Boiler stops and shows error 20; may occur intermittently during operation.

Safety: Turn OFF power and allow the boiler to cool before touching internal components.

• 1. Bleed air from system: If connected to a hydronic heating loop, bleed air from radiators, baseboards, and high points. Air pockets can cause localized overheating.
• 2. Verify pump operation: Listen and feel for the system circulator pump when there is a call for heat. If the pump is not running, check its power supply and any external controls.
• 3. Clean filters and strainers: Remove and clean the boiler inlet water filter and any external strainers to restore full flow.
• 4. Inspect high limit wiring: With power OFF, locate the high limit switch on the heat exchanger or outlet piping. Check for loose or damaged wires.
• 5. Professional diagnosis: If error 20 persists, the high limit switch or control board may be faulty, or the heat exchanger may be severely restricted. A Noritz-trained technician should test and replace components as needed.