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How to reduce boiler pressure

Nobody likes being under too much pressure, and your boiler is no different! If you’ve noticed that the pressure gauge dial on your boiler has gone into the red zone, or your heating system has shut down, it could be that high pressure in your boiler is to blame. In this blog, we’ll walk you through the steps to reducing the pressure in your boiler, which will help to prevent leaks and other problems with your heating system.

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How to reduce boiler pressure…

Nobody likes being under too much pressure, and your boiler is no different!

If you’ve noticed that the pressure gauge dial on your boiler has gone into the red zone, or your heating system has shut down, it could be that high pressure in your boiler is to blame.

Modern boilers require constant water pressure to function properly. There must be the correct balance of water and air within your central heating system in order for your boiler to be able to pump hot water to your radiators and taps.

And the balance must be just right. Too low, and it will cause problems, such as the dreaded, no hot water, situation. But if it’s too high, there’s an increased chance of leaks developing in your heating system.

So, in order to keep things ticking over nicely, your boiler needs to maintain the correct pressure in order to heat your home effectively.

What is the difference between water pressure and boiler pressure?

A woman with question marks above her head.The terms ‘water pressure’ and ‘boiler pressure’ are easily confused, but they’re two different things.

Water pressure refers to how quickly the water comes out of your taps, whereas boiler pressure refers to the pressure of the hot water which is inside your central heating system.

What is boiler pressure and why does it matter?

Your gas central heating system works by heating cold water and pumping it around your pipes and radiators.

Older heating systems may have used a small storage cistern in the loft to supply water into the heating system.

But most modern (and some older) heating systems are sealed. This means that there aren’t any vents to manage the water expanding and contracting as it heats and cools.

Instead, the system is pressurised to ‘push’ the water around your system.

To work efficiently, your boiler must maintain a stable pressure, which is determined by the volume of water in your heating system. It’s important that your boiler pressure is just right.

Pressure problems

Low boiler pressure can cause your system to cut out, preventing your central heating from working.

Alternatively, if there is high boiler pressure, the system will be strained beyond normal capacity and could potentially fail. Re-pressurising your boiler will help to keep your boiler working well.

However, not all boilers will need you to do this.

The Worcester Bosch Greenstar 8000 Lifestyle boilers boast an optional unique intelligent filling system. This new technology ensures that your system pressure is kept at its optimum level – which is great if you’re sick of the problems associated with high or low boiler pressure.

The intelligent filling system monitors the boiler pressure and automatically maintains the system pressure at around 1.3bar, removing the need for you to manually top up the boiler pressure.

How can I check my boiler pressure?

You can easily check your boiler’s pressure by reading the pressure gauge – which is usually located on the front of your boiler.

If you have a digital gauge, it will often flash if there’s high or low pressure.

If you have a hydraulic pressure gauge, the correct boiler pressure will be indicated with a green zone, and the maximum safe operating pressure with a red zone.


What is the correct pressure for a boiler?

When the heating system is cool, the pressure should be between 1 and 1.5 bar on the pressure gauge, and the indicator needle should be in the green section.

Most boiler manufacturers recommend a pressure of 1.3 bar (check your owner’s manual).

 If the pressure is too low, or below 0.5 bars, water has been lost from the system and must be replaced.

If the pressure is too high, anything over 2 bars, the needle will be in the red section.

It’s important to understand that your boiler pressure should go up when it’s switched on. However, the rise should be small, generally between 0.3 and 0.5 bar.

What causes your boiler pressure to become too high?

Although the pressure gauge on your boiler will alert you to the fact that it’s too high, unfortunately, it can’t tell you why. There are a few possible causes of your high boiler pressure:

The heating system has been overfilled

The simplest explanation for your high boiler pressure is if you’ve overfilled your heating system when trying to re-pressurise it.

When dealing with low pressure, it’s important to top the system up with more water via the external filling loop, to prevent damage to the component parts.

But if you’ve accidentally overfilled your system, it puts unnecessary strain on your heating system, and explains why your pressure is now too high. This is why it’s often better to call in a professional.

The way to fix this issue is by draining the excess water from the system. You can do this by letting it out through the radiator bleed valve, using your radiator key.

To do this, you’ll need to put a container under the bleed valve on your radiator and open the valve with your radiator key. Be careful because water will squirt out and be warned that this is quite a slow process!

The external filling loop has been left open

Another possibility is that you may have accidentally left the external filling loop open, or partially open.

The filling loop is where water enters the system from the mains, and consists of two valves. When both valves are open, the system lets more water in, and the pressure rises.

So, if you’ve left one valve open, this is your culprit for high pressure, because it will be leaking small amounts of water into the system.

This will mean that your boiler pressure will rise constantly, even when hot water and heating aren’t currently switched on.

Check both valves are closed – they turn by 90 degrees, and if the handle is aligned with the pipe, it means it’s open.

Faulty pressure relief valve

Pressure relief valves, or PRVs are designed to prevent a buildup of pressure inside your boiler.

These safety valves open to release pressure when the level becomes too high, and close again when the pressure has dropped to within a safe range.

If the PRV becomes faulty or breaks, your boiler’s excess pressure will have nowhere to escape from.

This will put excess strain on your boiler, until it eventually locks out and shows a fault code.  You should call a Gas Safe engineer to assess your boiler, and identify whether the PRV needs replacing.

Faulty expansion vessel

The expansion vessel is another crucial component of your boiler. It helps to keep the pressure consistent by acting as a sort of shock absorber.

It handles the expansion and contraction of water in your heating system as it heats and cools. If this is faulty or breaks, this could explain why the pressure in your boiler is too high.

Is high boiler pressure dangerous?

Although it’s alarming to be in the red zone on your pressure gauge, don’t panic – it’s not usually dangerous.

Your heating system is protected by a pressure relief valve (PRV), and most systems also shut down if boiler pressure gets too high. As always though, you should call a heating engineer if you are concerned.

How to reduce boiler pressure

A heating engineer will be able to help you if you think you have an issue with boiler pressure. But there a couple of simple things that you can try yourself first:

Reduce boiler pressure by reseting your boiler

The first thing to try is resetting your boiler. This is the IT equivalent of switching it off and switching it back on again! Check your manual on where to find the reset button on your boiler.

Reduce boiler pressure by checking that the filling loop is fully closed

As we discussed earlier, you should check that your filling loop is fully closed so that water is not being leaked into your system.

Reduce boiler pressure by bleeding your radiators

Radiators before and after being bled.You can release excess water from your system by bleeding your radiators.

To do this, you need to switch your heating off and allow the radiators to cool. This will protect you from being burned by any very hot water that comes out of your radiators during the bleeding process.

You should then prepare the area under your radiator by putting a towel on the floor to protect your carpets. You’ll need to hold a container underneath the bleed valve on your radiator to catch any water that is released.

You can now fit your radiator key to the bleed valve, which is usually at the top of your radiator. Slowly turn the key anti-clockwise to release air.

You should hear a hissing sound as the air escapes. Keep releasing air from the valve until you can no longer hear air escaping and some water begins to trickle out.

Once you have released the trapped air from your radiator, you need to close the valve quickly.

Once you’ve bled your radiators, you should reset your boiler and check the pressure.

Final thoughts…

If you’ve tried these steps but your pressure is still too high, you need to call in an expert to determine what the problem is, and whether you need any replacement parts.

Useful Links:

Viessmann guide to high boiler pressure