What does the Vaillant F23 error code mean?
F23 means the Vaillant boiler has detected that water flow or temperature isn't right and has shut down to protect the heat exchanger from running dry. It's commonly a circulation fault (pump, blocked system, air) rather than simple low pressure. An engineer is needed.
What causes the Vaillant F23 fault?
The F23 error on a Vaillant boiler is most commonly triggered by:
- A failed or seized central heating pump
- A blocked heat exchanger or heavily sludged system
- Air locking the pump or heat exchanger
- A faulty temperature sensor giving a false dry-fire reading
How to fix the Vaillant F23 boiler fault
Follow these steps. If the code returns after you have completed them, stop and call a Gas Safe engineer.
- 1.Do not keep resetting the boiler repeatedly — running a blocked pump dry can cause expensive damage.
- 2.Check the system pressure is at 1.0–1.5 bar (top up if it's low, as low water can contribute to F23).
- 3.Book a Gas Safe engineer to inspect the pump, sensors and heat exchanger.
Frequently asked questions about the Vaillant F23
What does F23 mean on a Vaillant boiler?
F23 is dry-fire protection. The boiler has shut down because it cannot confirm enough water is circulating, usually due to a pump fault, blockage, air lock or sensor issue.
Is a Vaillant F23 dangerous?
Not directly — it's a protective shutdown. But repeatedly resetting it can damage the pump or heat exchanger, so call an engineer rather than forcing a restart.
How is an F23 fault fixed?
An engineer will check the pump, inspect the system for blockages or air locks, and test the flow/return temperature sensors, replacing or power-flushing as needed.