Most UK combination and system boilers run happily with the pressure gauge reading between 1 and 1.5 bar when the heating is cold. Once the system warms up, the needle usually climbs a little, and anything up to around 2 bar while hot is normally nothing to worry about. If you are wondering what pressure should my boiler be, that 1 to 1.5 bar cold reading is the figure most manufacturers quote, and it is the first thing worth checking before you assume anything is broken.
A reading below 1 bar is the most common cause of a boiler locking out or refusing to fire. It usually means water has been lost somewhere in the system, often after radiators have been bled or a small leak has gone unnoticed at a valve. A reading that climbs above 2.5 to 3 bar, on the other hand, can point to an overfilled system or a faulty expansion vessel, and the pressure relief pipe may start dripping outside.
Topping the system up is straightforward on most modern boilers. Find the filling loop, usually a braided hose beneath the boiler, open the valves slowly, and watch the gauge rise to about 1.2 bar before closing them again. If you have just bled your radiators, expect the pressure to drop and top it up afterwards, since bleeding removes water as well as air. Check the gauge again the next day: a system that keeps losing pressure needs a heating engineer, not repeated top-ups.
Call a Gas Safe registered engineer if the pressure keeps falling, if the relief pipe is discharging, or if you cannot find the filling loop. Regular servicing keeps the expansion vessel charged and the pressure stable, and it is far cheaper than an emergency callout in winter. For a fuller explanation of readings, causes and fixes, see this guide to what pressure should my boiler be from Milo Magazine, and if your boiler is ageing and repairs are mounting up, it is worth reading about boiler replacement costs before you decide whether to repair or replace.
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