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How to drain a central heating system


1st Jan 2015 Home & Garden

How to drain a central heating system

Central heating systems sometimes have to be drained down – to repair a leak, for example. The following method is for an open-vented system, the most common type.


1. Switch off the boiler at the programmer.

After a few minutes, switch off the main electricity supply to the heating system. This is usually a fused spur switch located near the programmer.


2. Solid Fuel Fire

If there is a solid-fuel fire serving as a boiler, make sure the fire is out and the boiler is cold.


3. Water Supply

Shut off the water supply to the feed-and-expansion cistern (usually found in the loft). There should be a separate stop tap for this on the branch pipe from the rising main connected to the cistern’s ball valve.

draining central heating

If there isn’t a separate stop tap, or it is jammed and cannot be turned, stop the water flow into the cistern by tying up the ball valve to a piece of wood laid across the top of the cistern.


4. Locate Drain Valve

Locate the drain valve, which may be near the bottom of the boiler. There may be more than one drainage point on the system.

Clip a garden hose onto the outlet and run the hose to a drain outside. Open the drain cock so the water starts to flow. 

draining central heating


5. Locate all the points at which air is vented from the central heating system.

There will be radiator vents, a vent on the primary flow near the hot water cylinder in fully pumped systems, and manual or automatic vents in the loft if circulating pipes run there. There could be additional vents at other points as well.


6. Open the drain valve with a spanner, pliers or an Isle of Man key, turning counter-clockwise.

Water will then start to flow out of the hose at a fairly slow rate.

draining central heating


7. Start opening the venting points at the top of the system.

This will greatly speed up the flow from the drain valve. As the water level drops further, open the lower venting points until they are all open.

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