Glossary · Faults, servicing and gas safety
What does the F22 fault code mean?
F22 is the fault code a Vaillant or Glow-worm boiler shows for low water pressure in the heating system. Both makers give the normal range as 1 to 1.5 bar. If the gauge is under 1 bar, you can top it up through the filling loop. Above 1 bar, or if it keeps dropping, an engineer is needed.
Key points
- On Vaillant and Glow-worm boilers, F.22 means low water pressure. The normal range both quote is 1 to 1.5 bar.
- If the gauge reads under 1 bar, the manufacturers say you can repressurise through the filling loop if you are competent, following your manual.
- If the pressure reads above 1 bar when F.22 shows, both say it is a case for an engineer, not for topping up.
- Vaillant says a sudden drop may mean a major leak, and that if pressure falls again after repressurising you should not keep trying.
- Other brands use different codes. Check your own manufacturer's list.
What causes an F22 fault
On Vaillant and Glow-worm boilers, F.22 means low water pressure in the system. Both makers say the normal pressure range is 1 to 1.5 bar.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes A sealed heating system needs enough pressure in the water for the boiler to run safely, so the boiler shuts itself down when the pressure falls too low. Vaillant says low boiler pressure can be caused by water leaks or by bleeding radiators too much, and that the boiler may shut down as a safety precaution.[3]Vaillant, How to repressurise a boiler
The manufacturers’ lists give several possible causes. They are the pressure itself being below the normal range, a fault with the water pressure sensor, loose or faulty wiring to the pump or pressure sensor, a pump not working properly on open vented boilers and a faulty internal expansion vessel.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes Most of those are inside the boiler. Only the first is a job for a householder, and only in a limited way.
Other brands use different codes for low pressure, so check your own manufacturer’s list.
What you can check yourself
Look at the pressure gauge or display on the boiler. Then follow the manufacturer’s guidance for the reading you see.
| Gauge reading when F.22 shows | What Vaillant and Glow-worm say |
|---|---|
| Under 1 bar | Can be solved without an engineer if you are competent. Use the filling loop to raise the pressure slowly to between 1 and 1.5 bar |
| Over 1 bar | A case for an engineer. Do not try to solve it yourself, because opening the boiler casing can be dangerous |
Both makers say their guidance is used at your own risk.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes
Vaillant’s repressurising guide says to start by checking your boiler manual, because every boiler is different. As a general guide, it says to turn the boiler off and let the system cool, check the filling loop is securely attached, open the valves to let water in while watching the gauge, stop at the recommended level for your boiler, close the valves in reverse order and switch the boiler back on. Some boilers have a filling key instead of a loop.[3]Vaillant, How to repressurise a boiler Follow the manual for your model rather than this summary. Our entry on low water pressure covers the wider picture.
When to call an engineer
Vaillant says that if the pressure has suddenly dropped, a major leak may have occurred, so look for visible signs and call a Gas Safe registered engineer as soon as possible.[3]Vaillant, How to repressurise a boiler If you repressurise and the pressure falls as soon as you close the valve, Vaillant says there is likely a significant leak or a fault with the pressure release valve. It says to call an installer and not keep trying to repressurise, as this could cause further damage.[3]Vaillant, How to repressurise a boiler
Work on a gas appliance, and on the boiler’s internal parts, must be done by a Gas Safe registered engineer.[4]Health and Safety Executive, Domestic gas: frequently asked questions A boiler repair visit can find a leak, test the pressure sensor and check the expansion vessel and pressure relief valve.
Example
Mo lives in Morley and has a Vaillant boiler. One cold morning the heating is off and the display shows F.22. The gauge reads 0.7 bar. He checks the manual, finds the filling loop, and slowly raises the pressure to 1.2 bar. The boiler restarts.
Three days later the code returns. Following Vaillant’s advice, he does not top it up again. He books an engineer, who can look for a leak.
What people get wrong
The usual mistake is to keep topping up. Adding water treats the symptom. If the pressure drops again, the water is going somewhere, or a component such as the expansion vessel or pressure relief valve is failing. Vaillant says to stop and call an installer.
The other mistake is topping up when the gauge already reads above 1 bar. In that case both makers say it is a case for an engineer.[1]Vaillant, Boiler fault codes[2]Glow-worm, Boiler fault codes
The bottom line
F22 means low water pressure on Vaillant and Glow-worm boilers. If the gauge reads under 1 bar, you may be able to raise it to 1 to 1.5 bar using the filling loop and your manual. If it reads above 1 bar, drops again or fell suddenly, leave it to a Gas Safe registered engineer.
Common questions
What does F22 mean on a Vaillant boiler?
Low water pressure in the system. Vaillant says the normal range is 1 to 1.5 bar. Possible causes it lists include low pressure, a faulty pressure sensor, wiring faults, a faulty pump (open vented boilers only) or a faulty internal expansion vessel.
How do I clear an F22 fault code?
Vaillant says that if the gauge shows under 1 bar, you can raise the pressure using the filling loop to between 1 and 1.5 bar if you are competent to do so, following your manual. Above 1 bar, it is a job for an engineer.
Why does my boiler keep losing pressure after I top it up?
Vaillant says that if pressure drops as soon as you close the valve, there is likely a significant leak or a faulty pressure release valve. Call an installer and do not keep repressurising.
Is F22 dangerous?
The boiler has shut itself down as a precaution, according to Vaillant. Low pressure is not a gas fault, but the cause can be a leak. If you smell gas, leave the property and call 0800 111 999.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Vaillant. Boiler fault codes. Checked 11 November 2026. ↩
- Glow-worm. Boiler fault codes. Checked 11 November 2026. ↩
- Vaillant. How to repressurise a boiler. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