Boiler pressure explained

No one likes to be left without central heating or hot water, especially in the winter. If you're experiencing boiler problems one of the common causes is too high or too low boiler pressure. We've pulled together a comprehensive guide on all things to do with boiler pressure, including how to check your boiler pressure, how to re-pressurise your boiler, and other common questions about boiler pressure.

If you're looking to replace your boiler due to pressure problems, we have a wide range of boilers for your home to choose from, no matter your needs.

Why is boiler water pressure important?

Boiler water pressure is important because your boiler heats cold water that flows around a circuit of pipework and radiators through your home. If you have a combination boiler, the pressure will be maintained by a filling loop that connects to your water pipe. For your boiler to work as safely and efficiently as possible, your water pressure needs to be stable when the pipes expand and contract with the heat to ensure no problems arise when your home is being heated.

What is the correct pressure for a boiler?

What is the correct pressure for a boiler?

Your boiler pressure should be between 1 and 2 bars at different states. Anything below one bar is considered low pressure, whilst 2.75 bars and above is considered high pressure. However, it's important to check your boiler manual to find out exactly what your boiler water pressure should be, as it can vary between makes and models.

What pressure should your boiler be when the heating is on?

When your heating is turned on, your boiler pressure should be between 1.5 and 2 bars to be working at the best operating pressure. Most boilers will have coloured pressure zones to help you see if your boiler is pressurised correctly. Usually, your boiler pressure indicator will be in the green zone if it is at the correct pressure, but if it is too high or too low (above 2 or below 1), the indicator will be in the critical red zones.

Should the boiler pressure rise when the heating is on?

The pressure of your boiler should rise a little when the heating is on because the water in the pipes and radiators heats up and expands causing the pressure to increase in the whole system. So, when the heating is on you should notice the boiler pressure on the gauge to increase slightly but not above 2 bar.

Always check your boiler manual to find out the optimal working pressure when you turn the heating on to ensure it doesn't increase by more than 1 bar above its optimal working capacity.

What pressure should your boiler be when the heating is off?

When your radiators are cold, so when your central heating system is turned off, it's normal to see a water pressure of between 1 and 1.5 bar. This means the needle should stay in the green area of the pressure gauge. Depending on the make of your boiler, the recommended optimal working pressure by most manufacturers is 1.3 bar but always check your boiler manual to ensure your boiler is functioning at the optimal working pressure.

How to check your boiler pressure

Most combi and system boilers these days will have built-in pressure gauges, so you can easily keep a check on the pressure level of the boiler. Your gauge may display pressure using a dial (a hydraulic pressure gauge) or, if you have a modern boiler installed, it's probably a digital version. If it is the hydraulic pressure gauge, it will have the green zone and the red zone as mentioned above, if you have a digital pressure gauge (that looks a bit like a control panel), it will usually flash a warning signal if there is too low or too high pressure.

What happens if your boiler pressure is too high?

If your boiler pressure is too high, this does not mean that it is dangerous. The pressure in your boiler is controlled by the expansion vessel, which controls the balance of air versus water in your central heating system. Too little air in your expansion vessel and the pressure can become too high which can result in water coming out of your pressure relief valve. In most cases, the boiler will turn itself off if the boiler pressure gets too high. If this happens, make sure you consult with a qualified heating engineer.

How to reduce boiler pressure

If you notice that your boiler pressure is too high it could be because you have recently topped up the boiler pressure by too much or not closed the pressure relief valve properly. The pressure valves are usually found underneath your boiler and need to be closed tightly. Even if the valves haven’t been used recently, they may have come loose or been knocked accidentally causing pressure problems.

If your boiler pressure is still having problems, you can have a go at bleeding the radiators. Although this is an easy task, you may want to call out a boiler service engineer as the water can be boiling and potentially dangerous, especially if there are kids or pets around.

Reasons why your boiler pressure may have dropped and what to do about it

There are two major reasons why your boiler pressure could be too low. If you regularly check the pressure of your boiler, a slight pressure drop may not be of concern, but if there is a sudden drop that leaves you without hot water or heating, the following could be the main culprits.

Water Leak

A leak means water is being lost from your system and lowering the overall pressure. Unfortunately, the leak could be small and unnoticeable, but if it goes on for long enough, your pressure will slowly drop causing all manner of heating and hot water problems. Check around your pipes, radiators, and even your boiler for water or damp patches to see if there are any signs of a water leak.

Do not try to look inside your boiler though you'll need a Gas Registered engineer to do that. Also, if you do find a leak, or need to top up the water pressure regularly, a Gas Registered engineer should be called for advice on how to fix the problem.

Bleeding a Radiator

If you bled your radiator recently, you could have lost pressure in the boiler system. This occurs when the air was released from the radiators, lowering the pressure in your boiler system.

Is low boiler pressure dangerous?

Typically, low boiler pressure means water can't be circulated around your system or delivered to your taps and showers. As a result, the boiler will usually turn off and display an error code to alert you that there is an issue resulting in low pressure.

In most cases, low boiler pressure is nothing to be concerned about, but if you do have concerns you should always seek the advice of a registered Gas Safe Engineer.

How to repressurise your boiler with low pressure

It is possible to repressurise your own boiler, but as each boiler pressure system is different, it's important to check the manual that came with your boiler. You could also visit your boiler brand's website to see if they have any helpful tutorials and videos about topping up pressure on their systems.

To repressurise your boiler, you will be allowing more water from the mains supply to enter the system via the filling loop. The two main types of a filling loop are 'built in' and 'external'.

Filling loops may vary in design and filling process, but here's the basic re-pressurising process.

  1. Turn off your boiler and allow it to cool.
  2. Find the filling loop, or attach it to your boiler.
  3. Make sure you can see the boiler pressure gauge while using the filling loop.
  4. Open the valves on both sides to let water into your system.
  5. Wait until the pressure gauge reaches 1.5 bar then close each valve, one after the other.
  6. Switch your boiler back on and, if you need to, press the reset button.
  7. Don’t forget to remove the filling loop if it’s an attachment. Be careful in case there’s any water left in it.

If you are ever unsure or don't feel comfortable repressurising a boiler yourself, always call a Gas Safe registered engineer to help you.

How to release pressure from your boiler

If you are experiencing high boiler pressure, it is possible to release pressure yourself, however, if you are unsure, call a Gas Safe registered engineer to release the system pressure for you.

If your boiler continues to experience high pressure, a Gas Safe registered engineer should be consulted to determine whether a new boiler is needed.

Further help and advice

If you need any further help or advice on boilers, water pressure or deciding on the size of a new boiler in your home, we have a range of guides that will be able to help you.