Most combination boilers and many standard boilers are now installed as Pressurised Systems. This differs from the previous conventional boiler systems which were reliant on a water tank (often in the loft) with a ball float device to maintain water levels within the system. As the self filling water tank is omitted a pressurised system requires filling manually via a filling device, ( a ‘filling loop’). This allows a specific amount of water to be injected into the system via your mains cold water supply.

This water pressure is distinct from the pressure in your hot or cold water taps. The pressure in your mains cold water taps comes direct from the water mains and is maintained by your water company. The pressure in your hot water taps is created by the head of water in your heating cylinder or from the pump feeding your hot water supply.

How can you tell what the current system pressure is ?

Every pressurised system, regardless of the type of boiler (combination boiler or standard system), will incorporate a pressure gauge which you can read. This pressure gauge is the device with which the system water pressure in your boiler and radiators is monitored.

To maintain the system in a healthy condition the pressure gauge should be checked monthly. If when you check it, you find that the system pressure has fallen please follow the steps below to re-pressurise your system.

Your central heating system manual will advise of the pressure that your system runs at. Most systems should be pressurised to between 1 and 1.5 bar.

Topping up the system pressure

To top up your system and increase the pressure, you will need to locate your filling loop. It most usually resembles a stop tap and is connected to the central heating system by a metal hose. Occasionally you may find that one end of the this metal hose is not connected to the pipe work on your system. There may be a cap on the end of the pipe and it will need to be connected to the system to allow the water to be injected.

To connect the filling loop to the system, simply remove the cap from the end of the pipe work and attach the metal hose. You will then need to tighten this connection with your fingers, do not over-tighten with a spanner.

Sometimes this filling device may be hidden, behind a baffle near the boiler or perhaps inside a cupboard below the boiler. It will always be somewhere close to the boiler as it needs to be able to connect to the boiler pipework.

Another issue in finding the filling loop may be that some filling devices are an integral part of the boiler, you should have been advised of this by your boiler installer and you will need to refer to your boiler manual for the exact manner in which this system is repressurised.

If you have a Homecare contract don’t be too concerned if you can’t find your filling loop – ask the British Gas Service Engineer at the time of your next Annual Check.


To fill the system, use the tap you have located to open and close the filling loop. When the tap is opened it will allow fresh water to flow into your Central Heating system. As this happens you you will hear the water passing through the valves into the system. It is recommended that you open the valve slowly to allow the system to fill up gradually. When you do this a steady increase in pressure will be seen on the pressure gauge in the same way as you would see an increase on a car tyre gauge if you were inflating a tyre.

If you cannot see your pressure gauge while filling the system it is a good idea to have a friend look at it for you while you are turning the tap. When the recommended pressure is reached close the valve by turning it in the opposite direction to which you opened it.

There is no need to worry if your system does accidentally become over pressurised . All modern systems are designed with safety in mind and a pressure safety valve is incorporated into the plumbing. This acts like an overflow pipe releasing the excess pressure and allowing the system to revert to the recommended levels.

The safety valve may make a noise as it releases this excess pressure sounding like a thumping noise, again do not worry, this sound will stop when the system pressure reduces to a lower level.
If the system is free from leaks the water pressure should remain constant within the system in future months. If you notice that the pressure regularly falls you may have a pressure leak.

Bleeding radiators involving perhaps a small amount of air escaping from a system at the radiator bleed point can reduce the overall system pressure. As a result after bleeding your radiators you should remember to check your pressure gauge and fill the system as required.

Random water leaks will cause pressure loss within a pressurised central heating system and the severity of water leaks can vary. Very small leaks will cause pressure drops over a long time, possibly several months or even a year. Leaks of this magnitude may not be detectable as the water evaporates quite quickly although you may spot some residue following evaporation of the water.

Larger leaks may be more visible and will mean your system will require filling as frequently as once or twice a week. If this is the case you should check your system for leaks when it is cold paying particular attention to the areas around radiator and boiler valves. It is recommended that you check for leaks when the system is cold as heat causes expansion and can seal small leaks temporarily.


   

16 Responses to “Repressurising Systems”

  • rob:

    I have moved into a house and I cannot find a pressure filling point. My boiler is 15 years old, should it have this feature?

  • British Gas Homecare:

    If your boiler is 15 years old it could be a convential boiler type.

    Check in the loft. If, in addition to your large main household water tank, there is another small header tank then this will top up your central heating and you don’t have a sealed system.

  • Nicola Howard:

    Hi,
    We have Homecare 400 and our combi boiler keeps losing pressure. Will repressurising the system cure this or do we need to call out an engineer?
    Thanks
    Nicola Howard

  • British Gas Homecare:

    Boilers can loose pressure over time. Repressurising as indicated in this article and the manufacturers instructions shoul cure the issue.

