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Cold-water storage to cover a 24 h interruption of supply (CIBSE, 1986)
ranges from 451/person for offices to 901/person for dwellings and
1351/person for hotels.
Hot-water services
Hot water can either be generated by the central boiler plant and stored, or
produced close to the point of use by a more expensive fuel.
Central hot-water storage
The low-cost fuel used for the central heating plant is also used for the
hot-water services boiler. This is located within the main boiler house and a
large-volume storage cylinder is employed. A small power input boiler is run
almost continuously, winter and summer, under thermostatic control from the
stored hot water. Primary circulate pipes are kept short and well insulated.
This system can meet sudden large demands for hot water. Secondary cir-
culation pipes distribute hot water to sanitary appliances. A pump is fitted in the
seconds return; its function is to circulate hot water when the taps are shut and
it does not appreciably assist draw-off rates from taps. Connections from the
secondary flow the tap are known as dead-legs and are limited to 5 m of 15 mm
diameter pipe. This minimizes wastage of cold water in the non-circulating
pipework when running a tap and waiting for hot water to arrive.
Decentralized system
The decentralized system is mainly for small hot-water service loads dis-
tributed over a large building or site where it would be uneconomic to use a
central storage cylinder and extensive secondary pipework. Electricity or gas
can be used in small storage or instantaneous water heaters located at the point
of use. They are connected directly to the water main.
Electric instantaneous heaters have power consumptions of up to 6 kW
and produce water at 40 °C and up to 31/mm at 100% efficiency. Immersion
heaters are controlled by time switches and thermostats and are connected in
3 kW stages.
The indirect hot-water system
This is the basic layout of the combined central heating and indirect hot-
water service system. The cylinder is insulated with 75 mm fibre glass and
should have a thermostat attached to its surface at the level of the primary re-
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