Page 573 - 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 9 Sanitation systems

Flush pipe

50mm

195 mm

p Figure 9.15 A wash down WC pan

● The siphonic type: the flushing operation creates a vacuum, which
contributes to clearing the pan. There are two pan types:
1 The single trap siphonic WC pan (or ‘Malvern type’): this pan has a lower
outlet than other pan designs. It is usually installed only on replacements
as the design tends to look very outdated. They work by restricting the
flow of water from the cistern, which allows a build-up of water in the
pan, which is then forced through the restricted neck of the trap creating a
vacuum behind it and clearing the pan contents completely.
2 The double trap siphonic WC pan: very rarely sold in the UK since the
flushing volume of WC cisterns was reduced to six litres by the Water
Supply (Water Fittings) Regulations 1999. This kind of WC pan is very INDUSTRY TIP
quiet and extremely efficient at removing the pan contents. Unlike the
single trap siphonic pan, the double trap siphonic has an unrestricted Access the Water Supply
(Water Fittings) Regulations
outlet and two water traps. A special pressure reducing valve, called an 1999 at: www.legislation.gov.
aspirator (or bomb), is fitted to the bottom of the siphon. uk/uksi/1999/1148/contents/
When the cistern is flushed, a negative pressure is caused in the chamber made
between the two traps by the aspirator. The aspirator follows Bernoulli’s
principle (see page 381). It sucks out the air from the chamber as the
water from the flush passes through it, which causes the contents of
the bowl to be sucked through the two traps. The aspirator holds a little
water back to refill the second trap after the flush is complete.
Double trap siphonic WCs tend to be longer than wash down types
because of the extra water trap.

561

9781510416482.indb 561 29/03/19 9:05 PM