Boiler pressure problems are one of the most common issues UK homeowners face — and one of the most frustrating. Whether your boiler keeps losing pressure, the gauge is too high, or the pressure keeps rising for no clear reason, understanding what’s happening and how to fix it can save you time, stress, and money.
This comprehensive guide from JustServicesGroup walks you through everything you need to know about boiler pressure, including how to repressurise, depressurise, and prevent problems in the first place. We’ll explain what normal pressure looks like, the most common causes of issues, how to fix them safely, and when it’s best to call in a Gas Safe registered engineer.
What Is Boiler Pressure and Why Does It Matter?
Your boiler is part of a sealed central heating system. Instead of relying on gravity or open water tanks, it uses pressurised water to circulate heat efficiently around your home. That pressure is measured in bars, shown on a small gauge on the front of your boiler.
Most modern combi and system boilers in UK homes should have a cold pressure between 1.0 and 1.5 bar. When your heating is on and the water expands, it’s normal for pressure to rise slightly — up to around 2.0–2.5 bar. Anything much lower or higher than these ranges could indicate a problem.
Getting boiler pressure right matters because:
- ⚙️ Too low pressure can stop your heating and hot water from working.
- 🚨 Too high pressure can trigger safety shutdowns or even cause leaks.
- 💸 Incorrect pressure reduces efficiency, which can increase your energy bills.
Regularly checking your boiler pressure and acting quickly if it’s off can help you avoid bigger, more expensive issues later.
Signs of Boiler Pressure Problems
Whether pressure is too low, too high, or keeps changing unexpectedly, there are a few telltale signs homeowners often notice:
- ❄️ Radiators not heating properly or cold at the top
- 🔥 Boiler shutting down or displaying error codes
- 💧 Leaks or water dripping from pipes, valves, or the boiler itself
- 📉 Pressure gauge reading below 1 bar or above 2.5–3 bar
- ♻️ Pressure keeps rising even when the boiler is off
If you notice any of these symptoms, don’t panic — most boiler pressure problems are straightforward to fix once you know what’s causing them.
If you’re unsure or don’t feel confident tackling boiler issues yourself, our Gas Safe engineers at JustServicesGroup are here to help. We can quickly diagnose the cause and get your heating system back to perfect working order.
Boiler Losing Pressure – Why It Happens and How to Fix It
One of the most common boiler problems UK homeowners face is low boiler pressure — when the pressure gauge dips below 1.0 bar. If this happens, your boiler might still switch on, but your heating and hot water could stop working properly, or the boiler might shut down completely to protect itself.
The good news? Low boiler pressure is usually easy to fix. But first, let’s look at why it happens.
🔎 Why Boilers Lose Pressure
Here are the most common reasons your boiler might keep losing pressure:
- Leaks in the heating system
Even a tiny leak in a pipe, radiator, valve, or joint can slowly reduce pressure. You might notice small damp patches, rust marks, or water beneath radiators. In some cases, leaks are hidden under floors or behind walls — which is when it’s best to call a professional. - Bleeding your radiators
If you’ve recently bled your radiators, some water will have escaped from the system, which naturally reduces the pressure. This is totally normal — you’ll just need to top it up again. - Faulty pressure relief valve (PRV)
If the PRV is letting out water when it shouldn’t, the system will lose pressure over time. This often happens if the valve has become worn, dirty, or stuck partially open. - Expansion vessel issues
Inside most modern boilers is an expansion vessel — a small tank that helps regulate pressure as water expands when it heats up. If the vessel loses its air charge or fails, it can cause pressure to fluctuate or drop too low. - Internal boiler leaks
Occasionally, components inside the boiler (like the heat exchanger or seals) can degrade, leading to hidden water loss and pressure drops.
🧪 How to Check for Low Pressure Causes
Before repressurising your system, it’s a good idea to investigate why pressure is dropping:
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Check visible pipework and radiators for leaks or damp spots.
