Why Is My Boiler Losing Pressure?

If your boiler's pressure gauge keeps dropping below the normal range (usually marked 1-2 bar on the dial), it means water is leaving the sealed heating system somewhere. It's one of the most common boiler complaints, and often has a simple cause.

What usually causes it

• A small, slow leak - often at a radiator valve, pipe joint, or the boiler itself. Sometimes visible as a damp patch, sometimes not.
• Recently bled radiators - releasing trapped air also releases a little water, which drops the pressure slightly. This is normal and easy to top up.
• A faulty pressure relief valve or expansion vessel - these fail over time and need a professional to diagnose and replace.

What you can safely check yourself

Most boilers have a simple filling loop (a flexible braided hose, sometimes detachable) that lets you top the pressure back up to the normal range. Your boiler's manual will show exactly how - it's usually just opening one or two valves until the gauge reaches around 1.5 bar, then closing them again.

If you're topping up the pressure repeatedly (more than once every few weeks), that points to a real leak somewhere rather than normal settling, and it's worth getting looked at rather than just keeps topping up.

When to call a Gas Safe engineer

If you can see or smell a leak, if the pressure drops rapidly (within hours rather than weeks), or if topping it up doesn't hold, stop and get a Gas Safe registered engineer to look at it. Boilers involve gas and pressurised hot water - don't attempt any work beyond the filling loop yourself.