Most condensing 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.boiler pressure

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 current pressure of the system is ?

Every pressurised system, regardless of the type of boiler (combination boiler, combi 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.

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26 Responses to “Repressurise your System”

  • Sarah Sagert says:

    I wish the plumber who installed my system had given me a guide like this. I don’t remember them saying anything and now the system is six years old.

    I found this article made me aware and I set off to find the refill tap. It wasn’t easy as it was hidden behind a pannel which covered up the piping. Why do plumbers do this??

  • Boiler Systems says:

    Sorry to hear that Sarah. Sounds like it wasn’t a national installer who installed it?

  • Charles Wilson says:

    I have an old back boiler, I wish to replace it with a combi boiler.
    Can the new boiler use the existing piping. I am having new radiators installed.
    I live in a two bedroom flat.

  • Boiler Systems says:

    In general, the old piping for the back boiler unit will be removed when your new combination boiler is installed as it is plumbed in a completely different way.

    The pipes you have going to your radiators can remain in place as long as they are in a good state of repair. The combi system is sealed and this puts the system, and your old plumbing, under a 1 bar pressure so any weak or worn pipes and valves may leak.

  • Paul Willis says:

    We have a Worcester Bosch 28CDi combi boiler. The pressure is now getting low and I appear to have lost the white T shaped key that allows us to top up the water pressure. Can you tell me where I can buy a new one please?

  • Boiler Systems says:

    We do not stock these for retail but suggest you contact Worcester Bosch after sales service on 08457 256206.

    They should be able to provide you with a replacement or advise you of a local stockist.

  • anita heavens says:

    Can a boiler be installed in a loft?

  • Boiler Systems says:

    Yes, a boiler can be installed in a loft as long as the system is sealed.The loft itself has to have a drop down ladder attached to the hatch,the floor has to be boarded from the hatch to the boiler location and there has to be adequate lighting up there so the engineer can gain access quite easily to the boiler to repair or maintain.

  • Colin Marr says:

    Can a combi boiler be installed in a cupboard under the sink

  • Boiler Systems says:

    In theory it could, but finding one small enough to fit inside a floor cupboard could be a different matter. In addition to comply with the regulations the cupboard would have to be on an outer wall of the property for fluing purposes.

    To ensure that you comply with the regulations npower are able to send a Home Energy Consultant to your home to review the installation options and help to select the right boiler for your home. This visit is free with no-obligations on your part and will also enable an accurate quotation to be developed for a new boiler.

  • Brenda Hargreaves says:

    Help – I’ve topped up my combi boiler but it still won’t heat. Is there something else I need to do ? Is there supposed to be a pilot light – I can’t see one and once the motor starts nothing else happens ?

  • Boiler Systems says:

    It is very difficult to diagnose boiler faults over the Internet as there could be many reasons why a system is not heating up.

    Some boilers do not have pilot lights – they use instantaneous ignition.

    If you have a boiler maintenance agreement you should contact them on the telephone number you have been supplied with and ask them to check your boiler.

  • Paul Ormsby says:

    I have a Worcester Highflow 3.5 combi boiler (~1990)in which the pressure drops rapidly and erratically from above 1 bar to less than 0.2 bar and the pilot light goes out. I re-pressurise to above 1 bar and relight. The pressure seems to remain reasonably stable for a day or two then suddenly drops. I can find no evidence of a water leak any where in the radiator/pipe system. Could there be an internal problem in the bolier which causes these symptoms. The boiler is regularly serviced by an established compnay

  • Boiler Systems says:

    There can be many reasons why a central heating system is loosing pressure. We note that you have repressurised the system and this has not solved the problem. In the circumstances, the boiler being 17 years old, you should get someone on site to test the system.

  • Stuart says:

    I have a Gloworm Fuelsaver boiler and i’ve had nothing but problems with it. Having just moved into my new home it was not long before i had problems with the boiler. The boiler man said that it was not wired in correctly and that there was no permanent live feed from the boiler to the pump in the loft. As a result the over heat stat has been replaced twice and the pilot light goes out when the heating is switched off. Is this common? And if so were the regulations different when it was wired in in the first plce? The boiler is about 15 years old.

