Error F.5

Safety precautions:

1) Turn the boiler off and allow it to cool completely before touching any components. Do not remove covers that expose live electrical parts or the gas supply. If you smell gas, evacuate the property and call the gas emergency number immediately. Never bypass or defeat safety devices.

2) Isolate electrical power to the boiler or turn the room fuse off if you will be working near wiring. If you are uncomfortable with any step, stop and call a Gas Safe engineer.

Initial homeowner checks you can do:

1) Wait for the boiler to cool (30 minutes or until the display temperature is low). Locate the overheat thermostat — typically clipped to the flow pipe/top left inside the casing and has a small red manual reset button between two connectors. Press the red button once to reset if it has popped out. Restore power and observe.

2) Note what happens after reset: does the boiler run normally, shut down again immediately, or show F.5 again? Record any additional symptoms (pump runs continuously, boiler reaches temperature then cuts out immediately, radiators cold, unusual noises).

3) Check system water pressure on the boiler gauge. For combi/pressurised sealed systems this is usually around 1.0–1.5 bar when cold. If pressure is low, repressurise using the filling loop per the boiler manual. If you do not know how, stop and call a professional.

4) Ensure a couple of radiators are turned up so water can circulate. Fully closed TRVs or all radiators turned down can stop flow and cause overheating. Bleed radiators to remove air pockets if any radiators feel cold at the top.

5) Listen/feel for the circulating pump when the boiler is calling for heat: you should hear or feel a small vibration from the pump. If the pump does not run when the boiler calls or runs constantly irrespective of demand, that indicates pump or wiring issues and you should call a Gas Safe engineer.

Specific diagnostic and fix steps (homeowner-level):

1) Reset the overheat thermostat once cooled (press the red button). Restart the boiler and watch. If the boiler runs and stays running, monitor for a few cycles to ensure the fault does not return.

2) If the F.5 returns, check for lack of flow: open several radiator valves, bleed radiators, and ensure the system pressure is correct. After bleeding, re‑check and repressurise if necessary.

3) If the pump is not running when the boiler is calling, or if it runs when the boiler is off, do not touch pump wiring. Note the behaviour and call a Gas Safe engineer — pump wiring mistakes or pump failure are common causes of overheating.

4) Inspect visually (without removing protective covers) for obvious signs: blocked or restricted condensate or filters on DHW side (if fault occurs on hot water mode only), visible leaks, heavy corrosion, or debris around pipework. If you have recently flushed or drained the system, debris may have been dislodged and could be blocking the heat exchanger or filters — a professional clean may be required.

5) If the boiler trips only once and then runs OK after reset, it may have been a transient condition (airlock, temporary lack of flow). However repeated trips, inability to reset, or the reset button not staying in are clear signs you must get an engineer.

6) Avoid attempting to replace sensors, thermostats, heat exchangers, or internal wiring yourself. These parts require specialist testing (multimeter/NTC resistance checks, pump electrical checks) and safe gas work.

When to call a professional:

1) Call a Gas Safe registered heating engineer if F.5 reoccurs after a proper cool-down and basic checks, if the pump is suspected faulty or miswired, if the reset button will not stay depressed, if you suspect a blocked heat exchanger, or if there are any gas/electrical concerns.

2) Tell the engineer you have an F.5 overheat fault, what you observed when resetting, the system pressure, whether the pump ran, radiator state (bleeding/open/closed), and any recent work (system flushing, refilling). That information speeds diagnosis.

Do not attempt:

- Do not bypass the overheat thermostat or any safety device. Never work on gas valves, burner assemblies or remove sealed covers exposing live components. These tasks must be carried out by a Gas Safe registered engineer.

Summary: A single manual reset after the boiler cools is often the first step, and simple circulation issues (air, closed TRVs, low pressure, stuck pump) are common causes and sometimes resolvable by the homeowner. If the fault returns, the pump is not functioning correctly, the reset button fails, or you are unsure, arrange for a Gas Safe engineer to diagnose and repair the root cause.