Boiler Pressure Too High? Here’s How To Reduce It

Written by :
Last updated: October 31, 2023

While a small increase or fluctuation in pressure is normal, too much pressure in your boiler is sign that something is not right. An over-pressurised boiler will not be working effectively or efficiently and can lead to problems if left to its own devices. We take a look at what to do if you find that your boiler’s pressure is too high.

What Pressure Should My Boiler Run At?

While a small increase in boiler water pressure throughout the day is normal, raised pressure can mean your boiler system has become over pressurised. And, to know whether you may have a pressure problem with your combi boiler, it is important to know what is normal for your system.

The reason for some small pressure fluctuations is that when your boiler heats up water for your radiators and taps, the water expands as it gets hotter – hence why your boiler pressure should be different whether it is turned on or off. To manage and reduce boiler pressure, your boiler should have a pressure relief valve which controls any rise by releasing additional pressure from the system to bring it back into the normal range.

Combi boiler pressure is measured in bars and your boiler’s pressure at any given time is shown on the boiler pressure gauge, which you can read to know whether the pressure is as it should be. The boiler pressure gauge will most likely also show a colour indicator – green, amber, red – so you will know at a glance whether the pressure is too low, too high, or spot on. You want the reading to be in the green so that’s 1 – 1.5 bar of normal pressure inside your boiler, even when it is turned off. If you are in any doubt, read the manual from your boiler’s manufacturer and it should indicate the ideal recommended pressure in bars for your particular model.

Your boiler pressure is too high if the reading is near to or in the red zone, which means anything over 2.5 bar indicates over pressure. If the gauge reaches 3-4 bar, it means boiler pressure too high and you should see error codes on the gauge and ideally find that the boiler’s safety shut off valve has kicked in.

Your boiler can also suffer from low pressure – this is anything below 1 bar and indicates your boiler is losing pressure for some reason. If pressure is too low, it also means your boiler is not working efficiently and so it is a good idea to get professional advice from a heating engineer if the problem doesn’t rectify itself.

Problems That Lead To High Boiler Pressure

If you boiler pressure is too high, it can lead to damage and expensive repairs, so it’s important to be able to recognise when the pressure is too high and know when to call out a gas safe registered heating engineer. When reading the pressure of your boiler, you should always do it with the system turned on – the ideal reading should be between 1 and 1.5 bar. More than this, and you need to do some more investigation.

So, what causes high pressure in boiler systems? We take a look at the main problems you could encounter:

Too much water – one of the most common causes of high boiler pressure is that the heating system has been overfilled with water. You need sufficient water in your combi boiler system for it to operate correctly as it prevents it from getting too dry, which can damage the pump, heat exchanger and other potentially expensive to replace internal parts. However, an overfilled boiler puts too much water in the system and can add strain on the boiler, leading to high pressure. You can inadvertently overfill your heating system with too much water when topping it up via the external filling loop.

Too much or too little air in the system – your boiler’s pressure is controlled by the expansion vessel, which manages the balance of water and air in your central heating systems.  If you have too little air in the expansion vessel, and you may suffer from high pressure in your boiler. Too much air in the expansion vessel can also cause an over pressurised boiler and can lead to water leaking out of the pressure release valve.

Faulty components – with modern boilers technically sophisticated, the downside is that they have numerous components that can be vulnerable to damage or malfunction. And two of the most common parts to show faults are the pressure release valve and the filling loop. Should these parts be faulty or damaged, it is likely you will experience issues caused when pressure is too high, including it becoming over pressurised.

The pressure release valve (or PRV) can sometimes spring a leak, or the valve may not shut properly, which can reduce your boiler’s pressure over time. And if this valve unit malfunctions, it could mean the excess pressure cannot escape as efficiently as it should, leading to a high or over-pressurised situation.  This can also happen to your boiler’s auto air-vents or expansion vessels.  Meanwhile the filling loop, which is intended to open to let water in should the boiler’s pressure go too low, could be stuck open, meaning water is constantly flowing inside, and so increasing the pressure.

Most of your boiler’s parts may well be covered by your boiler’s warranty so if you suspect you have a faulty component, ask a qualified heating engineer to check it out, and replace if necessary so you can rectify your pressure issue, whether it is high or low.

