DIY How To

Draining a Central Heating System

Sometimes it is necessary to drain a sealed central heating system or a vented central heating system, this could be, to have a new boiler fitted, to add additional radiators, replace a faulty valve, or carry out some maintenance on the system.

Drain central heating system

This project is sponsored by Draineasy

Draineasy

The following method is a guide which should be used in conjunction with the system operating instructions.

Sealed systems are complex, and any work should be undertaken by a qualified plumber

Safety Advice

Turn off the boiler and allow the water to cool down.

Turn off the water supply to the central heating feed and expansion tank, either turn off the stopcock supplying the feed and expansion tank or lay a piece of wood across the tank and tie the ball valve up to it.

Some sealed systems have a make-up tank similar to a feed and expansion tank found in an open vented system. The difference is there will be a non return valve in the make-up line which stops the water expanding back into the tank when heated.
Turn off the water supply to the make-up tank, either turn off the stopcock supplying the make-up tank or lay a piece of wood across the tank and tie the ball valve up to it.

Expansion tank with ball valve tied up

If the system is filled directly from the rising main via a filling loop, it is a simple case of making sure the filling loop valve is closed, then drain the system as described below.

Locate the drain cock for the system, the usual place is in the return pipe near to the boiler, on the boiler itself or under the floor at the lowest point in the system. Connect a hose pipe and secure with a hose clip, put the other end of the hose pipe into an outside drain and open the drain cock. If you don't have a drain cock key, use an adjustable spanner. Open any manual relief valves to allow the water to drain out, until the flow stops. There will still be some water in the radiators, so starting at the highest radiator, slowly open all the radiator bleed valves, this will allow air into the system and allow the water to flow out. Drain cock

As the water flows out of the system check to see what colour it is.

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