How to Repressurise Your Combi Boiler: A DIY Guide

By The FixDelivered Team · Updated 2026-07-04 · 7 min read

A combi boiler that's losing pressure is a common household problem, often resulting in no heating or hot water. While it might seem daunting, repressurising your boiler is a task many homeowners can safely carry out themselves. This guide will walk you through the process, ensuring you understand why it happens and how to fix it, getting your home warm again.

Understanding Your Combi Boiler Pressure

Your combi boiler operates a 'sealed system' for your central heating, meaning the water within the heating circuit is kept at a consistent pressure. This pressure is crucial for the efficient circulation of hot water through your radiators and for the boiler to function correctly. Most combi boilers have a pressure gauge, usually a needle or a digital display, on the front panel.

The ideal boiler pressure when the system is cold (i.e., not running) is typically between 1 and 1.5 bar. When the heating is on and the water heats up, the pressure will naturally rise slightly, often to around 2 bar, which is perfectly normal. If the pressure drops significantly below 1 bar, your boiler may display an error code and stop working as a safety measure.

Common reasons for low boiler pressure include:

Bleeding radiators: Releasing air from your radiators will also release some water, causing a slight pressure drop.
Minor leaks: Even a tiny, unnoticeable leak in your heating system (pipes, radiators, or boiler) can lead to a gradual loss of pressure over time.
Recent maintenance: If a plumber has recently worked on your heating system, they might have drained some water, requiring a repressurise afterwards.
Expansion vessel issues: Less commonly, an issue with the boiler's expansion vessel can cause pressure fluctuations.

Safety First: Before You Start

Before you begin any work on your boiler, it's paramount to prioritise safety. Always read your boiler's instruction manual if you have it – it will provide specific guidance for your model.

Turn off your boiler: Switch your boiler off at the mains and allow it to cool down completely. This prevents accidental burns and ensures the pressure reading is accurate (cold pressure).
Identify your filling loop: Your boiler will have either an internal or external filling loop. Knowing which type you have is key to following the correct steps.
Be prepared for drips: Have a towel or absorbent cloth handy, as a small amount of water may escape during the process.

Crucially, for anything involving the internal workings of your boiler or gas supply, you MUST call a Gas Safe registered engineer. Never attempt to open the boiler casing or tamper with gas connections yourself. This guide is strictly for repressurising the sealed water system.

Tools You'll Need

Your boiler's instruction manual (if available)
A towel or absorbent cloth
(Possibly) A screwdriver, if your filling loop valves require one

Step-by-Step Guide: How to Repressurise Your Combi Boiler

The method you use depends on whether your boiler has an internal or external filling loop. Most modern combi boilers have an internal filling loop, making the process simpler.

Method 1: Internal Filling Loop (Most Common)

An internal filling loop is typically a short, braided hose with two small valves, often black or blue, located underneath your boiler and permanently connected.

Ensure the boiler is off and cold: As mentioned, switch off your boiler and let it cool down.
Locate the filling loop: Find the braided metal hose, usually silver, connecting two pipes underneath your boiler. Each end of this hose will have a small valve.
Open the valves slowly: Turn both valves (they might be levers or twist-knobs) anti-clockwise until they are fully open. You should hear water begin to flow into the system.
Monitor the pressure gauge: Keep a close eye on your boiler's pressure gauge. It will start to rise.
Close the valves: Once the gauge reaches the ideal cold pressure (typically between 1 and 1.5 bar), immediately close BOTH valves fully by turning them clockwise. It's vital to close both to prevent over-pressurisation and ensure the heating system remains sealed.
Check for leaks: Briefly inspect the filling loop and surrounding area for any drips.
Turn the boiler back on: Switch your boiler back on at the mains. It should now fire up and operate normally.

Method 2: External Filling Loop

An external filling loop is a separate, flexible hose that you connect to two designated points on your boiler only when repressurising. It is usually stored nearby, perhaps in a cupboard.

