Overview
The FA fault on Ideal Logic Max and Logic Plus regular boilers means the boiler has detected the flow and return temperatures or connections are reversed or the boiler believes flow and return are behaving opposite to expectations. The control electronics compare the flow and return thermistor readings and/or detect circulation problems; if the signal indicates the flow is colder than the return (or the sensors appear swapped), the controller flags FA to protect the appliance and stop normal operation. Common causes are incorrectly fitted or swapped flow/return thermistors, thermistor clips not properly secured to the correct pipe, plumbing installed with flow and return pipes reversed at the boiler connections, a pump circulation problem (pump running the wrong way, seized, or blocked), airlocks or lack of system water causing abnormal temperature behaviour, or wiring/PCB faults. Severity is moderate: the boiler will often lock out or cycle, reducing heating and potentially causing repeated restarts and longer warm-up times. It is usually safe to run short trials after basic checks, but repeated lockouts or overheating risk mean investigation is needed. Some checks are reasonable for a competent homeowner (check pressure, radiator bleeding, confirm pipes and valves open, visual check of thermistor clips). Anything that requires opening the boiler, changing pump wiring or rotation, swapping thermistors on the PCB, or working on gas connections should be carried out by a Gas Safe registered engineer. If basic DIY checks do not clear the fault, call a professional to avoid damaging the boiler or voiding warranty.
Troubleshooting Steps
Safety precautions
1. Before any inspection, switch the boiler off at the external isolator or fused spur and allow it to cool. Do not open the boiler casing or attempt internal electrical or gas work. If you smell gas, leave the property and call your gas emergency number immediately.
2. If you are uncomfortable with any step, stop and contact a Gas Safe registered engineer.
Initial checks a homeowner can do
1. Check system pressure on the boiler gauge when the system is cold. Target is usually around 1.0 to 1.5 bar for system boilers. If pressure is low, repressurise to the recommended level using the filling loop, following the boiler manual, then press RESTART.
2. Ensure all radiator and boiler isolation valves are open (flow and return service valves at the boiler and the circulating pump valves). Ensure any TRVs or zone valves are not closed for whole-system tests.
3. Bleed radiators to remove airlocks. Work from the lowest radiators upwards, then check pressure again and top up if required.
4. Put the boiler into a heating call (set CH demand) and carefully feel the two pipes that enter/leave the boiler marked flow and return (use caution, they can be hot). Note which one gets hot first and which stays cool.
Specific diagnostic and fix steps
1. Confirm symptom: If the pipe labelled flow stays cool and the pipe labelled return becomes hot during a CH call, that strongly suggests flow/return reversed at the plumbing, pump running the wrong way, or thermistors/clips swapped. Note this observation for the engineer.
2. Check thermistor clips: Locate the flow and return thermistor clamps (small sensors clipped to the pipes). Are they on the correct pipes according to the boiler diagram and firmly clipped to bare pipe metal (not insulated)? If a clip is loose or sitting on insulation, push it firmly to the pipe and reboot the boiler. Do not remove clips from the pipe if you are unsure how to refit them correctly.
3. Reset the boiler: After the above checks, reset the boiler using the restart button or power cycle. If FA clears and stays off, monitor for recurrence. If it returns, proceed to next steps.
4. Confirm circulation: With CH demand on, check radiators gradually get warm. If radiators stay cold and boiler pipes do not show expected flow temperature, there may be an airlock, closed valve, or pump fault. Re-bleed radiators and check pump isolation valves are open.
5. Pump checks (do not open the boiler): Listen for the pump running. If the pump is noisy, grinding, or feels like it is not moving water, note this. If you suspect the pump is running the wrong way (common after pump replacement), this usually requires an engineer. Some pumps have a rotation switch but many modern sealed wet-rotor pumps must be fitted with correct electrical phase or replaced if incorrect.
6. If thermistors appear to be physically swapped (flow thermistor connected to return terminal on PCB or vice versa) and you are competent with basic checks, you may visually inspect wiring at the external terminal block (with power off). If wiring appears obviously swapped, do not reconnect without confidence — call an engineer. Incorrect wiring changes inside the boiler can be hazardous and may invalidate warranty.
7. After any changes, repressurise if needed, restart the boiler and observe whether FA persists. Note temperature behaviour and any fault repetition patterns.
When to call a professional
1. Call a Gas Safe registered engineer if FA does not clear after the simple checks above, if you find pump noise/failure, if flow/return pipes seem reversed at the boiler, if thermistor wiring appears wrong or loose, or if the fault is intermittent and causing repeated lockouts.
2. Tell the engineer what you observed: system pressure, which pipe was hot/cold, whether radiators warmed, whether you bled radiators, whether the pump was noisy, and whether thermistor clips were secure. This information speeds diagnosis.
3. Do not attempt internal boiler electrical repairs, change gas components, or test/replace the pump without a qualified engineer. Such work requires isolation, specialist tools, and Gas Safe registration.
Final notes
1. FA often stems from installation/configuration or circulation issues rather than a catastrophic boiler failure, but it can lead to reduced heating, frequent cycling, and potential stress on components if unresolved. Prompt professional diagnosis is recommended if basic homeowner checks do not fix it.
2. If the boiler is under warranty or was recently worked on (new pump, PCB, etc.), contact the installer or manufacturer support and record any service history before the engineer visit.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Ideal Logic Max & Logic Plus Regular Boiler.