By Mohammed Miah, Gas Safe registered heating engineer
Most boilers run happily between 1 and 1.5 bar of pressure. If the needle on your pressure gauge keeps dropping below that, something is letting water, or air, out of a sealed system that's supposed to stay full.
The usual suspects
- A small leak somewhere in the pipework, a radiator valve, or the boiler itself, often too slow to see, but enough to drop pressure over days or weeks.
- A faulty pressure relief valve letting water escape when the system gets slightly too full.
- Air being released through an automatic air vent, which can look like a pressure drop.
- A recently bled radiator that hasn't been topped back up.
If you've bled a radiator recently, start there, topping the system back up to the correct pressure using the filling loop is often all it takes. If pressure keeps dropping again within a few days, that points to an actual leak rather than trapped air.
When to call an engineer
If you can't find a visible leak, if the pressure drops quickly rather than gradually, or if you're not confident using the filling loop, it's worth getting it looked at. We can pressure-test the system to find exactly where it's losing water and fix it properly rather than just topping it up on repeat.
