F22 Boiler Fault Code: What It Means on Every Brand

F22 is consistent across every brand that uses it — Vaillant, Glow-worm, Saunier Duval, Heatline and Protherm all share Vaillant Group control electronics, so F22 (shown as F.22 on some displays) always means low water pressure in the sealed heating system, typically below about 0.5 bar. It is the most homeowner-fixable code there is: re-pressurise to 1–1.5 bar via the filling loop with the system cold, reset, and only worry if it keeps coming back — recurring F22 points to a leak or a failed expansion vessel.

Up to: Lockout — engineer likely 5 brands use this code Meaning differs by brand

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F22 means	Severity	DIY?
GWGlow-worm	The F22 fault code on a Glow-Worm boiler signals that the water pressure inside your central heating system has dropped below the minimum threshold the boiler needs to operate safely —… Full guide →	Engineer may be needed	Some checks
HHeatline	The F22 code appears when your Heatline boiler detects that the water pressure inside the sealed heating system has fallen too low for safe operation — typically below 0.5 bar. Full guide →	Lockout — engineer likely	Some checks
PProthermshown as F.22	The F.22 fault code on Protherm boilers signals that the water pressure in your sealed heating system has fallen too low — typically below 0.3 bar — for the boiler to operate safely. Full guide →	Lockout — engineer likely	Some checks
SDSaunier Duval	The F22 code appears when your Saunier Duval boiler detects that the water pressure in the heating circuit has dropped too low — typically below 0.8 bar. Full guide →	Engineer may be needed	Some checks
VVaillantshown as F.22	F.22 is a low water pressure (system protection) fault. Full guide →	Lockout — engineer likely	Some checks

F22 brand by brand

Glow-worm F22

The F22 fault code on a Glow-Worm boiler signals that the water pressure inside your central heating system has dropped below the minimum threshold the boiler needs to operate safely — typically below 1.3 bar. When this happens, the boiler's built-in protection shuts the appliance down to prevent damage. You may notice a pressure reading as low as 0.3 bar flashing alongside the F22 code on the display. On some older Glow-Worm appliances, the same underlying fault may be displayed as F9 instead.

Causes, fixes & costs for Glow-worm F22 → · All Glow-worm codes

Heatline F22

The F22 code appears when your Heatline boiler detects that the water pressure inside the sealed heating system has fallen too low for safe operation — typically below 0.5 bar. Without sufficient pressure, the boiler cannot circulate water properly through the heat exchanger and radiators, so it locks out to protect itself from damage. While locked out, you will have no central heating and no hot water until the underlying cause is resolved and the boiler is reset.

Causes, fixes & costs for Heatline F22 → · All Heatline codes

Protherm F.22

The F.22 fault code on Protherm boilers signals that the water pressure in your sealed heating system has fallen too low — typically below 0.3 bar — for the boiler to operate safely. When this happens, the boiler locks out and shuts down to protect itself from running dry, which means you'll lose both central heating and hot water until the fault is cleared. You may also notice the pressure reading flashing on the display alongside the code. Protherm shares this fault code convention with other Vaillant Group boilers, so it carries the same meaning across that family of products. The fix is often straightforward — a simple top-up via the filling loop — but if pressure keeps dropping, there's usually an underlying problem that needs professional attention.

Causes, fixes & costs for Protherm F.22 → · All Protherm codes

Saunier Duval F22

The F22 code appears when your Saunier Duval boiler detects that the water pressure in the heating circuit has dropped too low — typically below 0.8 bar. Without sufficient pressure, the boiler cannot circulate water safely around the system, so it shuts itself down as a precaution. The result is a loss of both heating and hot water until the issue is resolved. Note: on certain Saunier Duval Condens models, F22 can instead indicate excessively high pressure (above 2.7 bar). If topping up pressure makes no sense given what you see on the gauge, check your specific model's documentation or call an engineer.

Causes, fixes & costs for Saunier Duval F22 → · All Saunier Duval codes

Vaillant F.22

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.

Causes, fixes & costs for Vaillant F.22 → · All Vaillant codes

Frequently asked questions

Is the F22 fault code the same on every boiler?

No. F22 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a F22 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a F22 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.