How to repressurise a boiler in five minutes

What pressure should a boiler be?

With the heating off and the system cold, most combi boilers should sit between 1 and 1.5 bar. When the heating is running it is normal for that to rise towards 2 bar. Below 1 bar, many boilers lock out and show a fault code.

Step by step: topping up with the filling loop

The filling loop is a short braided hose or a small built-in lever, usually underneath the boiler, with a valve at one or both ends.

• Switch the boiler off and let the system cool
• Check both ends of the filling loop are attached securely
• Open the valve slowly; you will hear water flowing in
• Watch the gauge and close the valve at about 1.2 to 1.5 bar
• Close both valves fully, then switch the boiler back on
• Reset the boiler if it shows a fault code

Do not overfill

If you go above 2 bar while cold, bleed a radiator to let a little water out until the gauge falls back. Too much pressure can make the pressure relief valve discharge water outside through the small copper pipe on the wall.

When low pressure means a bigger problem

Topping up once after bleeding radiators is normal. Needing to top up every week is not. It usually points to a small leak, a failing pressure relief valve or a faulty expansion vessel, and those need a qualified engineer to find and fix.