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Gas central heating keeps your home warm by heating cold water and pumping it around your pipes and radiators.  This whole system relies on the stability of boiler pressure in order to work efficiently.

Our guide runs through everything you need to know about boiler pressure, plus our recommendations for diagnosing and fixing low or high pressure

Boiler Pressure Explained

Boiler pressure, which is measured by a pressure gauge inside or underneath a boiler, maintains effective water flow around the home.  It basically ensures that hot water can flow properly through the entire heating system.

Your heating may not work correctly if you are experiencing boiler pressure issues. If the pressure is too low, hot water may not reach all areas of the heating system.  If it’s too high, the system may fail due to overload.

The good news is that many of these problems can be diagnosed and fixed with relative ease.

Why Is Boiler Pressure Important?

Most central heating systems are sealed. This means there are no vents to manage the water expanding and contracting as it heats and cools.

Instead, the system is pressurised – your boiler heats cold water and pumps it through all the pipework in your home.  It is important that the right pressure is maintained so that it can do this.

How Do I Check My Boiler Pressure?

Most Modern combi boilers will have a pressure gauge on the front showing what the current pressure is. Your boiler pressure will more than likely be displayed on one of the following:

  • A Dial – most commonly with a single needle and with low and high pressure shown by red areas on the dial
  • A Hydraulic Pressure Gauge – with one or 2 needles, with the red needle acting as a marker for where the pressure should be and a black one showing where it is
  • A Digital Display – some boilers display the pressure on an LCD screen and may flash or show an error message if the pressure is too high or low

One of these modes of measurement will allow you to check your boiler pressure so you know if any problems are present.

Boiler pressure can naturally fluctuate as the heating is used and water subsequently heats and cools. The pressure should stabilise itself once the system is switched off, so don’t worry if you see changes like these occurring.

What Pressure Should My Boiler Be?

The ideal pressure for your boiler is usually between 1 and 2 bars, but it’s possible it could go up to 2.5 bars when your heating is on.  Check your boiler manual to find out exactly what it should be for your boiler make and model.

Boiler pressure is below 1 = low pressure

Boiler pressure above 2.75 = high pressure

Low Boiler Pressure

You may be able to fix low boiler pressure yourself.  Firstly, run through these 2 initial steps:

  1. Check the pressure gauge to be certain that your boiler pressure is low
  2. Consult your boiler manufacturer manual if you have it (it may inform you of certain checks you need to make or steps you need to take that are brand specific)

Signs And Common Causes of Low Boiler Pressure

Unless you check your boiler pressure gauge regularly, you might not notice the pressure is dropping.  But if you suddenly notice one of the following it’s likely that Your boiler pressure is too low:

  • The pressure gauge is reading low when you check it
  • You have no heating or hot water
  • Your radiators aren’t heating up correctly


Common causes of low boiler pressure include:

  • Your boiler has naturally lost pressure over time
  • Your boiler has a leak
  • There is a leak somewhere in the pipework
  • Your radiators need bleeding

What Can I Do About Low Boiler Pressure?

Great news! It’s likely that you can increase your boiler pressure yourself in just a few simple steps.

Tip: Increasing boiler pressure is sometimes referred to as ‘re-pressurising your boiler’

If your boiler pressure is low, follow these steps:

  • Turn off your boiler
  • Locate your boiler’s filling loop. This is usually directly underneath the boiler and will have either:
  1. a) A keyless filling link which is two water pipes with valves on the ends, linked by a small, braided hose
  2. b) A keyed filling link which is internally located, meaning that you will need to consult your boiler’s manual to find it
  3. c) Standard external filling loop which looks like a silver braided hose
  • Open the valves by turning them to allow cold water to fill the boiler. You should be able to hear the water running and see the pressure gauge rising.
  • Wait for the pressure gauge to display a normal range and shut off the valves at around 1.5 bars. You may have to press reset on some boilers
  • If the boiler pressure maintains normal levels and no water is leaking (that you can clearly see), you can now turn your boiler back on
  • Allow the boiler to run and then cool down again. Re-check the pressure to ensure that it hasn’t dropped significantly
  • Grab a lovely hot shower or cosy up with the radiators on! You’re all set


You shouldn’t have to worry about increasing your boiler pressure all that often. If your pressure drops more than twice a year, consider thoroughly checking for leaks.  If you do find a leak, or continue experiencing low pressure, book a Gas Safe engineer to resolve the problem.

