Vaillant F.22 Fault Code: Low Water Pressure Explained
F.22 is a low water pressure (system protection) fault. Your Vaillant boiler has detected that there isn't enough water in the heating system to operate safely, so it shuts down to prevent damage to internal components such as the heat exchanger and pump. You'll typically lose both heating and hot water until pressure is restored. The lockout usually triggers when system pressure falls to around 0.3–0.5 bar (the exact figure varies by model), well below the ideal running range of 1–1.5 bar. On some heat-only models, F.22 can also point to poor circulation caused by an airlock, a failing pump, or a scaled heat exchanger rather than a simple gauge reading.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Vaillant F.22 fault code mean?
F.22 is a low water pressure (system protection) fault. Your Vaillant boiler has detected that there isn't enough water in the heating system to operate safely, so it shuts down to prevent damage to internal components such as the heat exchanger and pump. You'll typically lose both heating and hot water until pressure is restored. The lockout usually triggers when system pressure falls to around 0.3–0.5 bar (the exact figure varies by model), well below the ideal running range of 1–1.5 bar. On some heat-only models, F.22 can also point to poor circulation caused by an airlock, a failing pump, or a scaled heat exchanger rather than a simple gauge reading.
Common causes
- Gradual pressure loss over time Common
All sealed heating systems lose a small amount of pressure naturally over months. It's completely normal to need to top up via the filling loop once or twice a year.
- A leak in the system Common
A sudden pressure drop usually means a leak somewhere — pipework joints, radiator valves, towel rails, or internal boiler seals. Even a slow weep can cause repeated F.22 lockouts.
- Recently bled radiators Common
Bleeding radiators releases water and air, which lowers system pressure. If you've recently bled radiators and then seen F.22, this is the most likely cause.
- Faulty or waterlogged expansion vessel Sometimes
The expansion vessel absorbs pressure changes as water heats and cools. If it loses its air charge or becomes waterlogged, pressure swings can repeatedly trip F.22.
- Faulty pressure sensor Sometimes
The sensor that reports system pressure to the boiler can fail or misread, causing an F.22 even when the gauge looks healthy. This requires an engineer to diagnose and replace.
- Failed pressure relief valve (PRV) Sometimes
A PRV that's passing or stuck open will let water escape, usually visible as dripping from the external discharge pipe outside, causing recurring pressure loss.
- Circulation fault (pump, airlock or scaled heat exchanger) Rare
On heat-only systems especially, F.22 can flag poor water flow through the boiler caused by a tired pump, trapped air, or limescale build-up in the heat exchanger.
How to fix it
- Check the pressure gauge DIY safe
Look at the pressure gauge or digital display on the front of the boiler. If it reads below around 1 bar (or near zero), low pressure is confirmed as the cause of the F.22 lockout.
- Repressurise via the filling loop DIY safe
Locate the filling loop (a silver braided hose with one or two valves beneath the boiler). With the boiler off and cold, slowly open the valve(s) and watch the gauge rise to between 1.0 and 1.5 bar, then close the valves firmly. This is a safe homeowner task.
- Reset the boiler DIY safe
Once pressure is restored, press and hold the reset button for around 5 seconds to clear the F.22 lockout. Do not reset more than 2–3 times — repeated resets without fixing the cause won't help.
- Monitor the pressure over the next few days DIY safe
If pressure holds steady and the boiler runs normally, it was likely a one-off (for example after bleeding radiators). Keep an eye on the gauge for a week.
- Check for visible leaks and PRV discharge DIY safe
Look around radiator valves, visible pipework and under the boiler for damp patches, and check whether water is dripping from the external pressure relief discharge pipe outside. Note what you find but do not attempt repairs.
- Stop topping up and call a Gas Safe registered engineer Gas Safe engineer
If pressure drops again within days, or the boiler keeps locking out with F.22, do not keep repressurising. The system needs professional investigation — leak repairs, expansion vessel recharge/replacement, PRV, pressure sensor and pump work must be carried out by a Gas Safe registered engineer.
Parts you may need
- Pressure sensor · from £40
- Pressure relief valve (PRV) · from £35
- Expansion vessel · from £70
- Filling loop hose · from £15
- Central heating pump · from £120
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£280, depending on the underlying cause.
Frequently asked questions
Can I fix a Vaillant F.22 fault myself?
Often yes, if it's simply low pressure. Topping up via the filling loop to 1.0–1.5 bar and resetting the boiler is a safe DIY task. However, if pressure keeps dropping you have a leak or faulty component, and that work must be done by a Gas Safe registered engineer — never attempt to repair a leak or internal part yourself.
Why does my Vaillant boiler keep showing F.22 even after I top it up?
Recurring F.22 means pressure isn't being held. The usual culprits are a system leak, a waterlogged expansion vessel, a passing pressure relief valve, or a faulty pressure sensor. Stop repeatedly topping up and book an engineer to find and fix the underlying cause — most are sorted in a single visit.
What pressure should my Vaillant boiler be at?
Aim for 1.0–1.5 bar when the system is cold. F.22 typically triggers when pressure falls to around 0.3–0.5 bar, depending on the model. Always check your specific boiler's manual for the exact recommended figure.
How much does it cost to fix an F.22 fault?
If a simple repressurise solves it, there's no cost. Common repairs such as a pressure sensor, PRV or small leak typically fall in the £100–£280 range including parts and labour, often after a diagnostic call-out fee. Larger jobs like a hidden leak repair or pump replacement can cost more, and a rare PCB or heat exchanger fault could run several hundred pounds — your engineer will quote before any work.