F.23Vaillant
Vaillant F.23: the temperature difference between flow and return is too great
Checked against Vaillant's own documentation: Vaillant: boiler fault codes list and 2 more sources. Last reviewed October 2026. How we check
Engineer only
The water leaving the boiler is much hotter than the water coming back. Vaillant lists a blocked or faulty pump, or a pump not pushing enough water, as the usual causes.
What you can safely do
There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.
Before you book an engineer
- Write down the code exactly as the display shows it: F.23.
- Find your boiler's model name. It is usually on the front panel or in the user manual. The table below shows why it matters: Vaillant codes can mean different things on different models.
- Note when it started and what the boiler was doing (heating, hot water, or starting up).
- If the boiler is still under warranty, check your warranty paperwork before you book anyone.
Call a Gas Safe engineer if
- Book a Gas Safe registered engineer.
Find a Gas Safe engineer near you
Opens the official Gas Safe Register search.
Applies to ecoTEC plus. Codes can differ between models, so check your manual.