Boilers & heating
Boiler losing pressure
Pressure that drops once after bleeding radiators is normal. Pressure that keeps dropping usually means a leak somewhere in the heating system or a fault with a boiler part, and needs an engineer.
What the gauge should read
Most combi and system boilers work at around 1 to 1.5 bar when the heating is cold, but check your boiler's manual, as the right figure varies by model. Many boilers stop working if the pressure falls too low.
Common causes
- Bleeding radiators, which lets out some water and pressure
- A small leak from a radiator valve, pipe joint or the boiler itself
- A faulty pressure relief valve, often shown by a dripping pipe outside
- An expansion vessel that has lost its charge
Topping up the pressure
Most boilers have a filling loop so the pressure can be topped up, and the manual explains how. If you top it up and it drops again within days, stop topping up and get it looked at: something is losing water.
Safety: Don't remove the boiler casing or adjust anything inside the boiler. Work inside a gas boiler must be done by a Gas Safe registered engineer.
What to tell the engineer
- The boiler make and model
- The pressure reading, and how fast it drops
- Any error code on the display
- Any damp patches, drips or stains near pipes and radiators
Related services
Sources
- Domestic gas: frequently asked questions, Health and Safety Executive
Checked 26 September 2026.