Boiler locks out with “Err 12” and may show abnormal supply/return temperature difference

Safety first: Turn off power before working on pumps or valves. Hot water can cause burns.

• Check circulator: On a call for heat, the main system pump should run. Feel the pump body; if it is hot but not moving water, it may be seized. If it is cold and silent, check power to the pump relay.
• Verify zone valves: If you have motorized zone valves, confirm that the zone calling for heat is actually opening (lever moves or end switch clicks).
• Bleed air: Air in the system can stop flow. Use purge valves or radiator bleeders to remove air until a steady stream of water flows.
• Clean strainers: If your Burnham boiler has a system strainer or dirt separator, isolate it, relieve pressure, and remove the screen to clean out debris.
• Reset and monitor: Restore power and call for heat. Watch supply and return temperatures; a very large difference may indicate continued flow problems that require professional balancing or pump replacement.