Boiler pressure problems are one of the most common reasons people call a plumber — yet in many cases, it's something you can resolve yourself in under 10 minutes. Here's everything you need to know.
What is boiler pressure?
Your central heating system is a sealed circuit filled with water. This water is held at a specific pressure — typically 1–1.5 bar when cold. When heated, the pressure rises slightly (to around 2 bar when hot). This is normal.
The pressure is shown on a gauge on the front of your boiler — usually an analogue dial or a digital display.
What pressure should my boiler be at?
How to re-pressurise your boiler
Most combi and system boilers have a filling loop — a flexible braided hose with a valve at each end that connects the cold mains supply to the heating circuit.
1. Locate the filling loop (usually underneath or behind the boiler)
2. Check both ends are securely connected
3. Open both valves (usually lever or screw type) slowly
4. Watch the pressure gauge — stop at 1.5 bar
5. Close both valves
6. If it has a built-in filling key (many Worcester Bosch models), simply turn the key
Why does pressure keep dropping?
If you need to re-pressurise regularly (more than once every few months), there is a small leak in the system. This is typically:
A plumber can pressure-test the system and locate the leak.
High pressure
If the gauge reads above 3 bar, the system is overpressured. This typically happens if you've re-pressurised too much. Most boilers have a pressure relief valve that opens at around 3 bar and releases water through a pipe to outside (often a small copper pipe discharging near the ground outside your property).
You can reduce pressure by bleeding a radiator slightly until the gauge drops to 1.5 bar.
If your boiler pressure problem persists, contact NBS Plumbing & Heating — **07507 133204**.