Low Boiler Pressure
Gas Safe Reg. 980201• Same-day across Bradford, Leeds & Yorkshire
Most boilers operate at 1.0–1.5 bar when cold. If yours is dropping repeatedly or sitting under 0.5 bar, the boiler will lock out and stop heating. Topping up via the filling loop is fine as a quick fix — but if it keeps dropping you've got a leak or a failed expansion vessel and you'll need an engineer.
Most likely causes
- 1Failed expansion vessel (most common cause of repeat pressure loss)
- 2Slow leak on a radiator valve, towel rail or hidden pipework
- 3Pressure relief valve passing — water dripping from outside discharge pipe
- 4Auto air vent stuck open
- 5After bleeding radiators — system needs topping back up
What you can safely check first
Underneath the boiler look for a silver braided hose with two black handles. Slowly open both until pressure reads 1.0–1.5 bar, then close them. Don't overfill above 2.5 bar.
Walk every radiator and check valves underneath, plus any pipe joints in airing cupboards or under floors. Even a tiny drip can drop pressure overnight.
Look for a copper pipe coming out of an external wall. If water is dripping from it, the pressure relief valve is passing — you need an engineer.
- Pressure drops within hours of topping up (likely a hidden leak)
- Water visible from outside discharge pipe
- Boiler showing F1, F22 or low-pressure fault codes after repeated top-ups
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