What is normal
Most boilers sit at around 1 to 1.5 bar when cold and rise a little when the heating is on. If it climbs above about 2.5 to 3 bar, something is wrong. Check your manual for your boiler’s range.
Common causes
The most common cause is the filling loop being left open or not fully closed after topping up, which keeps adding water. Another is a failed expansion vessel, which normally absorbs the pressure rise as the water heats. A faulty valve or gauge can also give a high reading.
What you can check
Look for the filling loop, usually a flexible braided hose under the boiler, and make sure its valve is fully closed. If you added too much water, let the system cool, then release a little by opening a radiator bleed valve slightly. Our guide to why a boiler loses pressure explains the filling loop.
The pressure relief valve
If the pressure goes too high, a safety valve can release water through a pipe that usually ends outside. Dripping from this pipe is a warning sign, so do not ignore it.
When to call an engineer
Call a Gas Safe engineer if the pressure keeps rising, climbs quickly when the heating comes on, or the relief pipe is dripping. Expansion vessel faults and valve repairs are not DIY jobs.
Boiler pressure running high? Our Gas Safe engineers cover South Manchester and Cheshire.
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