DIY How To

Fill a sealed central heating system

The following method is a guide which should be used in conjunction with the system operating instructions. Sealed central heating systems are complex, and any work should be undertaken by a qualified plumber

Fill a sealed central heating system


Sealed system with make-up tank

Before you begin to fill the central heating system, make sure all the radiator bleed valves and drain cocks are closed. Restore the water supply to the make-up tank in the loft. As the radiators fill with water, air will become trapped at the top of the radiators. When the make-up tank has stopped filling with water, add a corrosion inhibitor to the make-up tank as per manufacturers instructions.

Open the radiator bleed valves one at a time starting at the lowest. When the water starts to come out of the valve, close the valve and move onto the next radiator. If there are bleed valves on the pump or above the boiler it may also be necessary to bleed these.

Sealed system with filling loop

Before you begin to fill the central heating system, make sure all the radiator bleed valves and drain cocks are closed. Open the valve on the filling loop, when the pressure reaches 1.5bar close the valve on the filling loop. As the radiators fill with water, air will become trapped at the top of the radiators.

Open the radiator bleed valves one at a time starting at the lowest. When the water starts to come out of the valve, close the valve and move onto the next radiator. Keep checking the pressure gauge in between filling the radiators, when the pressure drops below 1bar open the filling loop to increase it back to 1.5bar.

When all of the radiators have been filled check the pressure gauge, if the pressure is below 1.5bar open the filling loop and let the pressure build up to 1.5bar, then close the filling loop and turn the heating on. Allow the heating to completely heat up which should release any air in the system through the relief valves, turn off the heating and the pump and allow the system to cool down. Check the pressure and if necessary open the filling loop to increase the pressure to 1.5bar

Add a corrosion inhibitor as per manufacturers instructions. For sealed systems there are cartridge type inhibitors that add the inhibitor via a radiator bleed valve. If in doubt seek help and advice from a qualified plumber.

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