James Elston

James Elston

Published March 9, 2024

When your boiler loses pressure, it can be due to a variety of reasons. Your whole boiler unit is a pressurised system that needs to remain airtight to function. The most likely cause of a boiler losing pressure is a break in the airtight seal somewhere within your boiler and central heating system.

In this guide, we will cover why your boiler could be losing pressure, a step-by-step look at how you can repressurise your boiler, and how you can diagnose the exact cause of the leak. We will also look at when you should repair or replace a boiler with this issue.

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 may cause your heating system to lose pressure. When you release the trapped air during the process, the boiler’s pressure drops. There may also be a drop in the amount of water in the system when you bleed it, adding to the pressure drop.

Fortunately, you can resolve this boiler pressure loss by using your filling loop to bring your boiler and heating system back to the correct pressure (more on this later).

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

If there is a leak somewhere along your heating system, it will result in a loss of water which will reduce boiler pressure. A common culprit is a copper pipe along your central heating system that has suffered corrosion or rust, causing leaks to form.

Check along your system’s pipework for water puddles, damp patches and water stains. Take special care around joints and curved pipes.

Also, check along your radiator valves to make sure they are sealed and not the source of your boiler losing pressure.

You will be able to find different types of epoxy and putty sealants that can help you make a temporary repair on leaking pipes, but it is safest to contact a professional heating engineer for the repairs. The simplest way to get the best prices on local engineers is to get in touch with Eco Happy for quote comparisons.

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 system or conventional 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 heating 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.

You can also check out our guides on Vaillant boiler fault codes and Ideal boiler fault codes. If you have another model boiler, a quick search online can point you in the right direction.

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.


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.


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.

James Elston

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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