In order to work effectively and efficiently, you boiler needs to operate at the correct pressure. If the pressure is too low, you may not be able to heat your home properly or suddenly find that you do not have hot water when you were expecting it. If the pressure is too high, on the other hand, there may be a serious fault in the system that left unresolved could cause costly damage and may present a danger. In most cases, excessive pressure will cause your boiler to shut down - leaving you without heating or hot water. 

Read on to learn how to check that your boiler pressure is correct and what to do if it is not.

What is the correct pressure for my boiler?

Most gas boilers have an operating pressure of 1 - 1.5 bar. On most boilers, this will be shown as a green zone on the pressure gauge. Remember, this is the pressure that they tend to sit at when there is low or no demand - if you are using the heating or hot water, then you may see the pressure increase. An increase of more than 1 bar, when the heating is in use and up to temperature could indicate a fault with the expansion vessel or pressure release valve.

Where can I find the pressure gauge?

On gas combi boilers and system boilers, the pressure gauge will be found in the control panel and should be easy to identify.

For heat only (‘regular’) boilers installed on a pressurised system, the pressure gauge will be installed in the pipework, located just underneath the boiler. Open vented central heating systems, which can be recognised by the feed and expansion tanks in the loft, are not pressurised and therefore will not have a pressure gauge.

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How do I increase my boiler pressure?

How to increase boiler pressure is a very common question for homeowners. The first thing you should do before proceeding is consult your operating manual to ensure that it is safe for you to perform the procedure yourself. If you are happy to proceed, you can then repressurise your boiler using the following steps:

  • Locate the filling-loop and pressure gauge. The filling loop handles should be at a 90° angle to the flow of the pipe.
  • Check that you can see the pressure gauge whilst accessing the filling loop. The correct pressure level will often be shown in green on the gauge for modern systems, however you should check the pressure for your specific model in the manual prior to beginning the procedure.
  • Turn off the boiler.
  • Turn both filling loop handles until they match the direction of the pipe - you should now hear water flowing.
  • Observe the pressure gauge to make sure you don’t over-pressurise the boiler. If you cannot see the gauge at all times, it is recommended to ask someone to assist before proceeding. Once the correct pressure is reached, make sure the handles are fully closed by returning them to the 90° position.
  • Turn the boiler back on and check if the pressure remains steady.

If your boiler maintains its pressure once you have followed these steps, then the problem is likely to have been resolved and there is no need to take further action.

For more information, see our article what to do if your boiler loses pressure.

If at anytime you are unsure, we recommend that you seek assistance from a Gas Safe registered engineer.

What if I over pressurise my boiler?

If you accidentally over-pressurise the boiler, don’t panic. The simplest way to to reduce the pressure back to the correct level is to bleed your radiators. The procedure is fairly straightforward, but it is a time consuming job that is best avoided.

My combi boiler pressure increases when the heating is on - what should I do?

It is not unusual for your combi boiler’s pressure to increase when the heating is on or the hot water is running, but the pressure should generally not increase by more than 0.5 - 1 bar when there is demand on the system. The reason that the pressure gauge jump ups is that the water inside your boiler expands as it heats up.

Your boiler is designed to handle these rapid jumps in pressure, and has an in-built pressure relief valve for safety. This means that even if the pressure gauge is in the red area or looks far beyond safe limits, there is generally no reason to worry. The boiler pressure gauge should usually fall back to safe levels within a few minutes once the heating and water are off.

If your boiler pressure remains high but it is still working, you do not need to panic. Firstly, check that you have not inadvertently left the valve or filling loop open. If everything is in order with the loop, then it may be possible to reduce the pressure by bleeding your radiators.

If you have followed these steps and you are still not happy with your boiler pressure, then you will need to get a trained engineer to take a closer look.

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