How to Repressurise a Combi Boiler Re September 29, 2021

How to Repressurise a Combi Boiler

Baxi Boiler

How do I repressurise my combi boiler?

Have you noticed that your radiators are not getting as hot as they usually do? Or are you having problems with your hot water? Or has your heating and hot water turned off? This could be a result of a loss in boiler pressure which needs repressurising. 

If you are experiencing cold spots on your radiators, this could be a result of a build-up of debris in your central heating system. We have a blog that covers these issues, so head on over, once you’ve finished reading this blog 😉

If your boiler pressure levels have dropped, you may need to repressurise your boiler to ensure that it continues to work as efficiently as possible. If you are experiencing these issues, don’t panic as the majority of the time, you can repressurise your boiler yourself, without having to call out a Gas Safe Engineer.

Throughout this blog, we will be discussing everything you need to know about repressurising your boiler, with a step-by-step guide on how to do so.

A combination boiler is the most common boiler type. In fact, over half of all boiler installs in the UK are combis. The combination boiler is named for its ability to provide a combination of both heat and hot water for your home. 

Combi boilers are connected directly to the mains water supply coming into your home; they heat this water directly, eliminating the need for a separate water tank (as with a system or regular boiler). This saves a great deal of space, especially in flats and smaller homes where space is limited. 

Combination boilers are more economical as they only heat the water that you require. Many people that have older boilers (like a back boiler) switch over to a combi boiler when they purchase a new one.

In simple terms, the water is supplied directly from the mains, a gas burner is ignited when the tap is turned on, the heat exchanger transfers the energy to the water which heats it up, ready for you to use. 

If you like to learn about the process in-depth, check out our blog on ‘how do combi boilers work’.

At the front of your boiler, you will find your pressure gauge. When your boiler is off and cool, this is an easy way to check your boiler’s pressure.

A hydraulic pressure gauge will indicate the following: 

  • Standard operating pressure with a green zone
  • Maximum safe operating pressure with a red zone

You may find that your boiler has a digital boiler pressure gauge, which will show you if the pressure is too high or too low by flashing the reading on the LCD screen.

Ideally, the pressure of your combi boiler should be between 1 and 1.5 bar.

If your pressure reads at less than 1 bar, you may have lost water from the system due to a leak. 

If the pressure reads at 2.75 bar or above, which is too high, you should look into bleeding your radiators. This will help to release the pressure and bring it back down to a suitable level. 

If you are going to attempt to bleed a radiator yourself, make sure that you have a way of catching the water to prevent the walls or floor from being ruined. Take caution when undertaking this job as the water can be very hot. If you’re unsure about anything, it’s a good idea to call a qualified engineer to assist you.

Before you attempt to repressurise your combi boiler, it’s a good idea to check your boiler manual for instructions to ensure whether it is safe to do it yourself. Every type and model of boiler is slightly different so it is important to double-check what steps you need to follow for your particular boiler model.

Once you’re ready, you can use the following steps to complete the process:

  • Step 1: It’s important to switch your boiler off before re-pressurising it. If possible, let it cool down for 4 to 6 hours.

  • Step 2: Carefully pull out the tray from underneath your boiler and remove the key that you will find attached to it. You may find the key is secured to the tray with a clip. If so, all you need to do is unclip it to remove it.

  • Step 3: Next to the square manifold nut, you’ll find the key manifold keyhole. Insert your filling key into the key manifold. You will have been given the filling key during installation. 

  • Step 4: Turn the key approximately 45 degrees to the unlocked position. 

  • Step 5: As the pressure begins to rise, the arm on your boiler’s pressure gauge will begin to move up. Turn the manifold nut clockwise when the gauge reaches 1.5 bars. Keep an eye on the gauge to make sure it settles at this pressure.

  • Step 6: If the arm moves into the red section on your gauge, don’t panic! To lower the pressure again, simply turn the release knob on a nearby radiator. If you need to do this, take care not to be too close when turning the release knob on the radiator because the release valve will release hot air.

  • Step 7: When you’ve turned both the manifold nut and the manifold key back to the locked position, remove the filling key and put it back in the tray underneath your boiler.

  • Step 8: Switch your boiler back on. The pressure may fluctuate slightly at first, but it should settle within a minute or two.

Re-pressurising your boiler using a filling loop

  • Step 1: Switch your boiler off and allow 4 to 6 hours for it to cool down. When your boiler has cooled, check the filling loop hoses are secure (they may need tightening).
    Top Tip! A radiator, expansion tank or pressure relief valve leak could also cause a pressure drop, so it’s worth checking these things, too.

  • Step 2: Open the filling valves. These can be unlocked by tap handles, or with a screwdriver, and will be found near to where the filling loop hoses connect to the boiler. As you turn the valve counter-clockwise, you should start to hear water. 

  • Step 3: When the pressure gauge on your boiler reaches 1 bar, it’s time to close the valves.

  • Step 4: When the pressure on the gauge has settled at 1 or 1.5 bars, switch your boiler back on.

There are two types of filling loop: external and built-in. You will need to attach the external filling loop. It should be attached during Step 2 and then remove it before turning the boiler back on in Step 6.

This is an image of a traditional filling loop that is built of three main elements. 

In this image you can see:

  • The non-return valve
  • The isolation valves
  • The filling link

A filling loop connects the boiler to the mains to fill or pressurise the boiler. Cold water is able to enter through the non-return valve, pass through the filling link and exit via the isolation valve. The non-return valve does exactly what it says on the tin. It acts as a barrier ensuring that no dirty water is able to pass back through the entering isolation valve. 

