Heating System Drain Down
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Summary: Draining central heating, draining radiators, flushing central heating, flushing radiators.
These instructions are for an open vented system only. If your system is sealed please find a qualified plumber.
Sometimes it is necessary to drain down a heating system. This may be because you need to introduce an agent to clean it and flush it through, removing sludge etc or simply to change a radiator. In all events it is necessary to turn off the boiler. Turn off the gas or electricity supply to the boiler or in the case of a back boiler or solid fuel, make sure it is out.
In your loft you will have a tank called a feed and expansion tank. Its recognisable by a pipe coming in at the top which is the expansion part of the equation. If the water gets too hot in your system the steam can expand, via the pipe, into this tank.
The water is fed into this tank from the mains via a ball valve. This valve is the same as the one in your toilet cistern ( see our tanks and cisterns project ) When the water rises it lifts the ballcock. The ball is attached to an arm which closes the valve when it is lifted. When the tank is full of water, no more can get in because the valve is closed. When some water is drawn off, the ball drops, the valve opens and water rushes in to replace it.
To ensure no water comes in while you are draining the system down it is an easy job to place a piece of timber across the tank, lift up the valve arm and tie it to the timber. This will close the valve.
Now connect a hosepipe to the drain nozzle (marked outlet on the top image) and run it outside to a suitable point. Make sure, especially in winter, that the water does not run onto the road or pavement where it could freeze and cause accidents. Open up the drain valve and let the system run through.
The water will run faster if you open any bleed valves on the radiators starting with the radiators at the top of the building. As the water level drops you can open the bleed valves in the downstairs radiators also.
When no more water comes out of the hose you must check that the system is finished before you remove radiators or start wiork. There is a chance that some air has got into the system and locked the water from escaping. Go into the loft and fill the tank with about 6 inches of water by loosening the arm you have tied up. This should, in a few seconds, start running out of the hose. If it does not you have an air lock and should connect the other end of the hose to the cold tap and send a short blast of water back into the radiator you are draining from. Make absolutely sure the hose is well attached to the radiator drain nozzle.
Refilling the System
Close the draincock on the radiator and all the bleed valves that you have opened. Untie the string in the feed tank and let the water fill up the system via the tank. Wait until the tank stops filling and go downstairs to bleed the bottom level radiators. When they have been bled, repeat with the top radiators upstairs. This should ensure your system is filled.
Double check the work you have done to ensure you have retightened all nuts etc and turn on your power supply and relight the boiler. As the water heats up you may hear some knocking sounds as any air expands in the system. The radiators will probably need bleeding again once the system is fully heated.