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Look under the boiler for signs of water discharge.
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Inspect the pressure relief valve (usually a copper pipe pointing outside). If you see dripping water, the valve may need replacing.
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Monitor pressure over time. If it keeps dropping after you top up, there’s likely a leak or internal fault.
If you spot a leak or suspect an internal issue, don’t attempt repairs yourself – call a Gas Safe registered engineer.
How to Repressurise Your Boiler (With a Filling Loop)
If the pressure is simply too low, you can usually fix it by topping up the water in the system. Most combi and system boilers in the UK use a filling loop — a flexible, braided metal hose with small valves at each end.
Here’s how to do it safely:
🔧 Step-by-Step: Repressurising with a Filling Loop
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Turn off your boiler
Make sure the heating and hot water are off and the boiler has cooled down before you start.
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Locate the filling loop
It’s usually found underneath the boiler and connects the mains cold water to the central heating system. You’ll see two small valves, often with black or blue handles.
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Attach the filling loop (if detachable)
Some filling loops are built-in, while others need attaching. If yours is removable, connect it securely at both ends.
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Open the valves slowly
Turn both valves slowly to allow cold water to enter the system. You should hear water flowing.
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Watch the pressure gauge
Keep an eye on the gauge as the pressure rises. Stop once it reaches around 1.2 to 1.5 bar.
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Close both valves fully
This is important — leaving a valve slightly open is a common reason for boiler pressure creeping too high later.
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Remove the filling loop (if detachable)
Disconnect it and store it safely for next time.
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Turn your boiler back on
Once repressurised, switch your boiler back on and check that it’s running normally. Your heating and hot water should now work properly again.
✅ Top tip: After repressurising, keep an eye on the pressure for the next 24–48 hours. If it drops again quickly, you may have a leak or another underlying issue that needs professional attention.
How to Repressurise a Boiler Without a Filling Loop
Some older boilers — or certain system setups — don’t have a traditional filling loop. Don’t worry, you can still repressurise them, but the process might look a little different.
Here’s how:
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Locate the built-in filling key or internal valve
Some Worcester Bosch and other boilers use a key system or internal knob instead of an external filling loop. You’ll find it on the underside of the boiler.
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Insert the key and unlock it (if required)
Push and twist the key into position until it locks in place.
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Turn the valve to let water in
As with a standard filling loop, water will enter the system and pressure will rise.
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Close the valve and remove the key
Once the gauge reads around 1.2–1.5 bar, close the valve and remove the key if applicable.
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Restart the boiler and check operation
Your boiler should now be back to normal pressure and functioning correctly.
If you’re not sure how to repressurise your specific model, always check your manufacturer’s manual or contact a Gas Safe engineer for help.
🚨 When Repressurising Doesn’t Work
If you’re topping up your boiler frequently — for example, more than once every few weeks — that’s a sign something isn’t right. Constant pressure loss points to leaks, a faulty expansion vessel, or another internal fault.
In this case, it’s time to call in the experts. Continuing to repressurise a faulty system without fixing the root cause can cause more damage over time.
Boiler Pressure Too High – Causes, Symptoms, and Fixes
While low pressure is more common, some homeowners experience the opposite problem: boiler pressure that’s too high. If your gauge is reading above 2.5–3 bar, especially when the boiler is cold, that’s a red flag.
A boiler running at too high a pressure can trigger safety shutdowns, cause leaks, and put unnecessary strain on internal components. Thankfully, once you know why it’s happening, you can usually bring it back under control.
🔎 Symptoms of High Boiler Pressure
Some of the most common signs that your boiler pressure is too high include:
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📈 The pressure gauge is consistently reading above 2.5 bar (or above 3 bar when hot)
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💦 Water is dripping from the pressure relief valve (PRV) outside your property
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🔥 The boiler switches off unexpectedly or displays an error code
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💧 Radiators or pipe joints are leaking
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🔁 The pressure keeps climbing even when the heating is off
⚙️ Common Causes of High Boiler Pressure
Here’s why pressure might be too high in your system:
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Overfilling the system
The most common cause is simply adding too much water when repressurising. Even a slight overfill can push the pressure above normal levels.
