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BoilerCodes
F.23
Vaillant fault code

Vaillant F.23: the temperature difference between flow and return is too great

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.

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.

What F.23 means on each Vaillant model

From Vaillant's own fault tables. Your model name is usually on the front of the boiler or in the user manual.

ModelMeaningCauses listed for engineers
ecoTEC plusSafety switch-off: temperature difference too greatPump blocked, insufficient pump output, air in the boiler, flow and return sensors connected the wrong way round

Common questions about Vaillant F.23

Can I reset F.23?

Resetting won't fix the cause. A pump or circulation problem needs an engineer.

Nearby Vaillant codes

F.22F.28F.75F.29F.83F.72F.20F.25F.27F.61F.62

See all 36 Vaillant codes

Sources