BoilerFaultCodesTechnical description
Under-floor system temperature fault
What causes this fault?
This fault occurs when the water temperature entering your underfloor heating loops exceeds a safe threshold, triggered by a safety thermostat. The most common reasons are a failure in the external mixing valve that regulates heat or a pump failure that prevents hot water from moving through the system quickly enough. It is designed to prevent excessive heat from warping floorboards or damaging sensitive pipework.
This fault is more common in autumn and winter when the underfloor heating is first switched on after a long period of inactivity, which can lead to seized pumps or stuck valves.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.