    If you find that the boiler system regularly looses pressure after repressurisation you should have the system checked by a qualified engineer.

  • sean smith:

    Hi. We have a Ideal response 120 combi-boiler whereby it has a visual pressure gauge. I had to re-pressurise the system a couple of days ago and ever since, it wont stop filling up and we have had water continuously pouring out of the overflow. Do you have any suggestions on what might be causing the problem and what i should do to rectify it? Many thanks. Sean

  • British Gas Homecare:

    It sounds that a valve is jammed as the water should stop pressurising the system once the pressure valve that you opened is once again closed.

    If you have previously taken out one of British Gas’s Homecare agreements with the appropriate level of cover we recommend you request a visit from a British Gas engineer who should be able to identify the cause and adjust / repair your system.

    If you do not have an agreement, British Gas do offer an ‘on demand / emergency repair service. Telephone now on 0808 178 2098 for a Quotation. No matter how big the problem the fixed price quoted includes all parts, labour and call out charges.

  • Richard:

    I have an Ideal Response 120 Combi Boiler and the pressure is constantly at 3 (recommended to be between 1 and 2). This means that two or three times a week it does not fire and heat the water on demand. When it does fire I hear the ‘bang’ as the safety valve releases and it works ok for a few days. How do I reduce the pressure in the system and keep it down and what is causing the problem? Also, if the pressure is too high is this what is causing it not to fire? I have had the system checked, PCB replaced and the element cleaned.

  • British Gas Homecare:

    Richard, Whilst you say you have had the PCB changed, the system checked and the element cleaned none of these seem to have cleared the problem. Call back the plumber who did this work, tell them they have not solved the problem and ask them to complete the job you have paid them for.

    You will appreciate that we cannot solve plumbing and heating problems based on a few snippets of information within a web site comment.

  • wendy:

    Hi, I have no hot water coming through my shower,I have a Baxi combi 105e….abd a brisbane shower…. hot water is coming through the taps ok…. does this mean their is a prblem with the shower only or is it to do with the boiler

  • British Gas Homecare:

    It is very difficult to diagnose central heating systems remotely, particularly on the limited information available. You do not advise whether the central heating is hotting up and hence we cannot ascertain whether it is a shower issue or a boiler issue.

    If you have taken out an approiate British Gas Homecare service contract you should telephone the helpline number you have been provided with and an engineer will call to help remedy your problem. If you have not taken out any boiler service agreement British Gas have a heating repair service which could help. If you telephone 0808 178 2098 and explain your issue you will be provided with a no obligation fixed quote for fixing the problem.

  • Jan:

    Our hot water has recently only been staying hot for a few minutes before going cold. I also regularly have to turn the tap on and off several times or put the heating on to get the hot water to start up at all.Could this be due to low pressure? The pressure is around 0.9 and i have tried to repressurise it but nothing happens when i turn the stop tap on the filling loop.

  • British Gas Homecare:

    It could be low pressure, that certainly could be a reason why the filling loop cannot increase the pressure in the water system above 0.9 bar – but this may not be the only reason.

    If you telephone British Gas they will be able to provide you with a fixed price quote for attending and working on the problem.

  • bilal:

    hi, my boiler is letting out combustion inside the house, what should i do?

  • British Gas Homecare:

    It is not clear what you mean but if you suspect that your boiler exhaust gases are being vented into the house ten this is a very serious matter. You should switch off the boiler and open the windows to let fresh air into the house. Also ensure that the other members of your household are not exposed to the gases.

    Without delay get a Gas Safe (was CORGI) registered plumber to visit the house and arrange to make repairs to the flue, they can also advise you on any actions you should take in the interim.

  • Kathryn:

    Hi,
    I have repressurised my combi boiler, but I can’t remove the flexible loop without all the water pouring back out. Doesn’t seem to matter which way the taps are (closed or open), and doesn’t matter if I turn the water off at the mains.
    What am I doing wrong?
    Thanks

  • British Gas Homecare:

    It is very difficult to say without seeing your specific installation. We assume that this problem did not exist before you attached the flexible loop but happened after you had pressurised the system.

    Some systems have a valve at both ends of the flexible loop connection. Have you opened both but closed only one? If you say water is pouring out of the Central Heating System then whether the water mains are on or off is not material, the water pressure in the central heating system is driving the water out and your valve is not sealing this off. Have you perhaps over pressurised the system and is this water the excess pressure being released, if so it should stop after a short while. Other than that iIt sounds that a valve is jammed .

    If it is none of the above and you have previously taken out one of British Gas’s Homecare agreements with the appropriate level of cover we recommend you request a visit from a British Gas engineer who should be able to identify the cause and adjust / repair your system.


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