Boiler displays F23, may cycle frequently, radiators unevenly warm

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Insufficient circulation, undersized or blocked pump, closed radiator valves, sludge in system, incorrect hydraulic balancing

Safety: Allow system to cool before touching pipework.

Open TRVs: Ensure thermostatic radiator valves are open, especially on radiators near the boiler.
Check pump speed: If the pump has adjustable speed, set it to a higher setting as per Viessmann guidelines.
Bleed system: Remove air from radiators and high points.
System cleaning (technician): If F23 persists, the system may be heavily sludged. A professional should flush the system, clean strainers, and possibly fit a magnetic filter.

Medium 3/5

Circulation Pump

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