Boiler Losing Pressure – Causes & How To Fix It

Last Updated on February 3, 2025

When your boiler loses pressure, it can be due to a variety of reasons. A drop in pressure can be caused by leaks, faulty components, or air trapped in your radiators.

In this article, we will cover why your boiler is losing pressure, and provide a step-by-step guide on how you can repressurise your boiler. We will also discuss when you should repair or replace your boiler.

Why Is My Boiler Losing Pressure?

Let’s look at a few common causes for your boiler losing pressure.

Did you bleed your radiators?

Bleeding your radiators is essential for maintaining an efficient heating system, but it can also cause your boiler to lose pressure. When you release trapped air, the pressure naturally drops because air pockets are replaced with water, leading to a slight reduction in system pressure. Additionally, the process may remove small amounts of water from your system, further contributing to the pressure loss.

To fix this, simply repressurise your boiler using the filling loop until it reaches the optimal pressure level (typically between 1 and 1.5 bar). Regularly checking your pressure gauge after bleeding your radiators ensures your heating system runs efficiently without sudden pressure drops.

Make sure that after a bleed, you balance your radiators properly to ensure the correct flow of water for even heating.

Leak in central heating system

A leak in your central heating system is a common reason for a drop in boiler pressure. Even a small leak can lead to a gradual loss of water, reducing system pressure and affecting your boiler’s efficiency.

How to Identify a Leak in Your Heating System

• Inspect pipework – Look for water puddles, damp patches, or stains along copper pipes, especially around joints and bends, where corrosion or rust can cause leaks.
• Check radiator valves – Loose or faulty radiator valves can also be a source of leaks. Ensure they are properly sealed and not dripping.
• Monitor boiler pressure – If you keep needing to top up your boiler’s pressure, a hidden leak may be the cause.

How to Fix a Central Heating Leak

For a temporary fix, you can use epoxy sealants or plumbing putty to patch small leaks. However, for a safe and long-term solution, it’s best to call a Gas Safe registered heating engineer to locate and repair the issue professionally.

Ignoring a leak can lead to bigger problems, including water damage and costly repairs—so act fast to keep your heating system running efficiently.

Leak in the boiler

If you can’t find any issues along the pipework of your central heating system, there may be a water leak from the boiler itself. This will cause your boiler to lose pressure for the same reason as any other leak: once the water level drops, there will be a large amount of boiler pressure loss.

Check below your boiler for any leaks. If you have a conventional or system boiler, check the cold and hot water storage tanks for leaks. Keep in mind that the leak may not be visible.

Making temporary repairs on leaking pipework is one thing. Repairing a leaking boiler unit is something else. We suggest that you contact a Gas Safe registered engineer to investigate the source of the leak and make the necessary repairs.

Damaged expansion vessel

The expansion vessel consists of a diaphragm made of rubber and a compressed air chamber above it. When the boiler is heating water, it will expand and increase the boiler pressure.

The purpose of the expansion vessel is to allow that hot water some room within the boiler to expand. This is why you will see the boiler’s pressure gauge go up when it is on and working. That pressure is what pushes water throughout the central heating.

If you have a damaged expansion vessel, like a hole in the diaphragm within this chamber, the boiler will lose pressure when it heats up. The loss of pressure could also be due to one of the air valves in the expansion chamber being damaged or not sealed properly.

Contact a registered engineer to do a proper fix.

Faulty pressure relief valve

The pressure relief valve is an important safety device. If the boiler system pressure becomes too high, the pressure relief valve will open up and allow the boiler to lose pressure quickly, bringing it down to a safe level.

If your boiler is losing pressure during normal operation, that valve may have malfunctioned. This can mean that it has lost its air-tight seal, or it is stuck in an open position, affecting boiler pressure.

As this valve is an internal boiler component, you will need to arrange for a qualified boiler engineer to evaluate and repair this fault.

The pressure release valve is a key component to keep your boiler functioning effectively and safely. If it is not repaired properly, it could put your boiler and your safety at risk.

Damaged internal boiler parts

As the entire boiler is a closed system, failure on any number of internal components can cause low boiler pressure. It is impossible to diagnose and repair these issues without the help of a registered Gas Safe engineer or boiler engineer.

