Overview
The FU fault on Ideal Logic Max and Logic Plus regular/system boilers means the boiler has detected a temperature difference greater than 50°C between the flow and return pipes. In practice this indicates poor or no circulation of hot water through the heating circuit so the flow pipe is very hot while the return is much colder. The boiler will often lock out or disable central heating to protect the appliance when it sees this excessive differential. Common causes are restricted or blocked pipework, airlocks, a seized or failing pump, closed or partially closed isolation valves, very low system pressure, or faults with the flow/return thermistors or wiring. Some simple checks and basic actions can be done safely by a homeowner (pressure, bleeding radiators, checking valves, reset), but if the fault continues you should contact a Gas Safe registered engineer because more intrusive checks and repairs (pump replacement, flushing, thermistor or PCB work, gas or flue checks) are specialist and carry safety and warranty implications.
Troubleshooting Steps
Safety precautions:
- If you smell gas, do not touch the boiler — turn off the gas at the meter, open windows, leave the property and call the gas emergency number immediately.
- Switch the boiler off at the programmer and at the fused spur before touching any plumbing or electrical components. If you are unsure, leave the boiler off and call a professional.
- Do not open gas or sealed components, do not attempt soldering on gas pipes, and do not dismantle the pump, heat exchanger or gas valve unless you are Gas Safe qualified.
Initial checks a homeowner can do:
1. Note the boiler display and whether the boiler is in lockout. Press the RESTART button once after checking the basics below — do not keep resetting repeatedly.
2. Check system water pressure on the boiler gauge. Aim for around 1.0–1.5 bar (follow your manual). If pressure is low, repressurise to the manufacturer’s recommended pressure using the filling loop (close the filling loop when done). After repressurising, press restart.
3. Check radiator thermostatic valves and lockshields are open on a few radiators, especially the radiators nearest the boiler. Ensure any designer manifolds or motorised zone valves are open.
4. Check isolation valves at the boiler (flow and return isolation taps) are fully open. These are often under lever handles near the pipe connections; confirm they are parallel with the pipe.
5. Bleed radiators to remove airlocks starting with the highest radiator in the house. Air in the system can prevent circulation and give a large flow/return differential.
6. Listen for the pump running when the central heating is called: you may hear or feel the pump hum or slight vibration on the pipes. If the pump is silent when CH is on, note that for the engineer. Do not remove or dismantle the pump yourself.
Specific diagnostic and remedial steps (homeowner-level and what to report to the engineer):
1. After performing the initial checks (pressure, valves, bleeding), reboot the boiler by pressing RESTART. If FU clears and heating works, monitor for reoccurrence. If it returns, record when and any actions you took.
2. If pressure keeps falling or you cannot maintain 1–1.5 bar, there may be a leak or faulty component — do not continue to top up repeatedly without locating the cause; call an engineer.
3. If some radiators stay cold but others heat, or flow/return temp difference persists after bleeding and repressurising, this points to restricted flow (blocked pipe, sludge or scale) or a seized/failed pump. Note which radiators are affected and any unusual noises — this information helps the engineer.
4. If you have access to simple temperature tools (e.g. an infrared thermometer), measure flow and return pipe temperatures at the boiler. If delta is still >50°C after you’ve done the basic actions, stop and call a Gas Safe engineer — do not try to bypass or adjust thermistors yourself.
5. If the boiler displays other faults alongside FU (flow/return thermistor faults like L4/F4 or L5/F5), include these codes when you call the engineer; thermistor/clamp faults or wiring faults can mimic circulation problems.
When to call a professional and what they will check:
- Call a Gas Safe registered engineer if the FU code persists after you have checked pressure, opened isolation valves, bled radiators and restarted the boiler, or immediately if you smell gas, cannot repressurise, hear pump seizure, or are unsure.
- The engineer will safely isolate the boiler, check pump operation (electrical supply, impeller seized, correct speed), inspect and test motorised valves, check and re-secure flow and return thermistors and wiring, test the boiler PCB and related controls, look for blockages, carry out system flushing or power-flush if necessary, and check for scale or heat-exchanger restrictions. They will also test gas supply and flue components if relevant.
Final notes:
- FU indicates a circulation issue rather than an immediate gas flame problem, but it can cause loss of heating and risk of component stress — treat it promptly.
- Do not attempt gas, electrical or pump internals work yourself; always use a Gas Safe registered engineer for repairs beyond the basic homeowner checks listed above.
- Provide the engineer with the exact fault code, the steps you have already taken, and any other fault codes or symptoms (cold radiators, noises, pressure loss) to speed diagnosis.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Ideal Logic Max & Logic Plus Regular Boiler.