The domestic hot water supply system consists of a boiler, a hot water storage tank and a cold water supply tank interconnected by pipes arranged as below left. A convection current of hot water from the boiler rises up the flow pipe A while cold water descends to the boiler through the return pipe B where it becomes heated in turn.
The hot water tank gradually becomes filled with hot water from the top downwards. It is important to notice that the flow pipe A leaves the boiler at the top and enters the top of the top tank, while the return pipe B connects the bottom of the hot tank to the bottom of the boiler.
Hot water for use in the kitchen and bathroom is taken from a pipe leading from the top of the hot tank. When hot water is run off an equal volume of water from the cold supply tank enters the hot storage tank at the bottom through the pipe C. The whole system is thus kept constantly full of water and no air can enter. The level in the cold tank is maintained by a supply from the mains which enters through a ball cock.
An expansion pipe rises from the top of the hot tank and is bent over at right angles to lead into the cold tan. This is a safety feature. Fire causing water to boil in the hot tank results in some of this flowing harmlessly into the cold tank. Dissolved air is also allowed to escape from the water as it is heated. This could otherwise cause air locks and may interrupt the water circulation.
The diagram above right illustrates the convection currents on the left. To begin the water in the lower flask is coloured with blue ink while the tubes and upper vessel are full of clear water. When the flask is heated, a layer of hot coloured water collects at the top of the upper vessel. Eventually the upper vessel is full of coloured water, and the water start descending through the cold tube.