How to Repressurise Your Boiler

Low boiler pressure is one of the most common reasons for heating and hot water problems. The good news is that repressurising your boiler is simple and safe to do yourself.

What Is Boiler Pressure?

Boiler pressure measures the pressure of the hot water circulating through your heating system. Most boilers need pressure between 1.0 and 1.5 bar when cold, rising to around 2 bar when the heating is on.

Signs of Low Pressure

Pressure gauge shows below 1 bar
Error code displayed (often E10, E119, F22, or similar)
Heating not working
Radiators cold at the top
No hot water

Before You Start

If you smell gas or suspect a gas leak, do NOT attempt any work. Turn off the gas supply, open windows, leave the property, and call the Gas Emergency line on 0800 111 999.

Make sure your boiler is cold before repressurising. Turn off the heating and wait at least 30 minutes.

Step-by-Step Guide

Step 1: Locate the Filling Loop

The filling loop is usually a braided silver hose with one or two valves/taps. It connects two pipes under or near the boiler.

Where to find it:

Combi boilers: Often underneath the boiler, already attached
System boilers: May be near the boiler or by the hot water cylinder
External filling key: Some modern boilers use a separate key that slots in

Step 2: Check the Current Pressure

Look at the pressure gauge on your boiler. It's usually a dial or digital display. If it shows below 1 bar (often in the red zone), you need to add pressure.

Step 3: Open the Valves

With the filling loop connected:

Slowly open the first valve – you'll hear water starting to flow
Open the second valve if there is one
Watch the pressure gauge

Open valves slowly and in small increments. It's easy to add too much pressure if you rush.

Step 4: Top Up to Correct Pressure

Keep the valves open until the pressure gauge reads between 1.0 and 1.5 bar. This usually takes 10-20 seconds.

Step 5: Close the Valves

Close both valves firmly. The water flow sound should stop completely.

Step 6: Remove External Filling Key (If Used)

If your boiler uses a removable filling key, remove it now. Store it somewhere safe – you'll need it again.

Step 7: Reset the Boiler

If the boiler was showing an error code, you may need to press the reset button. Turn your heating on and check it's working.

Common Mistakes to Avoid

Over-pressurising: Don't go above 1.5 bar when cold. If you do, you'll need to bleed a radiator to release some pressure.
Leaving valves open: Always close both valves completely to prevent the system overfilling.
Ignoring recurring pressure loss: If you're topping up frequently (more than once every few months), there may be a leak.

When to Call an Engineer

Repressurising should be a rare maintenance task. If you're doing it regularly, you likely have:

A leak somewhere in the heating system
A faulty expansion vessel
A faulty pressure relief valve

These require a Gas Safe registered engineer to diagnose and fix. Contact us for expert boiler repair.