Boiler Lost Pressure? How to Re-pressurise It Yourself

Knowing how to re-pressurise your boiler is a useful skill that can save you an unnecessary call-out.

No matter what type of boiler you have, re-pressurising it will: 

• Keep your home’s heating efficient
• Extend your boiler’s life
• Lower potential engineer costs.

How check your boiler pressure

Most conventional modern boilers have a digital pressure reading or bar indicator display.

Older boilers have round hydraulic pressure gauges with a dial and needle, which can usually be found underneath the boiler.

In either case, pressure typically sits within a normal range of 1.2 - 1.5 bars (including in a cold, switched off boiler). 

Modern boilers will typically display a fault/error message or lock out if pressure falls below 0.5 or rises above 2 bar.

Low pressure

Your boiler’s pressure is too low if the needle/bar reading has moved to below 0.5. 

This could be caused by a recent radiator bleed, water leak or expansion vessel problem.

Repeated pressure drops can sometimes be caused by sludge and debris in the system.

In these cases, a power flush may be needed to restore it properly.

High pressure 

 Your boiler’s pressure is too high if the needle/bar reading has moved above 2. 

This could be caused by many factors, including:

• Excess water in the central heating system
• A leaky valve
• A faulty expansion vessel
• Thermal expansion of water.

(Read our guide on reducing boiler pressure if you believe that’s your issue.)

How to re-pressurise your boiler: which method do you need?

There are two ways to re-pressurise a boiler. Which method you need depends on your boiler model.

• Using a filling key: Used on newer Worcester Bosch boilers. The filling key is usually stored in a pull-out tray underneath the boiler.

• Using filling loop: The more common method, found on most other boiler makes and models. The filling loop is a braided silver hose, usually found underneath or beside the boiler.

If you're unsure which your boiler has, check underneath it. 

 If there's a pull-out tray with a key, you have a filling key system. 

If there's a silver hose with valves at each end, you have a filling loop.

Re-pressurising your boiler using a filling key

Step 1: Switch off your boiler and let it cool

Switch your boiler off and let it cool down for 4 - 6 hours before re-pressurising it.

Step 2: Locate and remove the filling key

Carefully pull out the tray underneath your boiler and remove the filling key attached to it. 

You may find the key is secured to the tray with a clip. If so, unclip it to remove.

Step 3: Insert the key into the the manifold

The manifold is the valve assembly where water enters the boiler. Next to it you'll find a small keyhole - insert your filling key here.

Step 4: Turn the key to the unlocked position

Turn the key approximately 45 degrees until it reaches the unlocked position.

Step 5: Watch the pressure gauge and stop at 1.5 bar

Watch the pressure gauge. As soon as it reaches 1.5 bar, turn the manifold nut clockwise to shut off the water flow and stop the pressure rising any further. 

Wait a moment to make sure the gauge settles at this pressure.

Top Tip!

If you can’t keep an eye on the pressure gauge while dealing with the filling loop, ask someone to help you. 

Step 6: Release pressure immediately if the gauge hits the red zone

If the pressure gauge enters the red zone, stop filling immediately and switch your boiler off. 

Don't attempt to bleed your radiator at this stage. At high pressure, radiators can release scalding water.

Instead, allow the boiler to cool completely before attempting anything further. And if the pressure stays high, don’t use the boiler - a malfunctioning boiler can also pose a carbon monoxide risk - and contact a Gas Safe registered engineer. 

Step 7: Lock the key and return it to the tray

When you've turned both the manifold nut and the manifold key back to the locked position, remove the filling key and put it back in the tray underneath your boiler.

Step 8: Switch your boiler back on

Switch your boiler back on. The pressure may fluctuate slightly at first, but it should settle within a minute or two.

Re-pressurising your boiler using a filling loop

Step 1: Switch off your boiler and check the hoses

Switch your boiler off and allow 4 to 6 hours for it to cool down. 

When your boiler has cooled, check the filling loop hoses are secure (they may need tightening).

Before working on your filling loop, make sure you know how to shut off the water supply at the stopcock. 

This is essential if anything goes wrong during the process.

Top Tip! 

A radiator, expansion tank or pressure relief valve leak could also cause a pressure drop, so it's worth checking these things, too.

Step 2: Open the filling valves

Open the filling valves. 

These can be unlocked by tap handles, or with a screwdriver, and will be found near to where the filling loop hoses connect to the boiler. 

As you turn the valve counter clockwise, you should start to hear water.

Step 3: Close the valves at 1 bar

When the pressure gauge on your boiler reaches 1 bar, it's time to close the valves.

Step 4: Switch your boiler back on

Once the gauge reads between 1.2 and 1.5 bar, switch your boiler back on.

Aftermath: Is your boiler pressure fixed?

Once you've topped up the pressure on your system, your boiler should be working well.

But if it begins to lose pressure again, call a boiler repair engineer.

Consider boiler cover

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