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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing).
Published: January 28, 2020. Updated: February 16, 2024

Why is my boiler pressure too high?

Is your system or combi boiler pressure too high? There are several reasons for this, but if you respond quickly, it may not damage your system.

In this article you’ll find all the answers you need to questions like ‘What is boiler pressure?’, ‘Why is my boiler pressure too high?’ and ‘How to lower boiler water pressure’. They will help you understand the issue and get rid of it.

What causes high pressure?

High boiler pressure may be caused by several factors, including:

  • Overfilling of your heating system
  • Faulty boiler components such as an expansion vessel and filling loop
  • External filling loop is functional but still attached
  • Your boilers age
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What is boiler pressure?

Boiler pressure refers to the pressure of water inside your boiler. It’s what pushes your hot water through your property’s heating system. Therefore, if your boiler pressure is too high, it will cause issues. If it’s too low, then you’re going to struggle to get your radiators hot.

The pressure on your boiler may change for several reasons, but if it’s too high, the sooner you deal with it, the better.

If you don’t have any hot water, read our guide >

What pressure should my boiler be?

It’s important to recognise that not every boiler has the same requirements for water pressure. The general rule is that most central heating systems within the UK will need their pressure set between 1 and 2 bar – around 1.4 is ideal. This equals between 14 and 29 PSI (pounds per square inch).

How do I find out my boiler’s pressure level?

The only way to keep track of your boiler’s pressure is the gauge on the front. In some circumstances, a pressure guage may have been installed in another area. This might happen if a boiler is in the loft, for example. This saves you having to go up into the loft to look at the pressure gauge.

where is the pressure gauge on my boiler

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Why is my boiler pressure too high?

You’ll need to determine whether or not your boiler’s pressure is still rising, or if it’s been set too high and you need to bring it back down.

Overfilled system

An overfilled system is the most likely cause of boiler pressure too high for most people. Using a filling loop to pressurise the system adds more water. If you have added too much this means the pressure will be too high.

This could cause damage through pipe joints being more likely to fail (if it’s an older system), or cracks forming on the heat exchanger and other components

How to fix an overfilled heating system

Thankfully, solving this problem can be relatively easy, since all you’re essentially doing is draining some of the water from the system.

The simplest way is to bleed a radiator into a cup. This lets water out of the system reducing the pressure. You could use the drain valve, but this is more complicated and requires a hose to collect the water.

When removing water from the system, remember to keep checking the pressure gauge.

How to bleed a radiator

For detailed instructions on how you can bleed a radiator, please watch the short video. Alternatively, you can read our guide on radiators.

Essentially, use a screwdriver or radiator ‘key’ to partly unscrew the valve at the top of the radiator. Normal bleeding will let air out, but you will need to let some water out to reduce the pressure in the system.

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Faulty boiler components

The quality of your heating largely depends on the quality and condition of the boiler itself. There are several components found within your boiler that could be a reason for your boiler pressure being too high. You’ll need to check them to see if you have to deal with any other problems. These include:

  • Filling Loop
  • Expansion Vessel
  • PRV (Pressure Relief Valve)

What is a Filling Loop?

A boiler filling loop provides a temporary connection to the mains. It allows you to repressurise and fill your heated system with water.

Depending on the make of the boiler, these can either be external or internal. External filling loops may be detachable. In almost all cases, they have a metal braided design and sit right below your boiler.

You may also notice one or two black taps by, or on this filling loop. Double-check that these have been closed by making sure they’re perpendicular (90 degrees) to the pipe.

A filling loop that has been left slightly open is one of the more common causes of high boiler pressure. If you have an external filling loop, ensure the taps are completely closed.

Pressure Relief Valve Cross Section

What is a PRV (Pressure Relief Valve)?

The PRV is what is used to prevent a buildup of pressure inside your boiler. It’s designed to open and release pressure if it gets too high. Once it drops to a safe level, the valve closes again.

This is a cross-section of a typical pressure relief valve. If you look towards the far end, you’ll notice a light blue nut. This seals the water inside the system. It the pressure rises above normal the spring will push the nut/seal and let water out.

The main issue with a PRV is that it’s designed to be a last resort. Although most of the time they will reset to normal, there is a chance it could stick open. This would mean your boiler may not be able to reach pressure and you’ll have an issue with the pressure being too low. In this circumstance, you would need a new PRV.

Is it dangerous to have high boiler pressure?

In most cases, it’s not dangerous. Having a Pressure Relief Valve means your boiler able to release pressure if it gets too high. However, the situation can still worsen to the point that you may need to replace your boiler entirely. This is a last resort, but be aware that if you don’t take care of your boiler, things like this can happen.

Boilers with high pressure can struggle to stay operational if they’re not properly cared for. If untouched, an over-pressurised boiler can develop cracks, which then cause leaks. To stop this from occurring, always make sure to:

  • Check the pressure levels of your boiler regularly
  • Contact a professional who can give you appropriate advice and support
  • Be aware of how to bleed your radiator if your pressure is slightly too high
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What to do next?

We recommend trying to reduce the system pressure as quickly as possible, just in case.

Hopefully, it is an easy fix for you. But, if not, you may need a new boiler. Especially if there is an issue with the heat exchanger or pressure vessel.

It’s important that you don’t do anything you’re not qualified for in this situation.

Having a professional assess the issue means you can get that much-needed reassurance. It should also mean your warranty remains unaffected. Any tampering with the product is likely to affect your warranty, meaning you’ll be even more stuck finding a solution to the problem.

Get advice for a new boiler

Most boilers should last you up to 15 years before you need a replacement. But, if you know you’ve already reached that point, you need to speak directly with a heating expert for some genuine advice and support.

It’s also worth speaking to an installer with good relations with their manufacturers. In the case of Boilerhut, we are a Premium Viessmann Installer. We can provide you with reliable, experienced advice to help you decide which boiler is best for you.

If you’d like to find out more about how Boilerhut can help, you can either:

  • Call us directly on 029 2009 9898 to speak to one of our in-house advisors
  • Or hit the button

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