Why Does My Boiler Keep Losing Pressure? Common Causes and Solutions

While it is a common problem among homeowners, the issue of pressure loss in boilers can lead to no hot water or central heating which can be particularly disruptive and uncomfortable if it happens in the winter months. A boiler can lose pressure due to several reasons, the most common being leaks in the system, faulty pressure relief valves, or issues with the expansion vessel. In this article, you will gain a clear understanding of why your boiler may be losing pressure and what steps you need to take to address it to help you avoid being without heat when you need it the most.

What Is Boiler Pressure And Why Does It Matter?

In a heating system, a specific amount of water is required for circulation. Older heating systems stored this water in a header tank located in the loft, which allowed for expansion as the water heated up. Modern boilers, however, use an expansion vessel instead of a header tank to manage this function more efficiently (a huge design win in terms of reducing space requirements and simplifying maintenance for homeowners!)

Because water volume increases and decreases with temperature, the expansion vessel works by absorbing the pressure changes (the rise in pressure) of water when your boiler heats up the water in the system.

For boilers to run efficiently and safely, the boiler pressure needs to stay within a specific range (usually 1-1.5 bar). If you are noticing symptoms that the pressure may have fallen below the set range (such as no heating or hot water, unusual noises, you may need to top up the pressure in the system. We will teach you the steps on how to do this later in the article.

How Do I Check If My Boiler Pressure Is Low?

You can check the boilers’ pressure gauge reading. Your boiler manual will show you where the pressure gauge is located on your boiler. Typically, this will be on the front panel of your boiler and will show you how high or low the pressure is or if it is at an adequate level. Modern boilers tend to also have a screen which flashes an error code to indicate the cause of the problem. Check your boiler manual to see whether the error code for your boiler model corresponds with low boiler pressure.

If your boiler pressure is too low the pressure gauge will show as below one bar. If the pressure is too high, the pressure gauge will show as above 2 bar. Anywhere in between is deemed adequate pressure.

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Common Causes And Signs of Boiler Pressure Loss

There are multiple reasons why a boiler may start to lose pressure. However, the most common ones are as follows:

Leaks In The System

Any leaks in the pipes, radiators, or the boiler itself can cause the system pressure to drop. Leaks are not always obvious to spot and in addition to frequent drops in boiler pressure, you may notice:

Puddles of water around the boiler or damp spots near your radiators.
Damp patches on walls, ceilings, or floors near where the pipes run.
Strange hissing noises, or dripping, near the boiler or radiator.
Some or all your radiators or parts of the heating system not warming up properly.

Leaks are now always easy to find. The leak may be inside the circuit of pipes and radiators inside your walls, or it

could be around the connections on radiators. Look for obvious signs like damp patches and paint flaking around radiators and on the walls near to pipework. Sometimes the condensation on pipework can be mistaken for a leak, so, wipe the pipe clean and check for any drips after doing so to make sure. We would always recommend enlisting a professional to repair loose or broken pipework joints to ensure your leak doesn’t turn into a flood!

Faulty Pressure Release Valve (PRV)

The pressure relief valve is a safety component designed to release excess pressure if it builds up with the system. When the boiler’s pressure exceeds safe levels, the valve opens to release some water or steam to reduce the pressure level to a safe range. Other than a drop in boiler pressure, you may notice:

Constant dripping or leaking from the valve, it may not be closing properly.
Rust around the valve, this usually means the valve isn’t sealing properly.
Unusual whistling or hissing noises around the valve area.

Expansion Vessel Issues

As described earlier, the expansion vessel’s role in the heating system is to help maintain pressure by absorbing the rise in pressure when the boiler heats up the water (causing the water volume to expand) in the system. If the vessel isn’t working correctly, you could notice frequent pressure drops/need to repressurise, as well as:

Overpressure when heating. If the boiler pressure rapidly rises above the ideal range when the system heats up, the vessel may not be expanding as it should.
Water discharge from the PRV.
Constant dripping from the vessel or relief pipe – this could mean the diaphragm or bladder inside the vessel has failed.
No pressure change.
Odd noises such as gurgling or banging from the vessel or nearby pipes.
Visible corrosion or damage on the vessel itself.

Air In The System

Trapped air is never a good thing! Trapped air in radiators can have a big impact on pressure and can be caused by many things but the build-up of sediment and corrosion is often the biggest culprit. If air is trapped in your rads, the first sign you will probably notice is cold spots on your radiators and therefore, less warmth in your rooms.

