F23 Boiler Fault Code: What It Means on Every Brand

F23 reads the same across the Vaillant Group brands: the temperature gap between flow and return pipes is too large, which means water is not moving through the heat exchanger fast enough — low pressure, an airlock, a sludged system or a struggling pump. Check pressure and bleed radiators; beyond that it is a circulation diagnosis for an engineer.

Lockout — engineer likely 5 brands use this code

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 F23 means	Severity	DIY?
GWGlow-worm	The F23 code means your Glow-Worm boiler has detected an excessive temperature difference between the flow and return pipes and has shut itself down as a safety measure. Full guide →	Lockout — engineer likely	No
HHeatline	The F23 fault code on Heatline boilers indicates that the temperature difference between the flow pipe (water leaving the boiler) and the return pipe (water coming back) has exceeded 35°C. Full guide →	Lockout — engineer likely	Some checks
PProthermshown as F.23	The F.23 code on Protherm boilers signals that the temperature gap between the flow pipe (hot water leaving the boiler) and the return pipe (cooler water coming back) has grown dangerously… Full guide →	Lockout — engineer likely	Some checks
SDSaunier Duval	The F23 code appears when your Saunier Duval boiler detects that the temperature difference between the flow pipe (carrying hot water out of the boiler) and the return pipe (bringing… Full guide →	Lockout — engineer likely	Some checks
VVaillantshown as F.23	The F.23 fault code appears on Vaillant boilers when the temperature gap between the flow pipe (hot water leaving the boiler) and the return pipe (cooler water coming back) exceeds… Full guide →	Lockout — engineer likely	Some checks

F23 brand by brand

Glow-worm F23

The F23 code means your Glow-Worm boiler has detected an excessive temperature difference between the flow and return pipes and has shut itself down as a safety measure. Your boiler has two NTC thermistors — one monitoring the water temperature as it leaves the boiler (flow) and one monitoring it as it returns after circulating through your radiators. Some temperature drop across the system is perfectly normal, but when the gap between the two readings becomes too large, the boiler interprets this as a circulation problem and locks out to prevent overheating or damage. The boiler will remain off until the root cause is addressed and the fault is cleared.

Causes, fixes & costs for Glow-worm F23 → · All Glow-worm codes

Heatline F23

The F23 fault code on Heatline boilers indicates that the temperature difference between the flow pipe (water leaving the boiler) and the return pipe (water coming back) has exceeded 35°C. Under normal operation, the gap between these two temperatures should be much smaller. When it climbs too high, it tells the boiler that water is not circulating properly — the flow side is getting very hot while cooler water fails to return at the expected rate. As a safety measure, the boiler triggers a lockout and shuts itself down to prevent overheating or internal damage. The boiler will not restart until the underlying problem is resolved. Heatline sits within the Vaillant Group, and its Capriz, Advance, and Kalder ranges share design heritage with Vaillant products, meaning engineers familiar with Vaillant fault codes will recognise the F23 immediately.

Causes, fixes & costs for Heatline F23 → · All Heatline codes

Protherm F.23

The F.23 code on Protherm boilers signals that the temperature gap between the flow pipe (hot water leaving the boiler) and the return pipe (cooler water coming back) has grown dangerously large — exceeding 35°C (35K). The boiler interprets this as a sign that water is not circulating properly, raising the risk of localised overheating. Protherm's control board will tolerate a brief excursion, but once the fault has been logged five times it escalates to a full safety lockout requiring a manual reset. Because Protherm is part of the Vaillant Group, the F.23 definition is consistent across the Gepard (Cheetah), Panther and Leopard ranges and shares the same logic as Vaillant's own F.23 code. The underlying problem is nearly always related to poor water circulation, low system pressure, or a sensor giving inaccurate readings — not a gas-side fault — but it still demands prompt attention to protect the heat exchanger from damage.

Causes, fixes & costs for Protherm F.23 → · All Protherm codes

Saunier Duval F23

The F23 code appears when your Saunier Duval boiler detects that the temperature difference between the flow pipe (carrying hot water out of the boiler) and the return pipe (bringing cooled water back) has exceeded 35°C. This gap tells the boiler that water is not moving around the heating circuit properly — instead of flowing freely, it is stagnating near the heat exchanger, overheating, and triggering a safety shutdown. The boiler locks out to prevent damage until the underlying circulation problem is resolved.

Causes, fixes & costs for Saunier Duval F23 → · All Saunier Duval codes

Vaillant F.23

The F.23 fault code appears on Vaillant boilers when the temperature gap between the flow pipe (hot water leaving the boiler) and the return pipe (cooler water coming back) exceeds approximately 35°C. Under normal operation this difference should be modest, because water circulates steadily around the system. When the gap grows too large it tells the boiler that something is preventing proper circulation — water is being heated but not moving around the system efficiently enough. As a safety measure, the boiler triggers a lockout and shuts down to avoid overheating the heat exchanger or other internal components. The boiler cannot restart until the underlying problem has been resolved and, if necessary, a manual reset is performed.

Causes, fixes & costs for Vaillant F.23 → · All Vaillant codes

Frequently asked questions

Is the F23 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant the F23 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F23 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a F23 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.