Page 418 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

The City & Guilds Textbook: Plumbing Book 1

Note: The discharge may consist of high temperature water and steam. Asphalt,
roofing felt and other non-metallic rainwater goods may be damaged by very
high temperature hot water discharges.

Termination of the discharge pipework where the storage vessel
is sited below ground level
When storage vessels are sited below ground, such as in a cellar, the removal
of the discharge becomes a problem because it cannot be discharged safely
away from the building. However, with the approval of the local authority and
the vessel manufacturer it may be possible to pump the discharge to a suitable
external point. A constant temperature of 95°C should be allowed for when
designing a suitable pumping arrangement. The pump should include a suitable
switching arrangement installed in conjunction with a discharge collection
vessel made from a material resistant to high temperature water. The vessel
should be carefully sized in line with the predicted discharge rate, and should
include an audible alarm to indicate discharge from either of the pressure or
temperature relief valves is taking place.

3 SYSTEM SAFETY

AND EFFICIENCY

Hot water, by its nature, can be dangerous if:
● the temperature of the water is too high
● the delivery system does not contain a vent pipe to keep the system
at atmospheric pressure
l there are no means to accommodate the expansion of the water due to the
water being heated
● there are no means to relieve excessive pressure and/or temperature.
Because of this, various safety features must be built in to hot water systems to
prevent the water from:

● exceeding 60°C at the point of use
● exceeding 100°C at the point of storage
● over-pressurising the water beyond safe limits.

In this part of the chapter, we will look at the safety features that prevent
excessive pressure and water temperature.

Open vent pipe

Open vented systems contain a vent pipe, which remains open to the atmosphere,
ensuring that the hot water cannot exceed 100°C. The vent pipe acts as a safety
relief outlet should the system become overheated.
Water at atmospheric pressure boils at 100°C. However, once the water is
pressurised, the boiling point temperature rises. The higher the pressure, the
higher the boiling point. So, a system without a vent pipe to maintain 100°C
maximum is classed as an unvented system and this means that the water will
exceed this – often, as we will see later, with catastrophic consequences.

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