Vaillant Boiler F23 Error — Low Water Pressure Fix

What Does This Error Mean?

F23 is the low-pressure code on Vaillant boilers — system pressure has dropped below 0.5 bar and the boiler has cut out for safety.
Most heating systems lose pressure slowly over months, especially after winter.
Open the filling loop, watch the pressure gauge climb to 1.0–1.5 bar, close the loop.
Press the reset button.
Five minutes of work and F23 is cleared.

Affected Models

Vaillant ecoTEC plus
Vaillant ecoTEC pro
Vaillant ecoFIT pure
Vaillant turboMAX

Common Causes

Slow water loss through radiator bleed valves over months
Recent radiator bleeding lowered pressure
Pressure relief valve discharging due to expansion vessel issue
Leak somewhere in the heating circuit (radiator pipe, joint)
Pressure sensor reading low when actual pressure is fine (rare)

How to Fix It

Find the pressure gauge.

Look at the front of the boiler.
You'll see a circular gauge marked in bar — usually green between 1.0 and 2.0 bar.
If the needle is below 0.5 bar (in the red), F23 is correct.
Find the filling loop.

Look under the boiler for a silver braided hose connecting two pipes — that's the filling loop.
It has two small valves, one at each end.
Some installs hide it inside a small panel; check the bottom of the boiler for a removable cover.
Open both filling loop valves.

Turn each valve a quarter-turn (handle parallel to the pipe) to open.
You'll hear water flowing.
Watch the pressure gauge climb — slowly is best.
Stop adding water at 1.0–1.5 bar.
Don't go above 2.0 bar.
Close both filling loop valves.

Turn each handle perpendicular to the pipe.
Both valves must be fully closed — leaving them open will gradually over-pressurize the system.
Leaving them closed is the normal state.
Press the reset button on the boiler.

On most ecoTEC boilers, there's a small button (often labelled 'Reset' or with a flame icon) on the front panel.
Press it.
The boiler should re-light within a few seconds and F23 clears.
If F23 returns within days, you have a small leak somewhere — call a Gas Safe engineer.

Frequently Asked Questions

How often should I top up the pressure?

Once or twice a year is normal — typically before winter when heating ramps up.
If you find yourself topping up monthly, you have a leak somewhere.
Common leak locations: radiator bleed valves, pipe joints under floors, the pressure relief valve discharging outside.
A Gas Safe engineer can diagnose and fix.

Why does F23 keep coming back the same week?

Same-week recurrence almost always means a real leak.
Topping up to 1.5 bar and seeing pressure drop to 0.5 in days means roughly a liter of water is escaping somewhere.
Check radiator valves, look around for damp spots, and have a Gas Safe engineer inspect — small leaks become big leaks if ignored.