Ensure the boiler is off and cold: Switch off your boiler and let it cool down.
Locate the connection points: Find the two capped valves or connection points underneath your boiler, often with a small label indicating 'filling loop'.
Connect the external filling loop: Attach the flexible hose securely to both connection points. Ensure it's hand-tight to prevent leaks.
Open the boiler valves: Slowly open the two valves on the boiler where you've connected the hose. You might hear water starting to flow.
Monitor the pressure gauge: Watch the pressure gauge rise.
Close the boiler valves: Once the gauge reaches between 1 and 1.5 bar, immediately close BOTH boiler valves fully.
Disconnect the filling loop: Carefully disconnect the external filling loop from the boiler. Be prepared for a small amount of water to escape when you do this – have your towel ready.
Store the filling loop: Keep the external filling loop in a safe, accessible place for future use.
Check for leaks: Inspect the connection points for any drips.
Turn the boiler back on: Switch your boiler back on at the mains.

What to Do if Pressure Keeps Dropping

If you find yourself needing to repressurise your boiler frequently (e.g., once a week or more), it usually indicates a leak somewhere in your central heating system. This could be a visible leak from a radiator valve or pipework, or a less obvious, hidden leak within the boiler itself or under floorboards.

When to Call a Professional

While repressurising is a straightforward DIY task, there are several situations where calling a qualified professional is the safest and most sensible option:

You can't locate your filling loop: Some boilers have less obvious setups, or the manual might be missing.
You're unsure or uncomfortable: If at any point you feel uncertain about the process, don't proceed.
The pressure drops quickly or repeatedly: This strongly suggests a leak that needs professional diagnosis and repair.
There's a visible leak: Any water leaking from your boiler or pipework requires immediate attention from a plumber.
The boiler isn't working after repressurising: If the boiler still displays an error or fails to fire up, there could be an underlying issue.
You've over-pressurised the system: If the pressure goes above 2.5-3 bar, you might need to bleed a radiator to release some pressure, but if you're unsure, call a professional.

If you're unsure or suspect a leak, it's always best to call a qualified professional. FixDelivered can help you find a vetted local emergency plumber quickly. They'll diagnose the issue and get your heating back on safely. You can learn more about how it works.

As a rough guide for 2026, a plumber call-out to diagnose a persistent pressure drop might typically be in the region of £80-£150, varying by region and urgency, plus parts and labour for any repairs.

Preventing Future Boiler Pressure Issues

Regular servicing: Have your boiler serviced annually by a Gas Safe registered engineer. They can identify potential issues before they become major problems.
Bleed radiators correctly: If you bleed your radiators, do it slowly and check your boiler pressure afterwards, repressurising if necessary.
Check pressure periodically: Make it a habit to glance at your boiler's pressure gauge every few weeks, especially during colder months.

Repressurising your combi boiler is a valuable skill that can quickly restore your heating and hot water. By following these steps carefully, you can often resolve low boiler pressure issues yourself. However, remember that safety comes first, and for any complex issues or if you prefer professional assistance, remember FixDelivered can connect you with vetted local plumbers efficiently. Simply visit our homepage to get started.

Frequently asked questions

What is the ideal pressure for a combi boiler?

The ideal cold pressure for a combi boiler is typically between 1 and 1.5 bar. When the heating is on, it's normal for the pressure to rise slightly, often up to around 2 bar.

Why does my boiler pressure keep dropping?

If your boiler pressure keeps dropping, it usually indicates a leak somewhere in your central heating system. This could be a visible leak from a radiator or pipe, or a hidden leak within the boiler or pipework. It's best to call a professional to diagnose and fix a persistent pressure drop.

Can I repressurise my boiler myself?

Yes, repressurising your combi boiler is a common DIY task that many homeowners can safely perform. This guide provides step-by-step instructions. However, if you are unsure, uncomfortable, or suspect a more serious issue like a leak, you should always call a qualified plumber.

What should I do if I can't find my boiler's filling loop?

If you can't locate your boiler's filling loop, or if your boiler manual doesn't provide clear instructions for your specific model, it's best to contact a qualified plumber. They will be able to identify the correct method for your boiler and carry out the repressurisation safely.

Need a plumber now? Tell us your emergency and FixDelivered will find and confirm an available local plumber near you, usually within the hour.

More plumbing guides · How FixDelivered works