Boiler help
Why Does My Boiler Pressure Keep Dropping?
Common reasons a boiler loses pressure, safe checks to make and signs that require a Gas Safe registered engineer.
Boiler pressure commonly falls because of a leak in the heating system, a faulty pressure relief valve, trapped air after radiator bleeding or a problem with the expansion vessel. Repressurising may restore heating temporarily, but repeated pressure loss needs professional diagnosis.
Check what the gauge is telling you
Many sealed heating systems operate at roughly 1 to 1.5 bar when cold, but the correct range is set by the boiler manufacturer. Check the manual or the marking on the display rather than relying on a universal number.
If the pressure falls again shortly after topping up, avoid repeatedly adding water. Fresh water can dilute corrosion inhibitor and may hide an underlying fault.
Common causes of pressure loss
Look for evidence without removing the boiler casing or touching gas components.
- Small leaks at radiator valves, pipe joints or towel rails.
- Water discharged through the copper pressure-relief pipe outside.
- An expansion vessel that no longer absorbs pressure changes correctly.
- Air released during radiator bleeding without the system being topped up afterwards.
- A hidden leak under flooring or within boxed-in pipework.
When an engineer is needed
Arrange an inspection when pressure drops repeatedly, rises sharply while heating is on, water appears from the external discharge pipe, the boiler locks out or there are signs of a concealed leak. Only an appropriately Gas Safe registered engineer should remove the boiler casing or work on gas-related components.
Frequently asked questions
Is low boiler pressure dangerous?
Low pressure usually causes the boiler to stop operating rather than creating an immediate gas danger, but the cause may be a leak or component fault and should not be ignored.
How often should I repressurise my boiler?
Occasional adjustment after radiator bleeding can be normal. If you need to repressurise repeatedly, arrange a professional diagnosis.