Boiler displays E4 and shuts down; may occur after long run time

Safety first: Turn off power and allow the boiler to cool. System water can be very hot and under pressure.

Steps:

• Check system pressure and flow: Verify the system pressure gauge is within the normal range specified by Peerless (often 12–20 psi for residential). Ensure zone valves and isolation valves are open.
• Verify circulator pump operation: When there is a call for heat, feel the Peerless system circulator pump body; it should vibrate slightly and get warm. If silent and cold, check the pump power connection and fuses.
• Bleed air from system: Air-bound radiators or loops can stop flow. Bleed air from radiators and high points using bleed valves.
• Inspect high limit sensor: Locate the supply temperature sensor or high limit probe on the boiler outlet. Ensure it is firmly seated and wiring is intact.
• Descale if needed: If the boiler is older and runs hot quickly, the heat exchanger may be scaled. A professional should perform a chemical flush or heat exchanger cleaning.