How to Fix Low Boiler Pressure (Step-by-Step)

Here’s a step-by-step guide for homeowners to fix low boiler pressure safely:

Check the Pressure Gauge
- Locate the boiler’s pressure gauge, usually on the front panel.
- If the reading is below the recommended range (typically 1-1.5 bar when cold), the pressure is too low.
Turn off the boiler
- Before adjusting the pressure, switch off the boiler and let it cool down for safety.
Locate the Filling Loop
- Find the filling loop (a flexible, braided hose with a small valve) typically located under the boiler or nearby.
- Some systems have a keyed filling loop, while others may have a built-in filling loop with levers.
Open the Filling Valve(s)
- Slowly open the valve(s) on the filling loop, usually by turning them to the left (counterclockwise).
- Listen for the sound of water entering the system. The pressure gauge should start to rise gradually.
Watch the Pressure Gauge
- Allow the pressure to reach the recommended level (usually 1-1.5 bar when cold).
- Once the gauge shows the correct pressure, close the filling valve(s) tightly by turning them clockwise.
Check for leaks
- After closing the valve, inspect the area around the filling loop and radiators for any leaks.
- Also, monitor the boiler pressure over the next few days. If it drops again quickly, there may be an underlying issue such as a leak or faulty component.
Turn the boiler back on
- Once you’ve set the pressure and checked for leaks, turn the boiler back on and let it operate normally.
- You may need to reset the boiler, depending on the model.
Bleed radiators (if necessary)
- If air is trapped in the radiators, this can affect pressure. Bleed the radiators by using a radiator key to release trapped air.
- After bleeding, you may need to top up the pressure again slightly if it drops.

For a detailed guide on fixing low pressure, check out our comprehensive guide on ‘Fixing Low Pressure On A Valliant Boiler’.

Preventative Tips To Avoid Future Pressure Loss

Our best tips would be to book in an annual boiler service by a professional to catch any issues early before they escalate.

You should always try to bleed your radiators once or twice a year to release trapped air and keep the system balanced.

Lastly, keep your eye out for small leaks or signs of small leaks by checking around radiators and pipes occasionally.

When To Call A Professional

Some of the causes of low boiler pressure, like faulty PRVs or expansion vessels and hidden leaks, require technical skills and specialised tools, and we would always recommend enlisting the help of a professional to carry out repairs. Likewise, if you find you are needing to re-pressurise your boiler on a regular basis, or you are not confident in fixing low pressure yourself, contact a qualified engineer to inspect the system, get to the root of the problem, and resolve. If you are based within our service area and would like help with any aspect of your boiling or heating system, the team at Happy Dog Plumbing would love to help. Contact us at your convenience, and we’ll do the rest!

About the author:

Oliver Osmore

Oliver Osmore has been at the helm of Happy Dog Plumbing since it became part of the Maintracts Services family, bringing with him decades of experience in plumbing, heating, and electrical services. As Managing Director of both companies, Oliver is dedicated to ensuring Happy Dog Plumbing continues its proud tradition of delivering exceptional service and innovative solutions to customers.

Oliver’s approachable nature and problem-solving expertise allow him to work closely with customers, offering tailored solutions to even the most complex challenges. He is passionate about upholding the high standards that Happy Dog Plumbing has become known for, ensuring every job is handled with professionalism and care.

Since its founding, Happy Dog Plumbing has been a trusted name in installation, maintenance, and repair services. Under Oliver’s leadership, the company thrives as part of a larger, customer-focused team, staying true to its roots while benefiting from the expanded resources of Maintracts Services.

Low boiler pressure itself isn’t dangerous but can cause inefficient heating and disrupt hot water supply. However, consistently low pressure can strain the boiler, and underlying issues—like leaks or a faulty pressure relief valve—can lead to larger problems if not addressed.

Most boilers operate best between 1-1.5 bar of pressure when cold. Check your boiler’s manual for the recommended range, as it may vary by model.

It’s normal to occasionally re-pressurise your boiler, especially after bleeding radiators or routine maintenance. However, if you’re adding pressure frequently (more than every few months), it may indicate a leak or a problem with the expansion vessel or pressure relief valve.

Signs of a boiler leak include visible water around the boiler or near radiators, damp patches on walls or ceilings near pipework, and strange noises like hissing or dripping. If you notice these signs, it’s best to consult a professional, as even small leaks can cause long-term issues.

Homeowners can often top up boiler pressure themselves by using the filling loop to add water to the system. However, it’s essential to follow the manufacturer’s instructions and check the pressure gauge frequently while filling. If pressure drops continue, or if you’re unsure about the process, consult a professional.

The pressure relief valve releases water to prevent pressure from exceeding safe levels. If it constantly leaks or releases water, it may be due to a faulty valve, an over-pressurised system, or an issue with the expansion vessel. Frequent releases suggest that the valve or expansion vessel may need repair or replacement.

Cold spots on radiators may indicate trapped air or sludge buildup, which can impact heating efficiency. Bleeding the radiators can often resolve this issue. If the problem persists, consider getting the system flushed to remove sludge and restore proper circulation.