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Choosing the method by which hot water is supplied is often pre-determined by the choice of central heating system; which is ironic as heating systems in the UK are used on average for less than five months of the year. In addition the amount of heating required can be pre-determined, i.e. 18 degrees centigrade in the bedrooms, 21 degrees in the living room and so on.
Both of the boiler systems above are called "vented" systems. Because of this vent pie, they are open to atmospheric pressure and operate as "low pressure" systems. They both call for cold water from a cold tank stored, generally, in the roof space. Because they are low pressure, sometimes the flow from the taps etc is not as great as one might like and pumps can be introduced, both for the domestic hot water and the heating, to give greater flow.
. The principles of heating are the same, but because everything is under the pressure of the mains water, flow rates are much better...Many safety devices are built into this system to accommodate for the greater pressure and expansion of the water. Although a small tank may be found in the loft for venting and feeding your central heating, No cold water storage tank is necessary.