An F22 fault code on a Vaillant boiler means the system water pressure has dropped too low and the boiler has shut itself down to protect the heat exchanger from running dry.

In most cases you can fix this yourself in about five minutes by repressurising the system. If the pressure keeps dropping afterwards, you have a leak somewhere and that needs an engineer.

What F22 actually means

Your boiler needs a certain amount of water in the system to work safely. Vaillant boilers monitor this constantly. When the pressure falls below roughly 0.5 bar, the boiler stops firing and displays F22 rather than risk damaging the heat exchanger, which is the most expensive part in the appliance.

So F22 is not really a fault. It is the boiler protecting itself, and it is telling you the system has lost water.

How to fix a Vaillant F22 yourself

Check the pressure gauge first. It should read between 1 and 1.5 bar when the heating is cold. If it is sitting near zero, that confirms it.

1. Turn the boiler off and let it cool.
2. Find the filling loop, usually a silver braided hose underneath the boiler with a valve at each end.
3. Open both valves slowly. You will hear water flowing in.
4. Watch the gauge. When it reaches 1.5 bar, close both valves.
5. Turn the boiler back on. Press and hold the reset button for around three seconds.

The fault should clear and the boiler should fire normally.

What to do if F22 comes straight back

If the pressure drops again within days, or the fault returns immediately, stop topping it up and call an engineer.

A sealed heating system should hold its pressure for months. If it does not, water is escaping somewhere. Common causes are a leaking radiator valve, a failed pressure relief valve discharging outside, a corroded pipe under the floor, or a failed expansion vessel inside the boiler.

When you need a Gas Safe engineer

• The pressure drops repeatedly
• You cannot find the filling loop
• The fault will not clear after repressurising
• You can see water anywhere near the boiler or under radiators
• The boiler is making unusual noises alongside the fault

How Vantage can help

We are Gas Safe registered and cover Hertfordshire, Bedfordshire and Cambridgeshire. Our emergency line is answered 24 hours a day, and we hold specialist leak detection equipment, so if your system is losing pressure we can find out where without pulling up your floors.

Frequently asked questions

Can I keep using my boiler with an F22 showing?

No. The boiler will not fire while F22 is displayed. That is deliberate, and running it dry would damage the heat exchanger.

How often should I need to top up my boiler pressure?

Once or twice a year is normal. Any more often than that means you have a leak.

What pressure should a Vaillant boiler be at?

Between 1 and 1.5 bar when cold. It will rise a little as the system heats up, which is normal.

Is F22 expensive to fix?

If it just needs repressurising it costs nothing, you can do it yourself. If it is a leak or a failed expansion vessel, it needs an engineer to diagnose it.

What an engineer will do if repressurising does not hold

If the pressure will not stay up, the job is to find where the water is going rather than to keep refilling. An engineer will normally isolate the heating circuit and pressure-test it, which separates a leak on the pipework from a fault inside the boiler. That one test decides the whole direction of the repair.

If the circuit holds pressure when isolated, the loss is inside the appliance, and the usual culprit is the expansion vessel. A vessel that has lost its air charge cannot absorb expansion, so the pressure relief valve lifts and discharges outside every time the boiler heats up. It is a repair rather than a new boiler, and it is one of the most common causes of the fault returning.

If the circuit does not hold, the leak is in the system. That is where leak detection comes in, using thermal imaging and acoustic equipment to find it without lifting floors.

Stopping it coming back

• Check the outside discharge pipe. There is a copper pipe through an external wall near the boiler. If it is wet or dripping, that is where your pressure is going.
• Look at radiator valves for staining or a white or green crusty deposit, which is a slow weep that has been drying and evaporating.
• Note the date each time you top up. A pattern is diagnostic. Twice a year is normal, monthly is a leak.
• Do not overfill. Going well above 1.5 bar cold makes the relief valve discharge once hot, which loses the pressure you just added.

Related fault codes

Other Vaillant codes: Vaillant F75 · Vaillant F28 · Vaillant F29 · Vaillant F27 · Vaillant F20 · Vaillant F23 · Vaillant F83 · Vaillant F32 · Vaillant F62 · Vaillant F61 · Vaillant F72

The same problem on other makes (low water pressure): Ideal F1 · Baxi and Potterton E119 · Glow-worm F22

See also our guide to boiler keeps losing pressure, the full boiler advice section, and boiler repair.

When to call an engineer

If those checks have not cleared it, the next steps are engineer work and the boiler should not be reset repeatedly in the meantime. Call Vantage Plumbing and Heating on 01462 506374. We are Gas Safe registered under 956889, the line is answered by a person at any hour of any day, and you get a price before anyone starts.