FU Boiler Fault Code: What It Means on Every Brand

The FU fault code appears on 2 brands — Grant and Ideal — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 2 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 FU means	Severity	DIY?
GGrant	FU on a Grant Aerona³ heat pump (the 13kW and 17kW HPID13R32/HPID17R32 models) is a pump error: the circulating pump that drives water around your heating circuit is blocked, air-locked,… Full guide →	Engineer may be needed	Some checks
IIdeal	The FU fault code means your Ideal boiler has detected a temperature difference of more than 50°C between its flow pipe (hot water leaving the boiler) and its return pipe (cooler water… Full guide →	Lockout — engineer likely	No

FU brand by brand

Grant FU

FU on a Grant Aerona³ heat pump (the 13kW and 17kW HPID13R32/HPID17R32 models) is a pump error: the circulating pump that drives water around your heating circuit is blocked, air-locked, or working against a restricted water circuit. Grant's documented diagnosis splits on a voltage check: if the pump is receiving normal voltage but not doing its job, the pump is replaced; if the voltage reaching it is abnormal, the fault is upstream and the main PCB should be replaced. Without circulation the heat pump cannot move its heat anywhere, so the unit stops — protecting the plate heat exchanger from freezing or overheating depending on season.

Causes, fixes & costs for Grant FU → · All Grant codes

Ideal FU

The FU fault code means your Ideal boiler has detected a temperature difference of more than 50°C between its flow pipe (hot water leaving the boiler) and its return pipe (cooler water coming back). When that gap becomes too large, the boiler's control system concludes that water is not circulating properly through the heating system and shuts the boiler down in lockout to protect the heat exchanger from overheating. The boiler will not fire while this code is active. Ideal's own guidance suggests checking system pressure (aim for 1.0–1.5 bar), bleeding radiators to remove air, and attempting a single reset — but if the fault returns, an engineer visit is needed to investigate a possible circulation blockage or component failure.

Causes, fixes & costs for Ideal FU → · All Ideal codes

Frequently asked questions

Is the FU fault code the same on every boiler?

No. FU 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 FU 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 FU fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. For heat pumps, an MCS or F-Gas qualified heat pump engineer. 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.