How to Increase Boiler Pressure

If your boiler pressure has dropped below 1 bar and the boiler has locked out, you need to repressurise the system via the filling loop. Here's exactly how to do it.

Locate the filling loop

The filling loop is a flexible braided hose (usually silver or grey) typically located under the boiler, connecting to two isolation valves. Some newer boilers have the filling loop built into the bottom panel.

Step-by-step

1. Make sure the heating is off and the boiler is cool.
2. Locate the two isolation valves on the filling loop. They'll either have screwdriver slots or small handles.
3. Slowly open both valves — you'll hear water entering the system.
4. Watch the pressure gauge on the boiler — stop when it reads 1.2–1.5 bar.
5. Close both filling loop valves fully.
6. Press the boiler's reset button to clear the low-pressure lockout.
7. Turn the heating on and check the pressure holds when the system heats up (it will rise to around 1.5–2 bar when hot — this is normal).

If pressure drops repeatedly

If you're repressurising more than once a month, you have a leak somewhere in the system — possibly a weeping radiator valve, corroded pipe joint, or faulty pressure relief valve. Call a Gas Safe engineer to investigate before pressure loss causes a bigger problem.