BoilerFaultCodesTechnical description
18si Only • If the boiler loses water: the pressure will be indicated, see diagram 1 (a) and the running lamp (b) will flash red indicating a fault.
What causes this fault?
This fault occurs when the water level inside the central heating system drops below a safe threshold, often due to a small leak in the pipework or a radiator. It can also happen if air has been bled from the radiators recently or if the internal expansion vessel has lost its charge, causing water to escape through the safety valve. Without enough water pressure, the boiler cannot circulate heat safely and shuts down to prevent the heat exchanger from overheating.
This issue often surfaces at the start of autumn when the heating is turned on for the first time after summer, as the change in temperature reveals small leaks or pressure losses.
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.