Boiler locks out with A04; unit may start then shut down on high temperature, or may not start at all.

Safety first: Turn OFF power to the boiler. Allow the unit to cool before touching any components. Do not repeatedly reset a high limit without finding the cause of overheating.

Steps:

• 1. Check system water flow: Ensure all isolation valves to and from the Raypak boiler are open. Verify that zone valves are not stuck closed. Listen or feel for operation of the system circulator pump when there is a call for heat.
• 2. Inspect the circulator pump: If the pump is hot but not running, or is noisy and seized, it may have failed. Check the pump power connections and fuses. A failed circulator must be replaced.
• 3. Reset the manual high limit (if equipped): Locate the manual reset high limit on the boiler (often a red button on a sensor well). Press firmly until you feel a click. If it immediately trips again on the next call for heat, shut the system down and investigate flow and control issues.
• 4. Check for air in the system: Air-bound piping can stop flow and cause rapid overheating. Bleed air from radiators or air vents per system design.
• 5. Inspect high limit sensor wiring: Ensure the sensor probe is fully inserted into the well and wiring is intact. Replace the sensor if it is open or shorted (technician test).

Warning: Persistent A04 errors indicate a serious overheating or flow problem. If you cannot quickly identify a flow restriction or pump failure, contact a qualified boiler technician.