Draining Heating Systems
If you have a conventional indirect hot water system – perhaps installed in conjunction with a central heating system – you can drain the primary circuit, together with the radiator circuit if there is one, without draining the water from the outer part of the storage cylinder.
Because of the increased risk of corrosion that arises from water and air coming into contact with steel surfaces, a radiator circuit should be drained only when absolutely essential. When this has to be done – to add additional radiators, perhaps – you should tie up the ball-valve serving the feed-and-expansion tank and drain from both the drain-valve beside the boiler and from any drain-valves provided at low points of the system. You must, of course, let out or switch off the boiler before attempting this.
When refilling the primary circuit (or when refilling a direct system with boiler) it may help to prevent the formation of air – locks if you connect one end of your garden hose to the boiler drain-valve and the other end to the cold tap over the kitchen sink. Open them both up and the system will fill upwards, with air being driven out in front of the rising water.
As the central heating circuit refills, open up all the radiator vents – and any other air vents that there may be in the system – and leave them open until water begins to flow through them. It is a good idea, when refilling a central heating system, to introduce a reliable corrosion-proofer into the feed- and expansion tank to prevent future internal corrosion, but you can do this only if you fill the system from the top, not from the bottom.
Winter precautions
One final point: if you are leaving your home empty during the winter months, you should drain the main cold water storage cistern and, if you have a direct hot water system and will be away for more than two or three days, you should drain the hot cylinder, the boiler and its circulation pipes as well. Human memory is fallible. Leave a conspicuous notice on the boiler and by the immersion heater switch saying:
“SYSTEM DRAINED – DO NOT LIGHT BOILER OR SWITCH ON HEATER UNTIL IT HAS BEEN REFILLED”.
Because of the risk of corrosion already referred to, the primary circuit and any central heating system connected to it should not be drained in these circumstances. If you have a central heating system that is capable of automatic control, leave it switched on under the control of a frost-stat, This is a thermostatic control, usually positioned in a garage or in the roof space, that will bring the heating into operation when a predetermined, near-freezing-point temperature, is reached.
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