Page 419 - 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

The vent pipe will also assist with the expansion of water by allowing the
expanded water to rise within the pipe. The vent pipe must be sited over the
cold feed cistern in the roof space.

Temperature relief valve

In an unvented hot water storage system, where there is a risk of the water
temperature exceeding 100°C, a temperature relief valve must be fitted as
part of the three-tier level of safety. To evacuate the hot water away from the
building in the event of the water reaching 95°C, the temperature relief valve
must be connected to the discharge pipework.

The discharge pipework

This pipework is connected to both the temperature and pressure relief valves
via a tundish. It is designed to evacuate any discharged water quickly and safely
away from the building to a drain.

Thermostatic mixing valve

(TMV2 and TMV3)

The object of any hot water storage system is to store water at the relatively high
temperature of 60°C to ensure that it is free from any bacteria, to distribute the
water at 55°C and yet to deliver the water at the hot water outlets at the relatively
low temperature of 35°C to 46°C, to ensure the safety of the end user. The most
efficient way to do this is by the use of thermostatic mixing valves (TMVs).
TMVs (sometimes known as a thermostatic blending valves) are designed to
mix hot and cold water to a predetermined temperature, to ensure that the
water is delivered to the outlet at a temperature that will not cause injury but
is hot enough to facilitate good personal hygiene. There are three methods of
installing TMVs, as described below.

Single valve installations

This is probably the most common of all TMV installations. The maximum pipe
length to a single appliance is 2 m from the TMV to the outlet. Back-to-back
installations are acceptable from a single valve provided that the use of one
appliance does not affect the other, and that both appliances have a similar
flow rate requirement, e.g. two washbasins. Typical installations are those
listed below.
● Baths: it is now a requirement of Building Regulations Approved Document
G3 that all bath installations in new and refurbished properties incorporate
the use of a TMV. This would normally be set to a temperature of between
41°C and 44°C, depending on personal comfort levels. Temperatures above
this can be used only in exceptional circumstances.
● Showers: these installations usually require a temperature of not more
than 43°C. In residential care homes and other medical facilities, a
temperature of not more than 41°C should be used according to NHS
guidelines.

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