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turn. Water is stored at 65 °C, and when fully charged the thermostat closes the
motorized valve on the primary return. This «off» signal may also be linked
into the pump and boiler control scheme to complete the shut-down when the
central heating controls are satisfied.
Hot-water pipes are insulated with a minimum of 25 mm of insulation, as
are tanks exposed to frost. The primary system feed and expansion tank has a
minimum capacity of 501, and the cold-water storage tank has a capacity of at
least 2301.
Hot-water storage requirements at 65 °C are as follows: office, 51 per
person; dwelling, 301 per person; hotel and sports pavilion, 351 per person
(CIBSE, 1986).
Materials for water services
Corrosion protection is provided by ensuring that incompatible materials
are not mixed in the same pipework system, by recirculating the water in central
heating systems to reduce fresh oxygen intake, and by adding inhibiting chemi-
cals to the water. Hot and cold-water service systems are continually flushed
with fresh water, making it necessary to use galvanized metal, copper or
stainless steel.
Copper and galvanized steel should not be used in the same system be-
cause electrolytic action will remove the internal zinc coating and lead to fail-
ure. A galvanized cold-water storage tank can be successfully used with copper
pipework as the low temperature in this region limits electrolytic action. Heat
accelerates all corrosion activity.
Black mild steel is used in recirculatory heating systems, and an initial
layer of mill, which is metal oxide scale formed during the high-temperature
working of the steel during its manufacture, helps to slow further corrosion.
Discoloration of the central heating water from rust to black during use shows
steady corrosion. A black metallic sludge forms at low points after some years.
Large hot-water and steam systems have the mill scale chemically removed
during commissioning and corrosion inhibiting chemicals are mixed with the
water to maintain cleanliness and avoid further deterioration.
The formation of methane gas in heating systems during the first year of
use is due to early rapid corrosion, and radiators need frequent venting to main-
tain water levels. Proprietary inhibitors should be added to all central heating
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