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Filling loop left open
If the valves on your filling loop haven’t been closed properly, water will continue entering the system, increasing pressure over time.
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Faulty expansion vessel
If the expansion vessel inside your boiler loses its air charge, it can’t absorb the expanding water as it heats. This leads to pressure spikes and can cause the PRV to release water.
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Blocked or stuck pressure relief valve (PRV)
A malfunctioning PRV might not release excess pressure as it should, allowing it to rise too high.
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Incorrectly sized or installed system components
In some cases — especially after new radiator installations or boiler replacements — pressure issues can result from a mismatch between system size and boiler capacity.
How to Reduce Boiler Pressure (How to Depressurise)
If your boiler pressure is too high, the first step is to safely reduce it. The most common and straightforward way is by bleeding your radiators.
🩸 Step-by-Step: How to Depressurise Your Boiler by Bleeding Radiators
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Turn off your boiler and let it cool
Working on a hot system can be dangerous and less effective.
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Locate the highest radiator in your home
Start with the radiator on the top floor — air and excess pressure rise to the top of the system.
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Use a radiator key to slowly open the bleed valve
Hold a cloth or small bowl underneath to catch any water that escapes.
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Release water until pressure drops
Keep an eye on the pressure gauge as you bleed. Once it drops back into the 1.0–1.5 bar range, close the valve.
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Check the boiler and system pressure
Once finished, turn your boiler back on and monitor the gauge again. If it’s stable, you’ve solved the issue.
🧰 Alternative: Use a Drain-Off Valve
If pressure is very high or bleeding radiators doesn’t help, you may need to drain a small amount of water from the system using a drain-off valve (usually located on a pipe near the lowest point of your system). This is a slightly more advanced step and may be best handled by a professional if you’re unsure.
✅ Important: Never attempt to release pressure by opening anything inside the boiler casing. Doing so is unsafe and could void your warranty.
Combi Boiler Pressure Keeps Rising – What It Means
It’s normal for pressure to increase slightly when your boiler is heating water. This happens because water expands as it heats, and the system’s expansion vessel is designed to absorb that extra volume.
However, if your combi boiler pressure keeps rising significantly, especially when the heating is off, it suggests there’s an underlying fault.
🔍 Common Reasons Your Combi Boiler Pressure Keeps Rising
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Filling loop left slightly open
Even if the valve looks closed, a small amount of water might still be entering the system. Double-check both ends are fully shut.
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Overfilled system
If you accidentally topped up the pressure too much last time, that excess water will continue to cause high readings.
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Faulty expansion vessel
A failing vessel can’t absorb expanding water, so pressure builds rapidly when the system heats up.
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Pressure relief valve issues
If the PRV is blocked or stuck, it won’t release excess pressure, causing it to climb steadily.
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Mains water pressure too high
In rare cases, your home’s water supply pressure might be too high, pushing pressure into the heating system through the filling loop. A pressure-reducing valve (PRV) on the mains supply can fix this.
🛠️ What to Do If Your Combi Boiler Pressure Keeps Rising
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Double-check the filling loop — Make sure both valves are completely closed and the loop is disconnected if removable.
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Bleed your radiators — This releases excess water and helps reduce pressure.
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Monitor pressure over time — If it keeps climbing even after taking these steps, it’s likely a faulty expansion vessel or PRV.
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Contact a Gas Safe engineer — These components are internal and should only be repaired or replaced by a professional.
How to Prevent Boiler Pressure Problems
While boiler pressure issues are common, many can be prevented with a little regular care and attention. A well-maintained boiler will not only perform better but also last longer and cost less to run.
