Boiler Pressure Problems — Low Pressure, High Pressure & How to Fix Them

Understanding Boiler Pressure

Modern combi and system boilers are pressurised systems. The pressure gauge (usually on the front of the boiler) shows the water pressure in the heating system in bar. Most boilers operate correctly at 1–1.5 bar when cold, rising to about 2 bar when hot.

Low Boiler Pressure

Symptoms: Gauge reads below 1 bar; boiler may display a fault code (e.g., F22 on Vaillant, E118 on Worcester Bosch)

Common causes:

What to do: You can repressurise your boiler using the filling loop. Consult your boiler manual for the specific process. If pressure keeps dropping, there's likely a leak that needs professional investigation.

High Boiler Pressure

Symptoms: Gauge reads above 2.5–3 bar; pressure relief valve may be dripping outside

Common causes:

What to do: Do not try to fix this yourself beyond checking whether you can bleed some radiators. If the pressure relief valve is dripping water outside, call a Gas Safe engineer. A faulty expansion vessel is a common cause and usually straightforward to replace.

When to Call a Professional

Call Halo Heating if:

Call us — we cover West Sussex 24/7 for heating emergencies.