Page 399 - 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

● Over-sink heaters: as the name suggests, these are fitted over an appliance such
as a sink. The water is delivered from a spout on the heater. A common complaint INDUSTRY TIP
with this type of heater is that they constantly drip water from the spout. Most localised gas
This is normal as the heater must be open to the atmosphere at all times to instantaneous water heaters do
accommodate the expansion when the water is heated. The dripping water not contain a flue. The gases
is the expansion taking place and will stop once the heater has reached its from the appliance simply
operating temperature. disperse in the room where
they are installed. They are
known as ‘flueless appliances’.

Insulation
Open vent to allow
for expansion
Spout of the tap
acts as an open
Heater element vent allowing
expansion of
water to take place

Thermostat

Mains cold Hot water supply
water supply from the heater
to the tap to the tap
Swivel spout Cold water feed
with spreader feed
Water inlet
control valve Cold water
supply from
the tap to the
water heater

Under-sink
p Figure 6.33 A typical over-sink storage water heater water heater
● Under-sink heaters: the under-sink heater works in exactly the same
way as the over-sink heater. The main difference is that these heaters p Figure 6.34 A typical under-sink
usually require a special tap or mixer tap that permits the outlet to be storage water heater
open to the atmosphere at all times to allow for expansion. The inlet INDUSTRY TIP
of water to the heater is still controlled from the tap. Typical capacities
are up to 15 litres.
What is a ‘dead leg’?
Secondary circulation When a hot tap is opened, a
certain amount of cold water is
Secondary circulation is necessary to prevent the wastage of water due to the usually drawn off and allowed
excessive lengths of hot water draw-off from the storage vessel to the outlet. to run to drain before hot
Here, we will look at the various methods of providing secondary circulation water arrives at the tap. This
in hot water systems, including larger domestic systems. wasted, cold water is known
as a dead leg. Under the Water
Secondary circulation in domestic dwellings Regulations, dead legs must
Secondary circulation is required where the length of any draw-off pipework be restricted to the lengths
is excessive. British Standard BS EN 806 (and BS 6700) and the Water Supply given in Table 6.1a. If this is
(Water Fittings) Regulations give the maximum length a hot water draw-off not possible, then secondary
pipe may travel without a secondary circulation system being installed. These circulation is required. Dead
lengths are reproduced in Table 6.1a of this chapter. legs are a potential source of
Legionella and noise.

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