Boiler displays E6 and shuts down, often with noisy boiling sounds inside the unit

Safety first: Turn off power and allow the boiler to cool completely. Do not reset the high limit until the cause is identified.

• Step 1 – Verify system flow: Ensure all isolation valves on the boiler supply and return are fully open. Confirm that zone valves (if installed) are operating correctly.
• Step 2 – Check circulation pump: Listen for pump operation when there is a call for heat. If the pump is hot to the touch but not circulating, it may be seized. A technician should test and replace the pump if necessary.
• Step 3 – Inspect for sludge or scale: Dirty system water can restrict flow through the heat exchanger. Look for dark, sludgy water when bleeding radiators. System flushing and inhibitor treatment may be required.
• Step 4 – Reset high limit: Locate the manual reset high limit thermostat on the boiler (often a red button). Once the unit is cool, press the reset. If it immediately trips again on the next cycle, do not continue resetting—there is an unresolved overheating issue.
• Step 5 – Replace faulty high limit: If flow is good and temperatures are normal but the high limit continues to trip, the thermostat itself may be faulty and should be replaced with the correct Rheem part.

Warning: Persistent high limit trips indicate a serious problem. Continued operation can damage the boiler and create safety risks. Involve a qualified technician.