Boiler Pressure: Everything You Need to Know

What Is Boiler Pressure?

Modern sealed central heating systems are pressurised — they contain a fixed volume of water held under pressure. The pressure gauge (usually a dial or digital display on the front of the boiler) shows this pressure in bar. Correct operating pressure is typically 1–1.5 bar when the system is cold, rising to around 2 bar when hot.

Low Boiler Pressure

Low pressure (below 1 bar when cold) is the most common issue. Symptoms include the boiler switching off, displaying an error code, or failing to provide adequate heating. Common causes include:

What to do: Repressurise the system using the filling loop — a flexible hose connected between the cold mains supply and the heating system. Your boiler manual will explain the process. If pressure drops repeatedly within days, there is a leak that requires professional investigation.

High Boiler Pressure

High pressure (above 2.5–3 bar) is less common but can be more serious. It may cause the pressure relief valve to discharge water through an overflow pipe outside. Common causes include overfilling when repressurising, or a faulty expansion vessel.

What to do: If the pressure is only slightly high, bleeding a radiator may reduce it. Do not attempt to adjust the pressure relief valve. If the overflow pipe is dripping water continuously, call a Gas Safe engineer promptly.

When to Call a Professional

Call Halo Heating on 01483 676108 for boiler pressure problems across Surrey.