Page 331 - 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 5 Cold water systems

low-pressure hot water. For the valve to work correctly, the mains cold water
must have a pressure of at least 1 bar and a maximum pressure of 3 bar.
Pressures in excess of this will require a pressure reducing valve to be fitted.
The venturi mixing valve uses the extra pressure of the cold water supply to
increase the pressure of the hot water supply by using the venturi principle.
The operating principle is as follows.
As the cold water passes through the venturi tube within the valve, its velocity
increases and its pressure is slightly reduced. At this point, the hot water is drawn in
to the cold supply and mixed. As the mixed water leaves the venturi, the pressure
reverts to almost as high as the initial cold supply, giving a fairly powerful shower.

Increase temperature

Temperature control

Hot water inlet Cold water inlet

Thermal shutdown device
Boosted mixed (reduces flow to a trickle
water outlet if mixed water temperature
º
rises above 45 C)

p Figure 5.106 The working principles of a venturi boost shower mixing valve

Pressure-compensating mixing valves
This type of mixing valve gives greater temperature stability compared to
manual mixing valves. Some valves can be used on both high- and low-pressure
systems, while others are specifically designed for high-pressure system use.

Pressure-compensating mixing valves are manufactured with either of the
following two types of control.

1 Sequential control: starting the shower at a low temperature and
progressively turning the control towards hot gradually increases the
temperature and maintains a steady flow rate. When the temperature is
set, a balancing diaphragm reacts to subtle changes in water pressure and
maintains the correct hot/cold mix.
2 Dual control: these have a separate flow control and temperature control
mechanism. The temperature control mechanism consists of a metallic
shuttle that moves backwards and forwards inside a plastic mixing tube. The

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