Category - Boilers
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Will Scholfield, Engineer

31 Jan : Updated 18 Apr ● 6 min read

Boiler pressure: A complete guide

Understanding how your boiler works isn't always easy. With various components to consider, it's easy to forget about your boiler pressure.

In this guide, we'll be going through everything you need to know about boiler pressure so you can spot any potential problems right away. It's important for homeowners and landlords to keep an eye on their boiler pressure so that their central heating systems can run smoothly, always delivering hot water to taps, radiators and underfloor heating pipes exactly when you need them to.

What is boiler pressure?

Boiler pressure measures the pressure of the water in your heating system and boiler. You can check your boiler pressure using the pressure gauge - a small gauge/dial that’s usually found on the front of the boiler. This gauge will indicate the pressure within the system to identify any leaks or pressure drops.

Checking your boiler pressure can help you identify any issues like whether your boiler pressure is too high and at risk of causing a system overload, or too low which may cause issues with the system and the boiler.

Why is boiler pressure important?

The pressure of your boiler determines the effectiveness and efficiency of your entire central heating system.

Maintaining the correct pressure level is important for generating heat around your home, but what happens when your boiler pressure is too high or too low?

High boiler pressure

As your boiler produces more heat, your boiler pressure is bound to increase and cause your pipes to expand. It is normal for your boiler pressure to increase slightly, but it should stay within a certain range. If it becomes too high, problems could ensue, but how high is too high?

Signs your boiler pressure is too high

  1. The pressure gauge has moved into the red section
  2. Your central heating system has stopped working
  3. Filling valves are open or not fixed on tightly
  4. Water may be coming out from the safety discharge pipe on the outside of the boiler 

Why is your boiler pressure high?

For your central heating system to run smoothly, there must be a balance of air, water and space for vessel expansion. When there is not enough air in the expansion vessel, this can cause the water pressure to get too high.

Reasons why your boiler pressure may be too high:

  • Repressurising - if you did this more recently, then the pressure may not be at the right level
  • Faulty parts - A fault may be causing your pressure to increase
  • Too much water in your system
  • Boiler’s age - if your boiler is over 10 years old, then there’s a chance your boiler may be performing strangely

Once you've noticed that the pressure is too high, you must act swiftly as it can be very dangerous for boilers to operate with very high system pressure.

How to fix your high boiler pressure

If you want to reduce your boiler pressure, you must first check all of the above to assess what action needs to be taken. After turning off the heating and letting your radiators cool down, make sure the filling loop valves are tightly secure and then go forward with bleeding your radiators.

Releasing a little bit of water should help readdress the boiler pressure and reduce things down slightly. It's wise to check the pressure gauge after removing some water to see whether you're at the right level.

Once you're happy with things, you should be okay to resume as usual, although we would suggest keeping an eye on the boiler pressure gauge over the next coming weeks.

If there are any issues that arise or you feel there is something more serious that needs addressing, contact a Gas Safe registered engineer for help.

Low boiler pressure

At the other end of the spectrum comes low boiler pressure. Although not quite as serious as having a high boiler pressure, your health can still be impacted by living in cold conditions, especially during the winter.

Signs your boiler pressure is too low

  1. You have no running hot water
  2. Your radiators aren't warming up
  3. The pressure gauge has fallen

Why is your boiler pressure low?

Low boiler pressure is usually a sign that there is a leak or an open valve that is releasing air. You may not notice this at first but overtime your system will slow down and prevent you from warming up your house.

Reasons why your boiler pressure it too low:

  • Air or water escaping from a radiator when bleeding it
  • A leak from the system i.e. radiator valves or pipework

How to fix your low boiler pressure

When increasing your boiler pressure, you must first turn it off completely.

Underneath the boiler you should find the filling loop valve with levers either side or a keyed filling loop. Turn the valves to align with the braided hose and wait while the boiler pressure gauge rises.

Stop the valves once you've reached between 1 - 1.5 bar on the pressure gauge.

If there are no leaks or faults, your boiler should return to normal. Let the boiler run for a few hours and stop again so you can check to see whether the pressure has reduced or stayed the same.

If you find that there is a leak with your system, contact a Gas Safe registered engineer who can come and inspect the problem for you.

Does your boiler pressure keep dropping?

BOXT have got your back

BOXT have got your back

If you're not sure why your boiler pressure keeps dropping below the recommende­d level, take a look at our guide to help you solve the problem.

Why does my boiler pressure keep dropping?

Boiler pressure by type: combi, standard and system

As we know, boiler pressure can be caused by various different things but another factor you may want to consider is the type of boiler you own. Combi, standard and system boilers can all have different boiler pressure limits, so it's useful to know how your type is affected.

What pressure should a combi boiler be?

For combi boilers, the pressure gauge should read between 1 to 1.5 bars. If you have less than one bar, this may indicate that there is a leak or fault with your water system that may need addressing.

Less than one bar would suggest that your boiler may need repressurising and you may need to contact a service engineer to help resolve the problem.

What pressure should a standard boiler be?

Standard boilers don’t usually have a pressure gauge or tank as they have a water tank in the loft which fills the system and vents any expansion in the loft.

Sometimes a standard boiler can have a separate expansion vessel/tank fitted on the pipework that looks like a big red metal balloon. These would have a pressure gauge on the front and this gauge should be pressure of 1-1.5 bar.

What pressure should a system boiler be?

Much like combi boilers, system boilers have a built-in pressure gauge. Older versions will usually have a display dial while more modern system boilers will use digital ways of showing you the boiler pressure.

The gauge of a system boiler should be between 1 to 1.5 bars. When checking the pressure gauge, it's worth looking before and after you switch your boiler on so you can get a clearer understanding of how your system is operating.

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