Other parts of the boiler that could be the cause of a pressure loss include:

• Damage or worn seals and gaskets
• A damaged or faulty heat exchanger
• Automatic air vent failure
• A problem with your pressure meter

How To Repressurise Your Boiler

It is actually normal for a boiler to lose pressure over time. However, you can restore it to its optimal pressure by using the filling loop – a tap that you can use to refill your boiler.

As water comes through this pipe, the water level in your boiler will increase, increasing the boiler pressure. By keeping an eye on the pressure gauge, you can properly restore lost pressure.

Let’s look at how to do this step by step:

1. Switch off your boiler. Allow the whole unit to cool down before you attempt to repressurise it. You don’t want to over-pressurise a boiler filled with hot water. It can be dangerous and it won’t allow the boiler to reach the correct base pressure.
2. Locate the filling loop hose: a flexible pipe with two valve taps. This hose connects your boiler unit to the mains water supply.
3. Open both valves by turning them clockwise. This will immediately start filling your boiler system with water.
4. Keep an eye on the pressure gauge to make sure you aren’t overfilling the boiler (more on this below).
5. Close the taps by turning them in an anti-clockwise direction when you see the pressure land in the green zone. Do not overfill the boiler.
6. Monitor the system to see if the low boiler pressure issues return.

What should my boiler pressure gauge read?

The pressure gauge on a standard boiler should always be between 1 and 1.5 bars. Optimal pressure is indicated by the green zone on the boiler pressure gauge.

The indicator arrow should always be within this green zone. You can also check your boiler’s manual to see what your model’s optimal pressure should be.

How Do I Diagnose Why My Boiler Is Losing Pressure?

To spot these problems it may be easier to simply check your boiler’s error code being displayed. These vary from model to model, so you will need to consult your specific boiler’s user manual for its list of error codes and what they mean. Alternatively, check out our helpful guides for more information on fault codes specific to different boiler brands:

• Vaillant fault codes
• Ideal fault codes
• Glow-worm F28 error
• Glow-worm f75 error
• Baxi fault codes
• Worcester Bosch fault codes

Is Low Boiler Pressure Dangerous?

Low boiler pressure is not dangerous in and of itself; however, it can cause damage to your boiler if not addressed. A water leak, for example. can cause property damage or lead to electrical faults.

Low boiler pressure is not going to put your life in immediate danger, but it should be treated seriously.

Should I Replace Or Repair My Boiler When It Loses Pressure?

A loss of boiler pressure is a fixable fault in the majority of cases. As long as you hire a qualified engineer to do repairs, you should not have to worry about replacing the unit.

The only cases you would want to consider replacing your entire boiler is when:

• The cost of repairing your boiler, including parts and labour, makes up more than 70% of its price. For example, if you have a £500 combi boiler and you need to pay £200 to replace a faulty heat exchanger, plus another £100 for the technician to do the repair, it would make more sense to spend £200 extra to get a new unit that can deliver 10 years of fault free heating.
• If you are experiencing regular faults in your unit, a replacement model may be a cheaper alternative in the long run. This could save you from spending hundreds of pounds every year to repair a new break on the unit.
• If your boiler is more than 10 years old, it would be a good idea to replace the whole unit. The risk of a serious failure increases the closer the boiler gets to the end of its recommended lifespan.

FAQs

Can a boiler lose pressure without a leak?

Yes, many internal components in a boiler can fail and cause it to lose pressure. You might not be able to identify these faults with a visual inspection, but you can check the boiler’s error codes.

Ideally, a registered heating or Gas Safe engineer should be called to investigate the internals of your boiler.

Why does my boiler lose pressure when the heating is off?

A common cause of low boiler pressure when you turn your heating off is a faulty expansion vessel. During operation, heat will build up and increase the boiler pressure, keeping the pressure gauge in the green.

This makes it appear that there is no boiler leak. When you turn your heating off, and the pressure drops, only then will you notice that your pressure gauge drops below optimal levels.

Conclusion

A boiler losing pressure is a hard problem to diagnose and fix yourself. While boiler error codes are a great indicator of an underlying issue, the complex nature of a boiler means that there can be many other factors at play.

Contact Eco Happy and we’ll find the best local engineers to repair your boiler when there’s a pressure issue.