Boiler losing pressure? Causes and how to top it up

What the gauge should read

Most combi and system boilers run at 1 to 1.5 bar when cold. If the needle sits below 1 bar (often in a red zone), the boiler will lock out with a low-pressure fault code such as Worcester EA, Vaillant F22 or Baxi E119.

Why pressure drops

• Bleeding radiators removes air and water, lowering pressure slightly each time.
• A small leak on a radiator valve, pipe joint or under floorboards. Even a drip you never see will empty the system over weeks.
• A faulty expansion vessel: pressure climbs when hot and falls when cold, and the pressure relief valve may drip outside.
• A passing pressure relief valve: look for a wet copper pipe on the outside wall.

How to top up

1. Let the boiler cool.
2. Find the filling loop, usually a braided silver hose with one or two small taps under the boiler.
3. Open the taps slowly until the gauge reaches 1.2 bar, then close both firmly.
4. Reset the boiler. If it has a detachable loop, remove it and cap the ends.

When to call us

If you are topping up more than once a month, something is leaking. Repeated top-ups add fresh oxygenated water that corrodes radiators and shortens the boiler's life. We find and fix the cause, typically a valve or expansion vessel, on a single visit.