A modern combi boiler needs water pressure of around 1 to 1.5 bar when the heating is cold. If your boiler pressure is dropping and the gauge falls towards zero, the boiler will lock out to protect itself and you lose heating and hot water. The good news: many causes are simple, and you can check them yourself in a few minutes.
Safety first. Never open the boiler casing. If you smell gas, leave the property, call the gas emergency number on 0800 111 999 and do not use switches.
1. Read the gauge when the heating is cold
Pressure rises when the system heats up, so always check it cold, first thing in the morning. Between 1 and 1.5 bar is normal. Below 0.5 bar most boilers stop and show a fault code.
2. Did you bleed a radiator recently?
Bleeding radiators lets air out, and some pressure goes with it. That is normal. Top the system back up using the filling loop under the boiler, following the steps in your boiler manual. See our guide on how to bleed a radiator for the full routine.
3. Look for small leaks
Check under radiators, around radiator valves, along visible pipes and under the boiler for damp patches, drips or green and white crusty deposits. A tiny weep from a valve can drain the pressure over a few days.
4. Check the pressure relief pipe outside
Most boilers have a small copper pipe that runs through the outside wall. If it drips, the pressure relief valve is letting water out, often because the pressure went too high when the system was topped up. This needs an engineer.
5. Is the condensate pipe frozen?
Not a pressure problem, but in a cold snap it is the most common reason a boiler stops. If the plastic pipe outside is frozen, pour warm (not boiling) water along it and reset the boiler.
When to call an engineer
If you need to top up more than once a month, the pressure keeps dropping with no visible leak, or the relief pipe drips, book a boiler repair. A hidden leak or a failing expansion vessel is usually the cause, and both are quick fixes for an engineer.