How to repressurise a boiler?

UK Home Improvement

How To Repressurise A Boiler?

Your household boiler is one of the most important appliances in your home. 

The machine that powers your central heating system and keeps the property warm, it’s particularly important during the winter months of the year when the temperature plummets and your home can be susceptible to the cold. 

With this in mind, any problems with your boiler can be extremely frustrating especially if the pressure drops.

If your boiler pressure becomes too low, it can mean that your boiler will not be working as effectively, resulting in a colder and less comfortable home. 

In these instances, it may be necessary to repressurise a boiler.   

While it might sound like a complicated process, you don’t need to call in professional support for this kind of task. In fact, a boiler can be repressured by yourself if you follow the appropriate steps – saving you time and money.

In this article we’ll look at how to repressurise a boiler, outlining everything you need to know about the process so you can minimise any boiler downtime and get your home toasty and warm once again.


Signs My Boiler Has Low Pressure 

Before we go into detail on how to repressurise a boiler, it’s important to understand the common signs and symptoms that indicate your boiler pressure is too low. Low boiler pressure can lead to a loss of heating efficiency and, in some cases, even cause the boiler to shut down. This could mean that you lose access to central heating and even hot water in your home, a disaster at any time and even more so during the winter. 

Here are some common signs that your boiler may be suffering from low pressure:

  • Loss of heat – The biggest and most obvious sign of low pressure is a loss of heat. If your radiators are not heating up as they should, it could be a sign of low boiler pressure. Inadequate pressure may prevent the proper distribution of hot water throughout the heating system.
  • Strange noises – Unusual noises such as banging, clanking and gurgling from the boiler or your home’s radiators could also indicate low pressure. This is because air that has become trapped in the system could cause peculiar noises to occur.  
  • Visible leaks – If you spot any visible water leaks around the boiler or from the heating pipes it could be a sign that pressure has dropped. However, not all pressure loss is due to leaks as it can naturally decrease over time. Nonetheless, it’s always worth checking out. 
  • Loss of hot water – As well as powering central heating, your boiler is also responsible for the supply of hot water. If you notice a sudden or gradual decrease in the availability of hot water in your home it may be related to low pressure. 
  • Boiler warning lights – These days, many modern boilers come with warning lights or error codes that indicate when an issue has occurred. This includes issues with low pressure. Be sure to check the boiler’s display panel for any warning lights or error messages.   

If you suspect your boiler has low pressure, it’s essential to address the issue promptly. By following the steps we’re going to outline in this article you can get your boiler back up and running in no time, however, if the issues persist it could indicate a more invasive problem which needs an engineer or technician to rectify. 

What Causes A Loss Of Boiler Pressure? 

Now we’ve explained the various signs that might indicate your boiler has lost pressure and needs repressurising, let’s look at the reasons why a loss of boiler pressure has occurred in the first place. 

There are two main reasons why boilers lose pressure.

Firstly, a boiler is likely to lose pressure if a leak has occurred. As mentioned in the section above, any visible signs of a leak could be an indication that pressure has been lost and so leaks must be fixed as soon as possible. If you notice any damp spots near pipes, radiators or the boiler itself in addition to the loss of pressure, then this is likely the cause. 

If you do find a leak near your boiler, remember you should never attempt to look inside the boiler itself. This can be extremely dangerous and only a professional and qualified Gas Safe engineer should be doing this. 

Secondly, boilers can sometimes lose pressure after you’ve bled your home’s radiators. A necessary household task to keep radiators working efficiently, after you’ve bled them it’s always worth repressurising your boiler to ensure both systems are working in harmony.   

What Should Boiler Pressure Be?

Before starting the repressurising process, it’s important that you know what the optimal boiler pressure should be. 

Many boilers will come with a gauge that informs you of the boiler pressure. If you’re concerned about your boiler pressure being too low (or too high) then this is something you should consult if your boiler has one. 

The ideal pressure for any boiler is between one-two bars. Anything below one is considered low and this is when action should be taken. It’s worth noting that you may spot that the pressure gauge is at the lower level of one when your radiators are cold and the heating is switched off. This is pretty normal, but if it drops below one at any point it’s considered low and you should consider repressurising the system. 

Alternatively, if you notice that the reading is higher than 2.75 bars, the boiler’s pressure is too high. 

How To Repressurise A Boiler?

Now we’ve explained all the relevant boiler pressure information, you can go about fixing the issue. 

There are normally three ways to repressurise a boiler, and we’ll outline each method below. However, we’d always recommend keeping the specific boiler manual close by as your individual make or model could have a different process. 

Let’s look at how to repressurise a boiler. 

Use the internal filling key 

  1. Firstly, you need to locate the internal filling key on the boiler. This is a small plastic covering that resembles a tray. Take it out to locate the key. 
  2. At this point, you should see two white arrows that are located near the base of the key. Make sure they’re correctly lined up to the key in the manifold. 
  3. Now you need to find the manifold, which is black plastic and connected by your system’s cold mains. It goes directly into the boiler and through the appliance’s heating system. 
  4. Next, find the padlock symbols that are engraved next to the manifold’s plastic. This is important as it will allow you to position the internal filling key the right way.
  5. Now comes one of the most important parts of the entire process. You need to find the aforementioned white arrows on the key and turn them. They need to line up with the open position of the manifold. Once the key has been pushed inside, turn it to the closed position. Be sure to double-check that the key is completely pushed down before progressing to the next step.     
  6. Once the internal filling key is closed, turn the white plastic nut anti-clockwise. You should then start to hear the appliance filling with water.
  7. Once the above steps have been completed you can check the boiler’s pressure gauge. If it has successfully reached the recommended level between one-two bars, the repressurising has been successful. 
  8. Finally, lock the internal filling key and ensure the white nut is tightened.  

Use the external filling loop 

If you’re repressurising the boiler with an external filling loop, follow the steps below. Usually, this process is far easier than an internal filling key. 

  1. First, locate the cold water mains valve along with the heating system valve. Next to these valves, you should be able to see a silver hose, this is an important element of the repressurising system. 
  2. Use the hose and connect it to both valves.  
  3. Fully open the cold water mains valve and then the heating system one.
  4. Monitor the pressure gauge throughout this process, the bar should move into the sweet spot of between one-two bars. 
  5. Once the desired pressure has been achieved, close both valves. Keep the hose attached. 

Use the internal filling loop

The final method to repressurise a boiler is by using an internal filling loop.

This is the most simple way to repressurise the boiler and why we’ve left it until last. All you have to do is have a quick look at the bottom of the appliance. You should see a blue lever. Simply give the lever a pull and the pressure should automatically start to rise. Once it reaches the level you need, let go of the lever. 

And that’s it.

If for whatever reason the lever does not yield results, you are going to need professional help. 

Remember, while the steps we’ve outlined are fairly straightforward, if you’re unsure about anything always consult a professional engineer or technician. And above all, never open up a boiler yourself. For more DIY or home improvement tips and advice, take a look at the content across our website. 


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