No heating, no hot water, and a pressure gauge sitting in the red, it's one of the most common reasons UK homeowners search for boiler help. The good news is that repressurising a boiler is usually a five-minute job you can do yourself, using either a filling loop or a filling key, depending on your boiler.

This guide walks through how to check your pressure, top it back up safely, and work out whether it's a one-off blip or a sign of something bigger, like a leak.

The easier way to book quality trades

Download app

What does repressurising a boiler mean?

Modern boilers rely on water pressure to circulate hot water around your radiators and taps. Over time, that pressure drops, sometimes gradually, sometimes overnight, and when it does, your boiler may stop working properly, lock out, or show a fault light.

Repressurising simply means topping the system back up to a safe working level, usually using a built-in filling loop or filling key. It's routine boiler maintenance, not a repair, and most people can do it themselves in a few minutes.

How to check your boiler pressure

Before you touch anything, check the gauge on the front of the boiler. There are two common types:

• Digital gauges show a number in bar (the pressure unit). Anything below 1 bar usually means low pressure.

• Dial gauges have a needle and coloured zones, typically red at each end and green in the middle. A needle sitting in the left-hand red zone means the pressure's too low; the right-hand red zone means it's too high.

For most combi and system boilers, you're aiming for a resting pressure of 1 to 1.5 bar when the heating is off and the system is cold. Check your manual, since some brands and models vary slightly.

Why does a boiler lose pressure?

There are two main causes of low boiler pressure:

• You've recently bled a radiator. Bleeding lets trapped air escape, but it also releases a small amount of water, which lowers pressure slightly. This is completely normal.

• There's a leak somewhere in the system. This could be a dripping radiator valve, a leaking pipe joint, or a fault with the pressure relief valve. Leaks are usually the reason if you find yourself repressurising the same boiler every few days or weeks.

If you're topping up pressure more than once every few months, it's worth getting a heating engineer to check for a leak before it causes bigger problems, our guide on what to do when your boiler keeps losing pressure covers this in more detail.

Need peace of mind? Every Checkatrade job is backed by the £1,000 guarantee.

Up to £1,000. Eligibility criteria and T&C’s apply.

How to repressurise a boiler step by step

Not every boiler uses the same system, so find the method that matches yours below.

Using an external filling loop

Most modern combi boilers use an external filling loop, a flexible braided hose with a valve at each end, either permanently attached or stored nearby.

1. Switch the boiler off completely and let it cool for at least four hours.

2. Check both ends of the filling loop are firmly attached to their valves.

3. Open both valves (you may need a screwdriver or the valve's own handle) to let cold water flow into the system.

4. Watch the gauge closely and close both valves again once it reaches 1 to 1.5 bar.

5. Detach the loop if it isn't a permanent fixture, keeping a towel handy for drips.

6. Turn the boiler back on and check it fires up normally.

Using an internal filling key

Some boilers, particularly older combi models, use a filling key instead.

1. Confirm the pressure is low on the front gauge.

2. Switch the boiler off and leave it to cool for at least four hours.

3. Unclip the filling key from its storage tray underneath the boiler.

4. Push the key into the manifold keyhole next to the manifold nut and turn to unlock.

5. Watch the gauge rise, and turn the key back to lock once it reaches around 1.5 bar.

6. Reclip the key into its tray and close the panel.

7. Turn the boiler back on.

Keyless internal filling loop

A smaller number of boilers use a keyless internal filling loop, usually a coloured lever tucked behind a small access panel at the bottom of the unit.

1. Switch the boiler off and let it cool down.

2. Locate the lever behind the access panel.

3. Pull and hold the lever while watching the gauge rise.

4. Release it once the pressure reaches 1 to 1.5 bar.

5. Close the panel and switch the boiler back on.

Once you've repressurised, most boilers need a manual reset before they'll fire up again, usually done by pressing and holding a reset button for a few seconds. Check your manual if you're not sure where this is on your model.

How to repressurise common boiler brands

Worcester Bosch boilers

Worcester boilers typically use an external filling loop. Attach both ends to the valves, open the taps to let mains water in, and watch the gauge closely, Worcester models often recommend topping up to just over 1 bar rather than the full 1.5.