There are a few manufactures that include their own filling loop, that are installed during installation. Ideal is one of these. Here is an example of an Integral filling loop that is used to connect Ideal boilers. 

Worcester Bosch is also another brand that supplies their own filling loop, however, Worcester Bosch tend to use key-less filling loops on their boilers. 

By re-pressurising your boiler, you’re allowing more water to enter the system from the mains supply.

Don’t panic if you accidentally over-pressurise your combi boiler. As previously mentioned, bleeding your radiators will help to bring the pressure back down.

Keep an eye on the pressure of your combi boiler over the following days and weeks after re-pressurising your boiler, to make sure it doesn’t drop again. Once you’ve topped up the pressure on your system, your boiler should be working well, leaving you with a nice and toasty home for the colder months and providing you with plenty of hot water. But if it begins to lose pressure again, or you don’t have the time to give it a go yourself in the first place, call a boiler repair engineer.

A boiler will naturally lose a small amount of pressure over time, or if you have bled your radiators and air escapes your system pressure can drop. If you have to regularly repressurise your boiler then there is a good chance there is a leak somewhere in your system and you may want to call out a Gas Safe engineer to see if they can locate it, and hopefully, put a stop to it.

There isn’t a specific timeframe that states when you should re-pressurise your combi boiler.

You should re-pressurise it when the pressure has dropped to below 1 bar. This indicates that water has been lost from the system and needs to be replaced.

You shouldn’t need to do this frequently. If you’re doing it more often than you think you should be, contact your installer as your boiler could be leaking.

Depending on how low the pressure of your boiler is, you should allow water in through the valves for around 10 seconds. This should be enough water to get your combi boiler back up to pressure again. If it takes longer than this, you could have a leak in the system. 

The whole process, including attaching the filling loop, opening and closing the valves and removing the filling loop should only take a couple of minutes. It might take a bit longer if you’ve never done it before.

A combi boiler system should be installed as a pressurised system. However, if you have a conventional (heat only) boiler system, you may have a self-filling water tank (usually located in the loft) that controls the pressure for you.

The system works by heating cold water in the hot water storage cylinder which feeds into your taps. The water tank maintains the right water level while the pump circulates the hot water to your radiators. You should never have to re-pressurise a regular heat only boiler. 

Combi boilers need to be connected to the mains to increase the pressure. If your boiler doesn’t have a built-in filling loop, you will need to purchase an external one, which you will need to attach yourself. 

An older combi boiler might have a filling key rather than a filling loop. The key can be inserted into the keyhole and put into the unlock position. Use a spanner or wrench to turn the valve next to the filling key until you can hear water flowing through it. Once the pressure reaches the 1.5 bar, close the valve with the spanner and remove the key by turning it to the lock position. This prevents the valve from being turned on accidentally.

NHS Discounts

If you notice that your combi boiler pressure continues to drop immediately after re-pressurising it, this may be a sign of a leak in the system or a damaged pressure relief valve. 

If you’ve tried to re-pressurise the system once and it hasn’t worked, don’t keep trying, this could cause more damage to your combi boiler. In this case, it is better to get a qualified engineer to come and have a look for you. 

Although many argue that re-pressuring your combi boiler is not a complicated procedure, if you are unsure about anything at all, call an engineer to help you determine what the problem is. You don’t want to be left without hot water or heating.

You may have noticed that your boiler pressure rises when your central heating is turned on. This is because when water is heated it expands, meaning that as the water in your boiler begins to heat up, the water expands and causes the pressure within the central heating system to rise.

When your heating is on you may be able to see the pressure gauge dial on your boiler slightly increase, however, this should not reach above 2.0 bar.

Boilers are designed to quickly respond to changes in pressure and can release excess pressure via the pressure release valve. 

If your boiler pressure is too low, then you may experience your boiler going into ‘lockout’ mode, meaning that it will turn off and your display panel will have an error code on it. 

You can top up your boiler pressure yourself, following the steps above, however, if you’re unsure about what you’re doing, it’s always better to call an Engineer. 

A common cause of low boiler pressure is a leak within your central heating system, potentially within a boiler component, a radiator or pipework.

If you have attempted to repressurise your boiler already and still notice a drop in your boiler pressure, then you will need to get advice from a Gas Safe Engineer and ask them to come and check for leaks. If you have a leak then it will usually require them to reseal or potentially re-pipe your system where necessary.

If your boiler pressure gauge reaches the critical red zones, there may be a fault with your boiler. 

If your boiler is consistently displaying a very high pressure, for example between 3.5 and 4.0 bars, you should get checked out by a registered Gas Safe Engineer. 

Boilers are designed to deal with high pressure so if you notice that your pressure gauge is displaying this, then this means that there is a potential fault with a component that has a central role in boiler pressure modulation. This could be the expansion vessel or the pressure release valve.

As with all appliances, over time, their efficiency will naturally decline. If maintained properly and regularly serviced, then your boiler should last you between 10 and 15 years. 

However, if you’re unsure if the lifespan of your boiler has come to an end, here are a few things you can do to prepare yourself for getting a new boiler:

  • Check out reviews on the current best boilers on the market 
  • Weigh it up against the cost of a new boiler, covered for repairs under a 10-year warranty. Boiler finance is offered by WarmZilla. 
  • If your old boiler has an expensive repair required, is certainly worth taking the leap to a new boiler.

When you purchase with WarmZilla, your installation will include the necessary flue kit needed for your new boiler, a guarantee that WarmZilla customer care will sort for you, a FREE Neomitis RF digital wireless room thermostat, a system cleanse, magnetic system filter for ongoing protection, a chemical inhibitor, removal of existing boiler and parts and installation by a friendly, Gas Safe Engineer.

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