Here are some practical steps UK homeowners can take to reduce the risk of pressure problems:
🔄 1. Check Boiler Pressure Regularly
Make it a habit to glance at your boiler’s pressure gauge once a month — especially in autumn and winter when the system is working hardest. Catching a change early can prevent bigger issues.
🧪 2. Bleed Your Radiators Annually
Trapped air in radiators reduces heating efficiency and can affect pressure. Bleeding them before winter starts helps your system run smoothly and avoids pressure drops.
🧰 3. Book an Annual Boiler Service
Annual servicing by a Gas Safe registered engineer is one of the best ways to keep pressure stable. They’ll check crucial components like the expansion vessel, pressure relief valve, and internal seals — spotting potential issues before they become major problems.
💧 4. Check for Leaks Around Your Home
Even small, slow leaks can reduce boiler pressure over time. Keep an eye out for damp patches around radiators, valves, or visible pipework.
🧼 5. Powerflush and Add Inhibitors
Over time, sludge and scale can build up inside your heating system, affecting pressure stability. A professional powerflush and the addition of a chemical inhibitor help protect your boiler and radiators from internal corrosion.
🛠️ 6. Don’t Overfill When Repressurising
When topping up your boiler, aim for around 1.2–1.5 bar (cold). Overfilling can lead to high pressure and further problems down the line.
When to Call a Gas Safe Engineer
Some boiler pressure issues can be safely handled at home — like topping up pressure or bleeding radiators. But there are times when it’s safer and smarter to call in a professional:
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🔧 Pressure drops repeatedly despite topping up
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💧 You can’t find the source of a pressure loss
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🚨 Pressure keeps rising even when the boiler is off
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🛠️ The PRV is leaking or stuck
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🧯 The boiler shuts down frequently or displays fault codes
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🔥 You suspect an internal leak or component failure
Attempting DIY repairs inside your boiler can be dangerous and may void your warranty. A Gas Safe registered engineer has the tools and training to safely diagnose and fix internal faults – often saving you money in the long run.
Boiler Pressure FAQs – Quick Answers for Homeowners
Here are some of the most common questions UK homeowners ask about boiler pressure:
🔎 What should my boiler pressure be?
For most modern combi and system boilers, the ideal pressure when cold is 1.0–1.5 bar. When the heating is on, it’s normal for pressure to rise to around 2.0–2.5 bar.
📉 Why does my boiler keep losing pressure?
The most common causes are leaks in the heating system, air released when bleeding radiators, a faulty pressure relief valve, or problems with the expansion vessel.
📈 Why is my boiler pressure too high?
It’s often caused by overfilling the system, leaving the filling loop open, or faults with the expansion vessel or PRV. Water expanding during heating will also increase pressure slightly — that’s normal.
🩸 How do I reduce boiler pressure?
The simplest way is to bleed a radiator to let out water until the gauge reads 1.2–1.5 bar. If pressure remains high, a professional may need to check internal components.
🔧 How do I repressurise my boiler?
Use the filling loop to let water into the system until the gauge reaches the correct range. If your boiler doesn’t have a filling loop, use the built-in key or valve system. Always close valves fully afterward.
🔥 Is it safe to fix boiler pressure myself?
Basic tasks like repressurising or bleeding radiators are safe for most homeowners. But anything involving internal components or leaks should be left to a Gas Safe registered engineer.
Final Thoughts – Take Control of Your Boiler Pressure
Boiler pressure problems might seem daunting, but once you understand the causes and solutions, most are straightforward to resolve. Keeping your boiler’s pressure within the right range ensures your heating runs efficiently, your home stays warm, and your boiler lasts longer.
Remember:
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📉 Low pressure usually points to a leak or water loss
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📈 High pressure often comes from overfilling or a faulty expansion vessel
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🔧 Pressure that keeps rising or falling may need professional attention
Regular checks, annual servicing, and prompt action when problems arise will help you avoid costly breakdowns — and if you ever feel unsure, expert help is just a call away.