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 current pressure of the system 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 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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