exchanger. Once the boiler has cooled, press the reset button if present (some are manual reset). If HL returns quickly, stop resetting and call a professional.
• Consider scale or sludge: Older systems may have scale or sludge restricting flow through the Laars heat exchanger. This requires professional cleaning or power flushing.

When to call a professional: Repeated HL trips, noisy boiling sounds, or no circulation indicate a serious flow problem or heat exchanger fouling that should be diagnosed by a qualified technician.