E20 error, unit may shut down during operation; often associated with return temperature or delta-T issues.

Warning: Hot water can cause burns. Allow system to cool before opening vents or drains.

• 1. Check supply and return temperatures: Compare the displayed supply and return temperatures. A very large difference indicates poor flow.
• 2. Verify circulator operation: Ensure the system pump(s) are running during a call for heat. Feel the pipes on both sides of the pump for temperature change.
• 3. Open valves and strainers: Confirm all isolation valves are open. Clean any strainers or dirt separators in the system.
• 4. Bleed air: Air pockets can severely restrict flow. Bleed air from high points, radiators, and automatic vents.
• 5. Pump speed: If using a variable-speed circulator, increase speed or set to constant pressure mode as appropriate for the system.
• 6. Professional cleaning: If flow remains low and E20 persists, the primary heat exchanger may need to be descaled or flushed by a technician.