Page 597 - 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

Atmospheric pressure Water flowing from the appliance
INDUSTRY TIP

Connecting two appliances
on a single waste pipe often
leads to incorrect installation
on the primary ventilated
stack, and is regularly the
Water sucked
out of trap reason that baths pull the
trap of the washbasin in many
Negative pressure zone bathroom installations. This
'Plug' of flowing water will always become a problem
unless the pipe size leading
to the final branch connection
is increased to 50 mm (see
p Figure 9.80 Induced siphonage Figure 9.81).

32 mm
50 mm 40 mm

To prevent induced siphonage on a multiple appliance installation from a single waste
pipe connected to a primary ventilation system, the waste pipe must increase in size to
50 mm as shown before entering the soil stack

p Figure 9.81 Multiple appliance installations

Compression

When water is discharged from a WC at first-floor level, it falls rapidly to the
base of the stack. If the bend at the base of the stack has a tight radius, the
water momentarily stops flowing, causing the water to back up, which creates a
back pressure of air. The back pressure travels up the stack and moves through
ground-floor waste pipes, eventually blowing the water out of the traps.
The use of large-radius bends, or two 45° bends, at the base of the stack
(see page 552) prevents this from happening by allowing the easy flow of water
from the soil stack to the drain, allowing the water to maintain its forward
motion and velocity.

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