Boiler locks out with E09, high limit temperature exceeded

Safety: Turn off power and allow the boiler to cool completely. Do not reset repeatedly without finding the cause of overheating.

• Step 1 – Check system pressure and valves: Verify the system pressure on the boiler gauge (typically 12–20 psi when cold). Ensure all isolation valves on the Triangle Tube boiler supply and return are open.
• Step 2 – Verify pump operation: Call for heat and listen/feel the system circulator pump. It should vibrate slightly and get warm. If silent and cold, it may be seized or not powered.
• Step 3 – Bleed air: Air in the system can stop flow. Use automatic air vents and manual bleeders on radiators or manifolds to purge air. Top up system pressure as needed.
• Step 4 – Clean strainers: If your system has a dirt separator or Y-strainer near the boiler, isolate and clean it to restore flow.
• Step 5 – Reset high limit: Some Triangle Tube models have a manual reset high limit on the front or inside the cabinet. Press the reset button once after the boiler has cooled.
• Call a technician: If E09 recurs, a professional should check pump performance, inspect the stainless steel heat exchanger for scaling, and verify sensor operation.