  • Boiler Systems says:

    Sorry to hear of the problems you have with your boiler. It sounds like the original installation 15 years ago had some issues.

    The boiler regulations are constantly being reviewed by the government and updates developed as needed. The last major changes saw the need for gas boilers installed after 1 April 2005 to be SEDBUK A or B rated and this related to energy efficiency.

    In that year also there were changes to the electrical wiring regulations, these were introduced to cut down the number of accidents associated with badly installed electrics.

  • Michael Merrett says:

    I am looking to reclaim the airing cupboard which is in the third bedroom of our 3 bed semi. At the moment we have a 15 yr old Baxi back-boiler. with 2 growing children the demand for hot water on tap is only going to increase.

    I want to move the boiler and HW storage tank into the loft, do away with the CH and HW headder tanks and balence the cold and hot water pressure.

    I need to decide whether to go for an un-vented or vented (thermal store) mains preasure hot water system. Could you please tell me what the disadvantages /advantages of each system are? In addition please could you let me know if anyone can install both types of system?

  • Boiler Systems says:

    NAtional installers do install both types of system. The main difference between the system types is that an unvented system you will have hot water stored in the system under pressure. This required expert installation and regular maintenance.

    Reading your comment it appears that you have investigated the options in depth and have knowledge of this area. Notwithstanding this if you require further information npower are able to send a Home Energy Consultant to your home to review the installation options and help to select the right boiler for your home.

  • james mcculloch says:

    i bled the radiators while the heating was on and the pressure fell out of the boiler to 0, when try to fill the pressure back up by using the loop, nothing is happenig! i have tried bleading the rads again to see if i can chase maybe an air block out the system but to no availe…! can any one help please

  • Boiler Systems says:

    It is important to switch off the heating before bleeding the radiators or it is possible to import more air into the system.

    If this is the cause of your problem you could possibly remove the issue by bleeding the system again with the heating off. Have you been successful in using the loop to pressurise your system before in the recent past? or has the filling mechanism become blocked?

    If you are still without heating and the filling mechanism does not work it appears that you will need the assistance of a heating engineer.

  • Jonathan Rugg says:

    Having recently moved, the house is fitted with an old Combi Boiler.

    The pressure is 0.7 bar and unfortunately the previous owner didn’t leave the manual. Can I download the manual online?

    Also could you advise what pressure the boiler should run at and how to increase it. Cant obviously see an external feed loop or valve.


  • Boiler Systems says:

    The boiler is manufactured to the highest specifications by Worcester Bosch and they hold the download manuals.

    If you contact their technical helpline on 08705 266241 they should be able to help you.

  • Sandeep Mandir says:

    I have a Baxi solo boiler and Range Fortic F4 cylinder, both were switched off for over a month while i was having my bathrooms and kitchen changed. I have switched them back on and the boiler looks like its working and the cylinder is getting hot, however the hot water pressure is very very low from all the taps in the house (cold is excellent). There is no pressure gauge on the boiler or cylinder and i can’t work out what the problem is. Please help.

  • Boiler Systems says:

    It is very difficult to diagnose central heating systems remotely, particularly on the limited information available.

    If you have taken out a gas boiler service contract with a reputable company 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 npower have a heating repair service which could help. If you telephone them explain your issue you will be provided with a no obligation fixed quote for fixing the problem.

  • David Allen says:

    I have a Worcester combi boiler installed by a national company and I am on a boiler service contract with them.
    With the heating system switched off, the pump comes on for a few minutes on odd occasions (usually early in the morning) but no light is showing on the boiler and no heat gets to the rads. The system is working well otherwise.
    Why is this? Should I ask a service engineer to call?

  • Boiler Systems says:

    Its called a pump overrun…the pump kicks in every now and again to either disperse any heat the boiler may be holding or to just turn itself over as part of its inbuilt maintenance routine, you do not need to get an engineer out.

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