The short answer
The Vaillant F22 fault code means low water pressure: the system has dropped below 0.5 bar and the boiler has shut down to protect itself. You can usually fix it yourself by repressurising to 1.5 bar using the filling loop. A recurring F22 means water is leaking somewhere.
What the law requires
What does F22 mean on a Vaillant boiler?
F22 is Vaillant’s low water pressure lockout, and it appears on ecoTEC, ecoFIT and most other Vaillant models. Sealed central heating systems run at pressure, typically between 1 and 1.5 bar when cold, and the boiler constantly monitors it. When the reading falls below roughly 0.5 bar, the boiler cannot circulate water safely, so it stops and shows F22. It is one of the most common codes any London engineer sees, it is not dangerous, and in most cases you can clear it yourself in five minutes without tools.
The pressure gauge tells you where you stand before you touch anything. On most Vaillants it is a small dial or digital reading on the front panel. Below 1 bar is low; below 0.5 bar is where F22 lives.
How to fix a Vaillant F22 fault in 5 steps
Pump replacement
The working heart of the system, and the most common cold-season failure; supplied and fitted.
Find the filling loop.
On most Vaillant installations it is a silver braided hose beneath the boiler with a small valve at each end. Some ecoTEC models have built-in filling keys instead; your manual shows which you have.
Open both filling loop valves slowly.
You will hear water flowing into the system. Watch the pressure gauge as it climbs.
Close both valves when the gauge reads 1.5 bar.
Do not overfill; anything much above 2 bar cold is too high, and the pressure relief valve will dump the excess.
Turn the boiler back on.
The F22 code should clear on its own or after pressing the reset button, and heating should resume normally.
If the display still shows F22 with the gauge reading 1.5 bar, the pressure sensor itself may be faulty, and that is an engineer’s job.
Why does my Vaillant keep showing F22?
This is the question that matters, because F22 itself is only a symptom. If you are repressurising every few weeks, water is leaving the system, and it is going somewhere. The usual suspects, in order of frequency: a small leak in the pipework or at a radiator valve (often invisible, under floors or behind kitchen units), a pressure relief valve that has started weeping and is quietly dumping water down its outside discharge pipe, or an expansion vessel that has lost its air charge, which pushes pressure up and down with temperature until the relief valve vents it away.
Topping up forever does not fix any of these. It feeds them, and in the long run the constant introduction of fresh, oxygenated water accelerates corrosion inside the system. The honest rule: repressurising once or twice a year is normal life; once or twice a month is a leak, and a leak trace usually finds it within the hour.
Is it safe to keep resetting F22?
What does an F22 repair cost?
Vaillant F22, quick answers
Can I run the boiler with F22 showing?
No, and it will not let you; the lockout exists to protect the pump and heat exchanger. Repressurise first.
What pressure should a Vaillant boiler be?
Between 1 and 1.5 bar cold. It will rise a little when hot; that is normal.
Does F22 mean I need a new boiler?
Almost never. F22 is a pressure symptom, not a verdict on the boiler.
My filling loop is missing or will not turn. What now?
Some installations use a removable loop that a previous engineer took away, and some valves seize. Both are quick engineer fixes; do not force a seized valve.
Is F22 the same as F.22?
Yes, Vaillant displays it with the point; people type it both ways.