Boiler pressure dropping? Read this first

Boiler pressure should sit between 1.0 and 1.5 bar when the system is cold. If yours has dropped below 1.0, the boiler will usually lock out, error code F22 (Worcester), F75 (Vaillant) or 1F (Ideal) is displayed, and you lose heating and hot water at the worst possible moment. Here's how to diagnose and fix it.

**How to top up the pressure.** Find the filling loop — usually a silver braided flexi-hose under the boiler with two black levers. With the boiler off, open both levers slowly. Watch the pressure gauge climb to 1.2 bar, then close both levers fully. Reset the boiler. In most cases the system fires straight back up.

**The catch — it shouldn't need topping up often.** A sealed central heating system is, by design, closed. If you're topping up more than once or twice a year, you have a leak, not a top-up problem. Topping up repeatedly is masking the real issue and introducing fresh oxygen-rich water into the system, which accelerates corrosion of radiators and the boiler heat exchanger.

**Where the leak might be.** In order of frequency: (1) Radiator valve compression joints — look for green crystallised verdigris around the valve bodies; (2) The pressure relief pipe outside the house — if water drips or runs from this overflow pipe on the external wall, the expansion vessel has failed; (3) Hidden leaks under floors or in walls, especially around UFH manifolds and concealed pipework; (4) The boiler heat exchanger itself — rare on modern boilers under 10 years old.

**The expansion vessel test.** Press the schrader valve on the back of the boiler (or the external vessel if separately mounted) with a screwdriver. If water comes out, the expansion vessel diaphragm has failed and needs recharging or replacing — typically a 1-hour job for a Gas Safe engineer. This is the single most common cause of unexplained pressure loss on boilers 5+ years old.

**When to call a professional.** If the pressure drops faster than 0.3 bar per week, or you can see water around the boiler, valves or pipework, stop topping up and call a Gas Safe engineer. Continued top-ups will mask the leak until it's caused secondary damage to floors, ceilings or the boiler itself.

**Prevention.** An annual boiler service includes a check of the expansion vessel pre-charge, a flue gas analysis, and a visual inspection of every visible joint. £85–£120 once a year saves the considerably larger cost of emergency call-outs and premature boiler replacement.