What the gauge should say
Most combi boilers have a pressure gauge on the front panel. With the heating off and the system cold, it should read between 1 and 1.5 bar. When the heating is on, a reading of up to about 2 bar is normal.
If the needle sits below 1 bar, the boiler may lock out and show a fault code. Your heating and hot water then stop.
Common causes
- A small leak somewhere in the system, often at a radiator valve or under a floor
- Water released when you bled a radiator
- A worn pressure relief valve that lets water out through the pipe outside
- A faulty expansion vessel inside the boiler
How to top it up
Your boiler manual shows the filling loop, usually a small tap or key underneath the boiler. Open it slowly, watch the gauge climb to about 1.2 bar, then close it fully.
When to call an engineer
If you need to top up more than once a month, water is leaving the system. Call us and we will find out where it is going before it damages a ceiling or floor.