Page 425 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 6 Hot water systems

l Open vented double feed indirect cylinders with gravity or pumped
primary circulation: must be fitted with a minimum of a cylinder
thermostat and a motorised zone valve, which closes when the water in
the cylinder reaches a pre-set level.
● Open vented cylinders with no high-limit thermostat: can be fitted with a
temperature relief valve that opens automatically at a specified temperature
to discharge water via a tundish and discharge pipework safely to outside the
property.

The use of thermostatic mixing valves
As we have already seen, the maximum temperature of hot water in a dwelling
should not exceed 60°C but this is far too hot for bathing and showering. Water
with a temperature as low as 51.66°C can cause serious burns to a child if it is
exposed to the skin for two minutes or more.

In April 2013, new legislation under Building Regulation Document G required
that all new-build properties and renovations have temperature control to
baths not exceeding 48°C and all hot water storage cylinders where the stored
water may exceed 80°C (usually solid fuel-heated cylinders). All properties to
which the public have access, such as schools, hospitals, nursing homes and
so on, under the Care Standards Act 2000, require that the temperature of
water delivered to all hot outlets, except where food preparation is carried out,
be limited to 43°C. This is done by the use of thermostatic mixing valves for
appliances and in-line blending valves for storage cylinders.
A thermostatic mixing valve mixes hot and cold water together and supplies it
to an appliance at exactly the correct temperature. They use a temperature-
sensitive element, usually a wax cartridge, that expands and contracts to
maintain a specific temperature based on the temperatures of the hot and cold
water entering the valve. The length of pipe from the mixing valve to the taps
should be kept as short as possible.

The insulation of hot water pipework
When installing new hot water installations in domestic properties, pipes should
be wrapped with thermal insulation that complies with the Domestic Heating
Compliance Guide. There are four main considerations:
1 Primary circulation pipes for heating and hot water circuits should be
insulated wherever they pass outside the heated living space, such as below
ventilated suspended timber floors and unheated roof spaces. This is for
protection against freezing.
2 Primary circulation pipes for domestic hot water circuits should be insulated
throughout their entire length, except where they pass through floorboards,
joists and other structural obstructions.
3 All pipes connected to hot water vessels, including the vent pipe, should be
insulated for at least 1 m from their points of connection to the cylinder, or
at least up to the point where they become concealed.
4 If secondary circulation, such as a pumped circuit feeding bath and basin taps
in a large property, is installed, all pipes fed with hot water should be insulated
to prevent excessive heat loss through the secondary circulation circuit.
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