A stuck filling loop – if you find your boiler pressure is too high, even when the hot water or radiators are not turned on, then the filling loop, which is used to increase boiler system pressure, could be partially open. If this important valve is stuck open or hasn’t been able to fully close, then it will be constantly dripping water into the heating system, meaning the pressure will rise, even when the heating system is off.

Boiler age – if your boiler is more than 10 years old, and high pressure is a regular problem, then it may be time to get a new boiler. Boilers have to work hard and so are susceptible to wear, tear and degradation over time. With an aged boiler, ongoing repairs and replacement boiler parts are more likely to be only a temporary solution and a new boiler system will be the best solution to bring your heating back to its efficient best.

How to Reduce Your Boiler Pressure

There are ways you can potentially reduce boiler pressure before calling out an engineer to fix your boiler problem. While it is important to only use a heating engineer to do any part replacements or major repairs, working through the following steps as soon as you suspect a high pressure issue can make a difference:

Inspect your boiler – before taking the following steps to reduce your boiler pressure, it is worth carefully checking the boiler over to see if you can identify an obvious cause for the pressure problem. And this means checking that the filling loop is closed and so not letting water into the system as well as making sure the PRV (pressure release valve) is not stuck. If you see either of these is a problem, call out an engineer who can fix, replace or advise what needs to be done.

Otherwise, you should:

  • Switch off your boiler and hot water to let the whole heating system to cool.
  • Now check the boiler pressure gauge.
  • If it remains above 2 bar, you will need to reduce the boiler pressure, which can be done by bleeding the radiators. This means removing any excess air and water from the system, which may be raising the boiler pressure.
  • You need the heating to be keep switched off and cool for this process. Also check that the filling loop or relief valve is tightly closed before you start.

Bleeding your radiators

  • For each radiator, use the radiator key to turn the bleed valve at the top
  • Allow for the air and excess water to come out, retightening the valve once the hissing has stopped
  • Once all your radiators have been successfully bled, turn your central heating system back on
  • Return to your boiler and check the reading on its pressure gauge
  • It should be back within the normal range. If not, repeat the process until you are happy it is at the correct pressure setting
  • Keep an eye on the pressure reading over the next few days, to check that it is not creeping back up.

After following the steps above, the boiler pressure still doesn’t come down, or it creeps back up to a high pressure gauge reading, then call our a gas safe registered engineer who can make a more in-depth investigation of your boiler to determine the root cause of your high pressure problem.

Is High Boiler Pressure Dangerous?

In most cases, high boiler water pressure is unlikely to be dangerous, as today’s Combi boiler systems have a pressure relief valve which should trigger a shut down when the readings get too high. However, while it is an effective safety system, it is important to not rely on this pressure relief valve to resolve the problem. If you do experience high pressure in your boiler, indicated by error codes or raised pressure gauge readings are persistent, you need to get a gas safe registered engineer out to check your boiler and heating system for faults or damage.

Too much pressure inside your boiler can cause damage to internal components, and this can prove costly to put right. And left unfixed, high pressure will mean your whole central heating system is not working as efficiently as it should. And ultimately, a pressure issue could mean your boiler is past its best and a new boiler is your best option.

Does Boiler Pressure Go Up When Heating Is On?

You can find that your boiler pressure does rise when you switch the heating on, and this is not necessarily due to a fault. It can also happen if your boiler suddenly finds itself busy with multiple outputs, such as the radiators, while someone is running a hot bath when the dishes are also being washed. It is perfectly normal for the boiler pressure to rise by about 1 bar due to water expansion and it should lower once the user pressure is off – the hot taps are switched off, for example.

However, if the pressure doesn’t eventually go back to normal, or you are regularly reading a high boiler pressure of 3 bar or more, even when demand for hot water drops, then there could be a problem. And with any persistent high pressure readings in your boiler, you should switch the boiler off and to get an engineer out pronto for a full check of your central heating system.


  1. Boiler Pressure Too High? Here’s how to Fix it – Ideal Heating
Boiler Maintenance Guides
Boiler Servicing
How Old is My Boiler?
How Long Does My Boiler Last?
How To Reset a Boiler: What You Need To Know
Preparing Your Boiler For Winter
How To Descale a Boiler
How To Re-pressurise a Boiler
What is a Magnaclean System
Boiler Making Noises