BoilerFaultCodesYour boiler has lost water pressure and has shut down as a safety precaution to prevent damage.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Low water pressure
What causes this fault?
This fault occurs when the water level inside your central heating system drops below the minimum level required for the boiler to operate safely. The most common reasons are a slow leak from a radiator valve or pipework, or the air pressure inside the internal expansion vessel needs topping up.
This fault often appears when you first turn the heating back on for autumn, as internal components can stick or minor leaks become apparent after a summer of inactivity.
DIY fix possible
Locate the filling loop (usually two silver braided hoses) beneath the boiler.Gradually open the small black taps until you hear water entering the system.Watch the pressure gauge and close the taps once the needle reaches 1.5 bar.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F21 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Glow-worm 30ci Plus is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.