Vaillant F.22 fault codewhat it means and how to fix it
The boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.
Take care
What does the pressure gauge read right now, with the boiler cold?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
The gauge reads zero or is in the red
The system has lost a significant amount of water, which means an active leak rather than gradual loss. Top up to 1.2 bar so you have heating tonight, but book someone to find it.
Check under the boiler, around every radiator valve, and around the airing cupboard for damp before you refill — finding the leak while the pressure is low is much easier.
The gauge reads between 0.1 and 0.8 bar
Normal slow loss. Top up to 1.2 bar and reset. If it has been a year or more since you last did this, nothing is wrong.
Note the date. Topping up more than twice a year is a leak, not maintenance.
The gauge reads 1.0 bar or higher and F.22 is still showing
The pressure is fine, so the sensor reading it is not. That is either a failed pressure sensor or a blockage stopping water reaching it, and it needs an engineer.
Do not keep resetting. The boiler is refusing to fire because it believes it has no water, and repeatedly overriding that is how heat exchangers get destroyed.
It drops again within days of topping up
That rate of loss is a real leak or a failed expansion vessel. Continually refilling introduces fresh oxygenated water, which corrodes the system from the inside.
Stop topping up and get it diagnosed. A pressure test takes an engineer under an hour.
What F.22 means
F.22 is a dry-fire protection lockout. Vaillant boilers measure system pressure continuously, and if it falls below about 0.5 bar the control board stops the burner before the heat exchanger can run without enough water passing through it. Running a heat exchanger dry destroys it in minutes, so this cut-out is doing exactly what it should.
The pressure loss itself is the actual fault. A sealed heating system is closed: the water in it has nowhere to go, so if the gauge is falling, water is leaving somewhere. That might be a slow weep from a radiator valve, a pinhole in pipework under the floor, or a failed expansion vessel pushing water out through the pressure relief valve every time the system heats up.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Water lost from the sealed system | A weeping radiator valve, a joint under the floor, or a leaking towel rail. Slow enough that you never see a puddle, fast enough to drop the gauge over a few weeks. | Common |
| Air bled out of radiators without topping back up | Every time you bleed a radiator you remove volume from the system. Bleed six radiators and the gauge drops noticeably. | Common |
| Failed expansion vessel | The vessel absorbs the expansion of heating water. When its internal bladder fails, pressure spikes on heating and dumps out through the pressure relief valve, so the system loses a little every cycle. | Occasional |
| Passing or corroded pressure relief valve | Debris on the seat holds the valve slightly open. Look for a copper pipe outside the wall near the boiler that drips or shows limescale staining. | Occasional |
What to try yourself
Look at the pressure gauge on the boiler or on the pipework beneath it. Cold, it should read between 1.0 and 1.5 bar. If the needle is in the red or near zero, that is your fault confirmed.
Find the filling loop — usually a braided silver hose with a valve at each end, underneath the boiler. On some Vaillant units it is an internal loop operated by a blue or grey key slot at the base.
Open both valves slowly. You will hear water flowing. Watch the gauge and close them as soon as it reaches 1.2 bar.
Do not overfill. Above 2.5 bar cold you risk the pressure relief valve dumping water, and you will be back where you started.Close both filling loop valves fully and reset the boiler with the press your operating instructions give — one second on the fault clearance key of a 2012 to 2021 ecoTEC plus, pro or ecoFIT, longer than three seconds on an ecoTEC sustain or a three-digit CS/1-5. On the CS/1-5 you may not need to: its instructions say F.022 goes out and the boiler works again on its own once the pressure is back.
Write the date and the pressure on a sticky note by the boiler. Check it again in a week. If it has dropped again, you have a leak and it needs finding rather than topping up.
Get a Gas Safe registered engineer to look at your Vaillant F.22
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£450
Time on site
10 minutes
Topping up costs nothing and takes five minutes. If it will not hold, expect £90–£160 for a leak trace, £150–£280 to replace an expansion vessel, and up to £450 if pipework has to be lifted.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Expansion vessel (8 litre, sealed system)
EV-8L£35–£85Plumbing merchant
Price checked 28 September 2026
Pressure relief valve, 3 bar
PRV-3B£18–£42Plumbing merchant
Price checked 28 September 2026
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Related Vaillant fault codes
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: water pressure
- S.53Get it checkedNot a fault. Your boiler is in a deliberate waiting period because the gap between flow and return temperatures is too wide, which points at poor circulation.Related: water pressure
- F.73Get it checkedThe pressure sensor signal is out of range at the low end. Usually the sensor or its wiring, rather than the pressure itself.Related: water pressure
- F.74Get it checkedThe water pressure sensor is sending a signal that is too high to be genuine. This is a wiring or sensor fault, not high system pressure.Related: water pressure
- S.76Get it checkedA service message, not a lockout: the heating water pressure has dropped and Vaillant want it topped up. The boiler is still running, but below 0.5 bar it will stop with F.22.Related: water pressure
- S.41Get it checkedA status, not a lockout: the heating water pressure is too high, above about 2.8 bar. Usually an over-filled system or a tired expansion vessel, and the safety valve will discharge above 3 bar.Related: water pressure
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.22
Can I just keep topping it up?
For a while, but it is a bad idea. Every refill brings fresh oxygenated water into a sealed system, and that is what causes internal corrosion and radiator sludge. Topping up twice a year is normal. Twice a month means you have a leak that will cost more the longer it runs.
What pressure should a Vaillant boiler be set to?
Between 1.0 and 1.5 bar with the system cold, and 1.2 bar is the usual target. It will read higher when the heating is running — up to about 2.0 bar is normal and nothing to worry about. Set it cold, never hot.
Why does F.22 come back every winter?
Cold weather makes you bleed radiators, and bleeding removes water. It also means the system runs more hours, so any slow leak loses more. A boiler that only shows F.22 in January usually has a small leak that stays hidden the rest of the year.
Is F.22 dangerous?
The lockout itself is a safety feature working correctly, so no. What matters is not defeating it. If the boiler will not hold pressure, leave it locked out and get it looked at rather than resetting repeatedly to force it to fire.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.