High Boiler Pressure: What Causes It and How to Fix

If your boiler pressure is too high, it can mean there is a serious problem. It certainly means that you need to deal with the issue.

If the pressure rises too high, your boiler could explode. But it isn’t always a big a problem as you might think. In this short guide, we will outline four fixes that help resolve high pressure. We will also explain what could be causing the problem and what you can do about it.

High boiler pressure can be a big problem or it can be caused by something quite small and simple. The hard bit is working out what the cause is and therefore what is the best way to fix the problem.

That is where this guide comes in. In the following sections, we will tell you everything you need to know about fixing high boiler pressure including what your pressure should be, why it might have gone up, and share fixes to the four most common causes.

It’s only a five-minute read but by the end, you will hopefully be in the position to make an informed decision about how to fix the high pressure issue on your boiler.

What is the correct pressure for your boiler?

The correct pressure for a boiler will depend on the individual make and model of the boiler in question.

Typically, a regular household boiler will have a recommended pressure of 1.3 bar.

However, this is not true of all boilers so it is important that you check in your boiler manual or with your manufacturer to make sure you know what the pressure should be.

How high is too high?

If the pressure on your boiler edges much higher than 1.5 bar, this suggests there is a problem that needs addressing.

If it gets as high as 2 bar, this suggests a more serious issue while anything at 3 bar or above is a major worry.

If it gets up to these higher levels, it is likely that your boiler will automatically lockout as various safeguards are built into modern boilers that shut them down if there are potentially dangerous problems.

If this doesn’t happen, we would strongly advise you to switch the boiler off and contact a qualified gas engineer before switching it back on again. You don’t want to run the risk of your boiler exploding.

Reasons your boiler pressure could be too high

Assuming that your boiler has power, if your boiler pressure is higher than 1.3 bar (or the recommended level), there are a number of possible reasons this might be the case:

  • There is too much water in the system
  • There is a problem with the boilers pressure release valves
  • The external filling loop is letting in water
  • Natural expansion in your boiler’s components is increasing pressure.

An important distinction that you will need to establish is whether the pressure of the boiler is rising or whether it is set too high. This is an important difference and will help you determine what the cause of the problem is.

RELATED READING: Is your boiler leaking water from the bottom?

4 Reasons your boiler pressure is high and how to fix them

There are a lot of reasons why your boiler pressure could be high but in our experience, there are 4 issues which tend to cause the majority of high pressure incidents. In this section, we will explain all four and also explain how you can fix them:

#1 – Heating System Overfilled

The most common reason for a boiler showing a high pressure is when the heating system has been overfilled.

Boilers need to have enough water in the system to operate properly, but if there is too much it can cause problems. Anyone can top up their system using the external filling loop and it is not unusual for them to overfill.

This can cause a number of problems including high pressure and increased strain on the copper pipe joints and various seals in the system. An overfilled system can even cause the heat exchanger to crack.

How to fix:

If your heating system is overfilled you will need to drain some of the water out. There are two ways to do this:

  • Release water from a radiator bleed valve – Use a radiator bleed key as you normally would, but instead of just releasing air, you can release some water from the system too. Remember to have some sort of container to catch the water and a cloth to clean up afterwards. Close the key as soon as the pressure on your boiler reaches 1.3 bar – or the stated pressure in your manual.
  • Release water using the drain cock – If you can locate the drain cock, connect a small hose pipe and open the valve properly to allow water to escape. Again once the pressure drops to 1.3 bar (or your recommended pressure) close the drain cock.

A lot of people worry that they might release too much water from the system. This isn’t a problem because you can always use the external filling loop to top it up again.

#2 – Pressure rises when heating and hot water is on and off

If the pressure is rising on your boiler regardless of whether the heating and hot water is on or not, it is likely that your boiler’s external filling loop isn’t fully turned off.

If the loop isn’t fully closed, it will be constantly allowing a small amount of water into the system and this will cause the pressure to keep creeping up.

How to fix:

You will need to call out a qualified engineer for this one. They will be able to disconnect the external filling loop which will stop the pressure rising.

The engineer will also be able to examine your external filling loop to see what is causing it to stay partially open. If needs be, they will install a new loop which should solve the problem.

#3 – Boiler pressure goes up less than 0.3 bar only when heating or hot water is on

Another common issue with boiler pressure is when you find it rising by around 0.3 bar whenever the hot water or heating in your property is working.

This is actually something that is fairly normal. When the water in your heating system gets hotter, some of the parts in your boiler will expand because of the heat. This, in turn, makes the pressure in your system go up.

All modern boilers should be fitted with a pressure release system. This will kick in if your boiler pressure goes up too much and release some pressure to keep things safe.

How to fix:

If the pressure rises are small, you don’t need to worry too much. However, if they are 0.5 bar or higher, you should probably ask an engineer to take a look just to make sure the issue isn’t being caused by something more serious.

#4 – Boiler pressure goes up more than 0.3 bar only when heating or hot water is on

If your boiler’s pressure is rising by more than 0.3 bar when you switch on your central heating or your hot water, it is likely there is a problem with the pressure release element of the unit.

Different boilers use different components to release pressure. Depending on the make and model you have, it could use a pressure release valve, auto air-vents or expansion vessels.

If there is a problem with any of these, it can result in pressure not being released from the boiler. This will gradually build up and become more and more of an issue. Eventually, your boiler will lock out.

How to fix:

If there is a problem with your boiler’s pressure release component, you will definitely need to call out a heating engineer. They will be able to check all of the relevant parts and either fix the issue or install a replacement component.


High pressure in your boiler is not always a major issue, but it can be. That is why you should keep an eye on the pressure and if it does start to go up, you should resolve the problem at the earliest opportunity.

In this guide, we have explained how to tell if your boiler pressure is too high and what can cause the boiler pressure to increase. We have also detailed the best way to fix the issue. Some of these you can do yourself but others will require the help of a heating engineer.

Have you had problems with high pressure on your boiler? What did you do about it? Do you have any extra tips or recommendations for our readers? We always welcome the comments and experiences of our readers so please do share yours with us using the comment box below.

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