Alow-pressure hot-water system is described below. water in the boiler is heated, and consequently becomes less dense than the cold water in the higher parts of the system. the denser cold water sinks to displace the heated water, which is forced to rise. in this way, a circulation of water is set up-the process known as convection. the water rises up the primary flow (pf) into the hot-water storage cylinder, or tank, and is replaced by water descending the primary return (pr). if a temperature difference is maintained between the water in the primary flow and that in the primary return, then the process will continue. this is called the primaty circulation and it is made more effective by connecting the primary flow to the upper region of both the boiler and the cylinder, and the primary return to the lower region of each. if a draw-off point is opened, water will be forced from it because of the height of the water in the feed lank (often this tank is also the building's main cold-water storage cistern). the hot-water supply is drawn from the top of the storage cylinder, where it collects because its density is lower than that of the colder water. heating water causes the dissolved air contained in cold water to be released. consequently, a vent pipe has to be taken from the top of the storage cylinder and carried up above the level of the water in the feed tank. the pipe allows air to escape from the system and, as a result, prevents the formation of air locks. a safety valve, set to operate at a certain pressure, is fitted to the system. also, a draw-off point is fitted at the lowest part of the system so that it can be drained.
