Boiler
F11
The F11 error code on an Ideal boiler indicates a low water pressure fault — one of the most common boiler problems in UK homes and usually straightforward to resolve. In many cases you can fix it yourself in minutes, but persistent pressure loss can signal a more serious underlying issue.
What does this error mean?
Your Ideal boiler continuously monitors the water pressure inside the central heating system. When pressure drops below approximately 0.5 bar, the boiler locks out and displays F11 to protect internal components from running dry. The boiler will not fire until pressure is restored to the normal operating range of 1–1.5 bar (shown on the pressure gauge on the boiler itself).
Est. repair cost
£0 if resolved by repressurising yourself; £80–£150 for a Gas Safe engineer call-out; £200–£350 if expansion vessel replacement is required
🔍 Likely causes
🛠️ Step-by-step fix
Check the pressure gauge: Locate the gauge on the front or underside of the boiler. If it reads below 1 bar, low pressure is confirmed as the cause of F11.
Repressurise via the filling loop: Find the braided silver filling loop (usually under the boiler). Slowly open both valves to let mains water into the system, watching the gauge rise to 1.2–1.5 bar, then close both valves firmly.
Reset the boiler: Once pressure is correct, press and hold the reset button on the boiler for 3–5 seconds until the display clears. The boiler should fire up normally.
Check for visible leaks: Inspect all radiator valves, visible pipework, and the area beneath the boiler for damp patches, drips, or mineral staining. If you find a leak, do not repressurise — call a Gas Safe engineer.
Monitor the pressure over 48 hours: If pressure drops again within a day or two, you have a leak or a failing expansion vessel — stop repressurising and book a professional.
Do NOT attempt to adjust the gas supply, open the boiler casing, or touch any internal components — this must be done by a Gas Safe registered engineer.
📞 When to call a professional
Call a Gas Safe registered engineer immediately if you find any water leak, if the pressure relief valve is dripping outside, or if pressure drops repeatedly after repressurising. Tell the engineer: ‘My Ideal boiler is showing F11, I’ve repressurised it but it keeps losing pressure’ — this helps them arrive prepared. Never ignore recurring pressure loss; it can indicate a serious system fault.
⚖️ Repair or replace?
A Gas Safe engineer call-out to diagnose and fix a pressure fault typically costs £80–£150, or up to £250–£350 if the expansion vessel needs replacing. If your Ideal boiler is over 10–12 years old and also has other recurring faults, a new A-rated boiler (£1,500–£2,500 installed) may be more economical long-term than repeated repairs. For a boiler under 8 years old in otherwise good condition, repair is almost always the right call.