Boiler displays OF or Overheat Fault; unit locks out and may require manual reset.

Safety first: Turn off power and allow the boiler to cool completely. Do not repeatedly reset an overheat fault without correcting the cause.

• Check all isolation valves: Ensure the boiler supply and return valves and any closely spaced tees or primary loop valves are fully open.
• Verify circulator operation: With a call for heat, feel the piping on both sides of the Laars boiler circulator. If one side is hot and the other is cool, flow may be restricted or the pump may not be running.
• Bleed air: Air trapped in the boiler or near piping can stop flow. Use automatic air vents or manual bleed valves on high points and radiators to remove air (with system cool and pump off).
• Reset high limit: Some Laars models have a manual reset high limit on the outlet. Press the reset button once the boiler is cool. If OF recurs quickly, stop and call a technician.

When to call a professional: Persistent overheat faults often indicate internal fouling of the heat exchanger or serious flow design issues that require professional evaluation.