DESIGN: DEAR MR PLUMBER
57
Can you smell it?
We often receive complaints from projects about unpleasant smells in the building. The three
pillars of the National Building Regulations (NBR) are health, safety, and economy. SANS
10400-P (P2) describes how the drainage system must be designed and how it must perform.
By Vollie Brink, Pr Eng
Health is extremely important. The system must not
endanger the health of the occupants inside the
building, while the drainage system’s contents carry
potentially dangerous human waste and foul air, which
are high health risks.
The major challenge is to keep the foul air in the piping and
that is done with water traps. The piping must be tested
to be both watertight and airtight and it is why we have
ventilation pipes to extract the foul air from the piping and
vent it into the atmosphere above the roof of the building.
The ventilation piping does not only allow the foul air to
escape out of the drainage system, but it must also allow
fresh air to enter into the piping to keep the pressure in
the piping constant and equal to the air pressure outside
the piping. This is why some people use a two-way vent
valve instead of an open vent pipe. The ideal ratio of air and
effluent in a stack pipe is approximately 75% air and 25%
effluent, and in horizontal piping it is 50% air and 50%
effluent to ‘balance’ the pressure in the system.
As the effluent drops down in a stack pipe, it can cause
positive and negative pressures, which can disturb the
water slots in the fixtures and either ‘suck’ it in or ‘push’ it
from the fixtures. This is why the ventilation in the piping is
so important to keep the water slots intact and to keep the
unhealthy air out of the building. The appropriate ventilation
of a sanitary drainage system is crucial to keep the building
healthy and safe for occupants. However, people often
complain about foul smells in buildings and then you and I
are called upon to solve the problem.
The question is how to go about finding the foul air leakage.
Perhaps the following will help to seek the source of the leak:
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My experience is that in most cases, the source
is a floor drain of which the ‘odour trap’ has been
removed by the cleaner and not put back in position.
We have often found that where there is a grease trap
in a restaurant, they clean the grease trap, remove
the odour trap from the floor trap, chuck the contents
into the trap, and do not put the odour trap back.
This is illegal. The contents of a grease trap must
be removed from the site in an approved manner;
it is illegal to discharge it on site and in the piping
elsewhere from the grease trap. We have found that
with smoke tests, the grease traps have serious leaks.
www.plumbingafrica.co.za
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Another culprit is the vent valve where a two-way
valve is installed inside the building and occasionally
discharges the foul air — sometimes even into the
air-conditioning ducting, spreading it through the
building. This is a widespread problem. A two-way
valve shall not be installed inside a building or even
outside under a window, which is also common and
in contravention of the NBR.
Many architects do not want vent pipes to protrude from
the roof, and then the vent pipes inside the building are
connected to a common horizontal collector vent pipe,
of which the size is not big enough to allow enough air
Vollie Brink is one of the
in the system. This can cause a problem.
industry’s longest serving
Manholes are not allowed inside a building, and an air
wet service engineers.
leak from these manholes can cause serious health
He continues to serve on
problems and odours. A sump pump inside a building is
SABS committees and
also a source of smells; even a lift sump pump causes it.
has been involved in the
A crack in the piping or a loose pipe joint can cause
air leaks.
Green Building Council
The metal-type bottle trap on a urinal is a potential
Star rating system.
source of foul-air leaks. These metal bottle traps have
Brink continues to consult
a metal ‘skirting’ inside it to form the trap and the
for various organisations
urine then corrodes it and it leaks. These metal ‘skirts’
while enjoying a well-
must be replaced with non-metal ‘skirts’.
earned retirement.
The connection between an air-conditioning condensate
pipe and a drain pipe is also in many cases the source
of bad odours.
Pipe joints can leak and crack in the piping.
The position from where a vent pipe discharges is important: it must be above the
highest window or opening and “above the roof level”, as per SANS 10400-P.
The vent piping must not be near an air-conditioning intake.
The negative and positive air pressures in the drainage piping can also cause the water
traps in the fixtures to be ‘sucked in’ or ‘pushed out’.
A vent pipe must always rise and not drop and run horizontally and then rise again.
The depth of a water trap shall not be less than 75mm as per SANS 10400-P.
The vent piping must be airtight to prevent leaks.
When a smoke test is conducted, all the vent pipes must be blocked and all the traps
must be full of water. It is better to test sections and not a large system.
Always remember that the ‘bad smells’ are pathogens/germs and pose a health risk. An
open vent pipe is preferable to a vent valve, as vent valves are never serviced and the
spring-loaded valves are never replaced. In some cases, it is the source of bad smells.
Floor drains that do not have a waste pipe discharging into it dries up and causes
bad smells if it is not regularly filled with water.
All covers to be properly closed and tight.
Vollie Brink
If you have experience in how to check for and locate the source of bad smells, we can
add it to this list and share it with others in the plumbing business. PA
August 2018 Volume 24 I Number 6