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Vaillant Boiler F.23 Error Code

Too big a gap between flow and return temperature on Vaillant boilers.

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F.23
May need a pro

The water leaving the boiler is far hotter than the water coming back, which means it isn't circulating properly. Air in the system or a pump that's blocked or weak is the most common cause. A faulty diverter valve, expansion vessel or sensor can also do it.

Compatible models

Vaillant names no specific models on its UK fault page; the ecoTEC plus VU ..6/6-5 OVZ installation manual also lists F.23.

Also shown as

No other codes are listed for this fault.

Difficulty

Try the basic checks, but the usual fix is a technician's job.

Not sure you are reading the code right? See how to read Vaillant boiler codes.

Safety first: never take the casing off a gas boiler. If you smell gas, open windows, don't use light switches, turn the gas off at the meter if you can and call your gas emergency number from outside. A carbon monoxide alarm going off means the same: switch the boiler off, get fresh air and call.

How to fix Vaillant F.23

  1. Check pressure and bleed if needed

    If radiators are cold at the top or you hear gurgling, bleed them with a radiator key, starting nearest the boiler, as Vaillant's operating manual describes. Recheck the pressure afterwards and top up if it fell below 0.8 bar.

  2. Reset the boiler

    Hold the reset button for longer than 3 seconds and let it run for a while.

When to call a technician

Should the code return after bleeding, the pump or diverter valve probably needs looking at. That's a job for a Gas Safe engineer.

Parts linked to F.23

These are the parts an engineer usually checks for this code. They sit inside the sealed casing, so leave them to a Gas Safe registered engineer.

Frequently asked questions

Which Vaillant models show the F.23 code?

Vaillant names no specific models on its UK fault page; the ecoTEC plus VU ..6/6-5 OVZ installation manual also lists F.23.

Can I fix Vaillant F.23 myself?

Only the outside checks. The steps above are the ones Vaillant's user instructions allow; if they don't clear F.23, the fix is inside the boiler and needs a Gas Safe registered engineer (RGII in Ireland).

Other Vaillant boiler codes

Last reviewed 2026-10-21. Codes and steps can vary by model; check your owner's manual.

Sources we checked