Close both taps, detach the loop, and switch the boiler back on. If pressure keeps dropping with no obvious cause, our guide to common Worcester Bosch boiler problems covers other faults worth ruling out.

Vaillant boilers

Vaillant boilers show pressure on either a digital display or a physical gauge, depending on the model. The same 1 to 1.5 bar target applies, and most Vaillant boilers use an external filling loop, so the steps above will apply directly.

Baxi boilers

Baxi's range spans several boiler generations, so the exact method depends on your model, some use a filling key, others a loop. Check the panel underneath the boiler first, since that's usually where you'll find whichever fitting your model uses.

Worcester Bosch boiler problems and how to fix them: 2026 guide

Having problems with your Worcester Bosch boiler? Read our Worcester boiler troubleshooting guide, for some of the most common faults, solutions and when to call in the experts.

Learn more

Ideal, Glow-worm and other brands

Whatever boiler you have, the manual that came with it will show exactly where the filling point is and what pressure to aim for.

If you've lost the paper copy, most manufacturers publish manuals online by model number, which is usually printed on a sticker inside the front cover.

What if the pressure won't rise, or keeps dropping again?

A few things can stop repressurising from working properly:

• The valves aren't fully open, double check both ends of the loop or key are turned all the way.

• There's air still in the system, bleeding a radiator first can help water flow more freely.

• There's an underlying leak, if pressure keeps dropping within days of topping up, this is the most likely cause.

If you've bled a radiator recently, some pressure loss is expected, our guide on how to bleed a radiator explains how to avoid over-bleeding.

If you own a Worcester boiler and pressure is dropping with no visible leak anywhere, this is a surprisingly common and specific issue, see why is my Worcester boiler losing pressure with no leak for likely causes.

What does it cost if it's more than a quick top-up?

Repressurising itself doesn't cost anything if you do it yourself, it's just water. But if an engineer needs to visit, diagnose a leak, or the boiler needs a part, there are a few typical costs worth knowing:

A one-off boiler service averages around £100.

Boiler repairs typically range from £120–£750 depending on the fault, see our full boiler repair cost guide for a breakdown by job.

If your boiler is beyond an economical repair, our new boiler cost guide covers installation prices by boiler type

Boiler service cost calculator

If you want a quicker steer on what a service or repair might cost before you call anyone, Checkatrade's AI-assisted Job Estimate Calculator can give you a guide price based on real past jobs in your area.

Just answer a few quick questions about your boiler and the work needed.

Job Estimate Calculator

Get Started

Step 0/3

Repair or replace?

If your boiler is regularly losing pressure, getting old, or has needed several repairs recently, it's worth weighing up whether to fix it again or start looking at a replacement.

Our guide on whether to repair or replace your boiler walks through the main things to consider, including age, efficiency and repair history.

If it turns out to be more than a pressure top-up, you might need a Gas Safe registered engineer for the boiler itself, or a plumber if it turns out to be a pipework or leak issue elsewhere in the system.

Finding a heating engineer you can trust

Anything beyond repressurising, fault-finding, repairs, servicing or a new installation, needs a Gas Safe registered engineer.

It's a legal requirement for anyone working on a gas boiler, and it's worth checking a trade's registration number before booking.

Every checked trade on Checkatrade has a profile with genuine, verified customer reviews, so you can see how they've performed on similar jobs before you book.

The Checkatrade guarantee also gives you extra peace of mind on qualifying work.

Need reassurance? All tradespeople pass up to 12 checks before joining, and we keep checking after.

How to prevent your boiler losing pressure again

A few simple habits reduce how often you'll need to repressurise:

• Check the pressure gauge every month or so, especially over winter

• Bleed radiators gently, a little at a time, rather than leaving them open too long

• Get an annual service, which catches small leaks before they become bigger problems

• Keep an eye out for damp patches, drips or rust around radiators and pipework

Before you repressurise your boiler

• Boiler switched off and cooled

• Filling loop or key located

• Towels ready for drips

• Gauge checked before and after

• Engineer booked if pressure keeps dropping

FAQs

Checkatrade

Follow us

More Gas Boiler Servicing / Repair Articles

What to do if your boiler loses pressure

Should I repair or replace my boiler?

Boiler service costs in the UK (2026 price guide)

View all