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Understanding Boiler Pressure: The Complete Guide

Our complete guide runs through everything you need to know about boiler pressure. We can provide clear instructions for diagnosing and fixing low or high boiler pressure, or alternatively, our heating engineers are ready to help you should you require our assistance.


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What is boiler pressure?

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

Your heating system may not work correctly if you are experiencing boiler pressure issues. Too low and hot water may not reach all areas of the heating system. Too high and the system may fail due to overload.

Many of these problems can be diagnosed and fixed with relative ease, but sometimes an engineer is required.

Pro Tips:



  • Don’t confuse boiler pressure with water pressure!
  • Boiler pressure is the pressure of the hot water in your system.
  • Water pressure is how fast the water comes out of your shower and taps.

Why is boiler pressure important?

Your heating system is pressurised and relies on pressure stability to move water around your home efficiently.

Your boiler heats cold water and pumps this through the pipework in your home. It is important that the right pressure level is maintained in order for your heating system to compensate for pipes expanding and contracting as water heats and cools.

How do I check my boiler pressure?

So, you’ve read and understood how important boiler pressure is for your heating system, particularly in those bitter winter months! Now it’s time to diagnose the issue at hand.

Modern, popular combi boilers will have a built-in pressure gauge for easy reading. Your boiler pressure will more than likely be displayed on one of the following:

- A dial
- A hydraulic pressure gauge
- A digital display

One of these modes of measurement should simply display your boiler pressure and alert you if any problems are present.

Boiler pressure will 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?


Normal boiler pressure should be around 1.5 bars as standard.Your pressure gauge should indicate when there is an issue. This is usually shown by:

A needle on a dial pointing to a red, amber or green zone.
A digital gauge displaying an error message, code or symbol.

Low boiler pressure

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

  • Check the pressure gauge to be certain that your boiler pressure is low
  • 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

Your boiler pressure is likely to be too low if:

  • The pressure gauge reads low
  • You have no heating or hot water
  • Your radiators aren’t heating up correctly

Common causes of low boiler pressure include:


  • Boiler has naturally lost pressure over time
  • Boiler is leaking
  • Pipe/pipes are leaking
  • Radiator/s needs bleeding

How to increase 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: a) A keyless filling link which is two water pipes with valves on the ends, linked by a small braided hose; b) A keyed filling link which is internally located, meaning that you will need to consult your boiler’s manual to find it, or 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 an engineer visit  as soon as possible.


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Is low boiler pressure dangerous?


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, though through some research on our blog beforehand, you may be able to repair it yourself. Run through the 2 initial steps again first:

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

Signs of high boiler pressure


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

  • 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

How to reduce 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, or keyed filler, 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. You can do this yourself, but take care in doing so as it is easy to make mistakes. If you run into an issue or are uncertain about this task, contact a professional such as ourselves.
  • 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.

Our boiler pressure checklist

  • Correct pressure on gauge:
    Should be between 1 and 2 bars and within a green zone if there is one. It is normal for pressure to rise outside of these measurements when the boiler is heating and fall when cooling, so don’t panic right away if your pressure seems incorrect.
  • Boiler pressure dropping:
    Some boilers lose pressure slightly over a few months and are easily fixed. If you experience rapid pressure losses, check for leaks and drips immediately. Follow our steps to increasing pressure and contact a heating engineer if in any doubt.
  • Boiler pressure rising:
    Boiler pressure may rise slightly due to over-filling, so follow our above steps to correct the issue. If this happens spontaneously and significantly, contact a heating engineer to diagnose and fix the issue.
  • Boiler leaking:
    Secure the leak if possible and divert water away from any electricals or wires. Contact a professional immediately for an emergency boiler repair if significant.

FAQs


Boiler pressure relief valve is leaking


This will likely be due to your boiler pressure being too high and your relief valve is overloaded. It will be compensating for the increased pressure by attempting to release excess water and pressure.

If you successfully bring your boiler pressure back to a normal level, your relief valve may stop leaking. If it continues to leak, call a heating engineer. You may need a new valve all together. Even so, your best option here is to contact a plumber who can diagnose and fix the issue with ease.

Will a boiler work without pressure?


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.

This may also end up costing you a lot in heating bills. If you’re in any doubt about your boiler pressure, get an expert to come out and help you. It’s far better than being out of pocket and left in the cold.

Do boilers lose pressure over time?


Yes, your boiler will lose pressure over time. This is a natural occurrence 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. There are huge benefits to being on a boiler service plan too - this will allow you peace of mind when it comes to boiler issues, knowing that we’ll keep your boiler and heating system well maintained and working efficiently.

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