All modern combination boilers have pressurised central heating systems, the pressure in the system will be indicated on the boiler display or on a pipe which enters the boiler. The optimal rest pressure for most boilers is 1.2 bar (please check your boilers manual/manufacturers instruction). From time to time, the pressure may drop, and the system will need ‘topping up’. If the system is losing pressure on a regular basis, please consult a Gas Safe Registered Engineer. However, in most cases the system will lose some pressure over a 6 month period.

There are two basic forms of topping up the system, each one unique to the type of boiler.

Internal filling loop and key

Most Worcester Bosch boilers have this system. The filling key can be found clipped into the plastic tray underneath the boiler and should be kept here should an engineer require it. The tray can be removed by gently pulling it towards you and it will slide of the mounting rails. At this point you should consult your manual as it will clearly show where and how the key is to be inserted. The key is to be inserted into the filling loop mechanism and then turned as shown in the manual. Next to where the key has been inserted you will find a square looking valve actuator. This then needs to be turned to open the water supply. DO THIS SLOWLY! When the boiler gauge has risen show a reading of 1.2 bar, which should be in the green zone, the valve should be closed immediately, and the key turned and removed.

If you overfill the system and the dial is still in the green zone, you can bleed a little water off the system by bleeding some water off a radiator as described below.

If re-pressuring the system does not work or you overfill over it, please call your Gas Engineer for advice.

Never allow your boiler pressure to go into the red zone and seek help from your gas engineer.

At the end of this process to remember to store your filling key back in the plastic tray and return the tray to its position.

Filling loop

Mostly found on all other types of boiler. A filling loop is used to connect the incoming mains water to the return side of your central heating system. This filling loop will have only one or two valve handles on it. The default position for both handles should be in the off position i.e. at 90°to the pipe. AT NO TIME SHOULD ANY ONE OF THESE HANDLES BE LEFT IN THE ON POSITION. To fill your system, firstly fully open, the valve furthest away from the boiler (if two valves are fitted), slowly the other valve and the pressure gauge should start to rise. NEVER LET THE PRESSURE EXCEED 1.2 BARS. If you inadvertently do this, release the pressure by bleeding a radiator as below. At the end of the procedure ensure both valves are returned to the fully off position. If you are unsure about anything described above, please contact your Gas Engineer. Please always consult with the owner’s manual/manufacturer’s instructions.

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