The 4 likely causes — in the order we check them
- Visible leak at a radiator valve, towel rail or pipe joint
- Failed expansion vessel (the most common cause on boilers 5+ years old)
- Pressure-relief valve weeping outside through the overflow pipe
- Pinhole leak in copper pipework hidden under floors
Quick check you can do yourself
Look at the outside wall where your boiler is fitted. If you see water dripping from a copper overflow pipe, your PRV is weeping — usually a sign the expansion vessel has failed. Both are stock-part repairs we can complete in a single visit.
When to stop topping up and call
If you're topping the boiler up more than once a fortnight, stop adding water and call us. Repeated top-ups dilute the corrosion inhibitor and accelerate sludge build-up, which can lead to a costly powerflush.