Vaillant F.22 Fault Code
The F.22 fault means the water pressure in your central heating system has dropped too low for safe operation. It is the most homeowner-resolvable Vaillant fault — but only if there is no underlying leak.
What is the Vaillant F.22 fault code?
The Vaillant F.22 fault is triggered when the boiler's water pressure sensor detects that the pressure in the central heating circuit has fallen below the minimum operating threshold (typically 0.5 bar). The boiler locks out to protect the system from running dry. In many cases this is a simple repressurise job. However, if the system loses pressure repeatedly — more than once every 4–6 months — there is a leak somewhere in the system that needs finding and fixing.
What causes the F.22 fault?
All sealed central heating systems lose a small amount of pressure over months and years. A drop of 0.1–0.2 bar per year is normal. Below 0.5 bar the boiler will lock out.
A slow drip from a radiator valve, pump gland, or pipework joint can cause pressure to drop monthly or even weekly. Find and fix the leak before repressurising.
Bleeding a radiator or any system component releases water alongside air, reducing system pressure. Always check and top up pressure after any bleed.
The expansion vessel absorbs pressure changes as the system heats up. If the vessel loses its air charge, pressure fluctuates dramatically and the boiler may show F.22 even when cold pressure appears correct.
A stuck-open automatic air vent slowly allows water to escape. Common on older installations — these vents are cheap to replace.
If the filling loop is accidentally left open after repressurising, it can drain back or let in air, causing erratic pressure readings.
What you can safely check yourself
Locate the pressure gauge on the front of the boiler (or below it). If it reads below 0.8 bar with the system cold, it needs topping up. If it reads 0, the system may have lost pressure completely — check for visible leaks first.
Locate the filling loop — typically a flexible silver hose with two valves underneath the boiler. Open both valves slowly until the gauge reads 1.2–1.5 bar, then close both valves. Ensure both valves are fully closed before resetting. If you cannot locate the filling loop or are unsure, call an engineer.
After repressurising, press and hold the reset button for 3–5 seconds. Watch the pressure gauge over the next 24 hours. If pressure drops below 1 bar within a week, there is a leak that needs finding — call an engineer.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Pressure test the system to find hidden leaks in pipework, radiators, or fittings
- Inspect and test the expansion vessel — recharge or replace if failed
- Check automatic air vents and replace if sticking open
- Inspect the pump gland seals for weeping
- Check radiator valves (TRVs and lockshields) for drips at the gland nut
- Repair or replace leaking components and repressurise to correct level
Expansion vessel failures are particularly common in London properties served by Thames Water. The hard water chemistry accelerates corrosion inside the vessel membrane. Across South London — Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley — it is common to find 10–15 year old ecoTEC boilers with completely flat expansion vessels. Reion checks expansion vessel charge on every annual service call.
How much does a F.22 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Vaillant boiler showing F.22
Check the pressure gauge — if below 1 bar, repressurise via the filling loop to 1.2–1.5 bar before resetting.
Ensure the filling loop valves are fully closed after repressurising.
Press and hold the reset button for 3–5 seconds.
The boiler will start up. Monitor the pressure gauge — it should remain stable.
If pressure drops again within a few days, call a Gas Safe engineer to locate the leak.
F.22 alone is rarely a reason to replace a boiler. The only exception is if the pressure loss is caused by a cracked heat exchanger — which on an older boiler may make replacement more economical than repair.
Common questions about the Vaillant F.22 fault
Can I repressurise a Vaillant boiler myself in London?
Why does my Vaillant boiler keep losing pressure?
What pressure should a Vaillant boiler be at?
Is an F22 fault dangerous?
How do I know if my Vaillant boiler needs a new expansion vessel?
Other Vaillant fault codes
Need a Vaillant engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