Tip: we have found several useful videos on this process on YouTube so do see if there’s one for your boiler make/model should you need help with re-pressurising

No, low boiler pressure isn’t dangerous.

Low boiler pressure may indicate a leak, which on a large scale may be damaging, but these leaks are far more likely to be minor and can be easily repaired. Just be careful of water leaking near any wires or electrical connections.

High Boiler Pressure

High boiler pressure is a slightly more difficult issue to fix on your own.  Run through the 2 initial steps again first:

  1. Check the pressure gauge to be certain that your boiler pressure is high
  2. Consult your boiler manufacturer manual if you have it

Signs And Common Causes of High Boiler Pressure

In the same way it responds when the pressure gets too low – if your boiler pressure gets too high it may automatically turn off.  Your boiler pressure is likely to be too high if:

  • The pressure gauge reads high or is in the red zone
  • Your boiler is making a banging noise
  • A small copper directly outside your property from the boiler may be discharging water


Common causes of high boiler pressure include:

  • Too much water was added to the boiler in a previous attempt to correct low boiler pressure
  • Valves on the filling loop are open or broken, allowing water to continuously flow in
  • The expansion vessel may have failed on your boiler

What Can I Do About High Boiler Pressure?

Firstly, you’ll need to diagnose the specific issue that is causing you to have high boiler pressure. By running through some simple steps, you can usually figure out what’s causing the problem and work to resolve it.

  • Check the fill valves
  • Turn off the boiler and let it cool if needs be
  • Check that your filling loop valves are shut tight and fully closed
  • Check if your pressure gauge has returned to displaying a normal reading. If your pressure is still too high, move on to the next possible issue

Bleed Your Radiators

If the above has failed to work, you may need to bleed your radiators. Bleeding your radiators takes water out of your central heating system, thus lowering the boiler pressure.  You can do this yourself but take care in doing so as it is easy to make mistakes.

  • Turn off the boiler and let the system cool down
  • Use a radiator key to gently open each valve on your radiators, one at a time
  • Release any trapped air in your radiators in its entirety
  • Repeat this for each radiator, then re-check your pressure gauge. If it is still too high, you may need to repeat the bleeding process one by one, ensuring that you go back and check the pressure gauge after each bleed
  • Continue checking the gauge until it reads normal again, between 1 and 2 bars or in the green zone. If your boiler pressure ends up too low, repeat the above procedure on increasing pressure
  • Take a lovely hot bubble bath! You’ve earned it


If your boiler pressure is still too high, you should arrange a boiler repair and have a Gas Safe Registered engineer diagnose and fix the problem.

Is High Boiler Pressure Dangerous?

No, high boiler pressure isn’t typically dangerous.

A boiler pressure relief valve in your boiler will protect the system and prevent significant damage by allowing water to escape when pressure is too high. Your system may also shut itself off for safety if it detects seriously high pressure. You may also see water discharging to outside which is a safety feature.

To conclude – Your boiler and heating system will struggle to work properly without pressure. The water will not flow through the pipework effectively and you won’t be able to heat water up enough to become hot. If you’re in any doubt about your boiler pressure, get an expert to come out and help you.

Your boiler will naturally lose pressure over time, and it is extremely likely that you will have to repressurise your boiler at some point in your lifetime (or get someone else to do it!). You shouldn’t experience significant and consistent low pressure more than twice a year. If this is the case, book a boiler service to help diagnose the issue and keep your boiler and heating system well maintained and working efficiently.

At Horizon Lets we only work with Gas Safe registered engineers and are always on hand to help landlords and tenants with boiler issues.  Contact us today to see how we can help you manage your property:


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