Boiler displays E4 and shuts down, often after running for a short period

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

• Step 1 – Check system pressure and valves: Verify that the hydronic system pressure is within Rheem’s recommended range (often 12–25 psi when cold). Ensure all isolation valves around the boiler and heating circuits are fully open.
• Step 2 – Confirm circulation: When the boiler calls for heat, the system pump should run. Listen for pump operation and feel the supply and return pipes. If the pump is silent or the pipes remain cool, the pump may be seized or airlocked.
• Step 3 – Bleed air: Bleed air from high points, radiators, and automatic air vents. Air pockets can stop flow and cause rapid overheating.
• Step 4 – Inspect outlet temperature sensor: With power off, locate the outlet water temperature sensor on the boiler’s supply pipe. Check its wiring and connector. Measure its resistance and compare to Rheem’s chart. Replace if out of specification.
• Step 5 – Check for heat exchanger blockage: If flow is confirmed but E4 persists, the primary heat exchanger may be scaled or partially blocked, especially in hard water areas. A qualified technician should inspect and, if needed, chemically flush or replace the heat exchanger.

Warning: Do not repeatedly reset an overheating boiler. Overheating can damage components and create scalding hazards.