LEARNING CONTENT
In Lesson 7, building hot and cold water supply systems were described with pipes to provide sufficient running
water to all of the fixtures throughout a building or project. Following the flow of the water through the plumbing
system, the next step will be to design a system to dispose of the sanitary waste and wastewater.
Wastewater, sometimes referred to as sewage, is used water. It comes from almost all sections of the building,
including bathrooms, kitchens, and laundry areas, and in commercial projects, equipment being serviced.
Because organic waste in wastewater tends to decompose quickly, one of the primary objectives of the sanitary
drainage system is to dispose of decaying wastes rapidly, before they cause objectionable odors or become
hazardous to health.
A. SANITARY DRAINAGE SYSTEM
Conventional Sanitary Drainage and Vent System
A sanitary drainage and vent system, sometimes referred to as the drain, waste, and vent (DWV) system,
is a network of pipes that remove wastewater from a building. In this section the terminology and function of each
of the parts are explained, but first a discussion of system operation is needed. (See Figure 14.1.)
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In typical plumbing system operation, the sanitary drainage side of the system consists of traps at each fixture,
and fixture branch, stack, and drain pipes that carry wastewater away from the plumbing fixtures and out of the
building. Water transports wastes into the sanitary drainage piping and out of the building sewer line, leading to
a community wastewater treatment plant or to a private sewage treatment system. Gravity is the driving force
behind wastewater flow, so the sanitary drainage system is known as a gravity system.
The vent system side of the system introduces and circulates air in the system to maintain atmospheric pressure
in the drain lines and ensure adequate gravity flow of wastewater. Venting prevents a negative pressure (suction)
in the system that could suck water from fixture traps and allow sewer gases to infiltrate the building. The vent
system also exhausts sewer gases to the outdoors.
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The chief components of a sanitary drainage and vent system
are described as follows:
Traps
A trap is a U-shaped pipe that catches and holds a small
quantity of wastewater that is poured down a fixture drain.
The trapped water prevents gases resulting from wastewater
decomposition from entering the building through the drain
pipes and the fixture. (See Figure 14.2.) Traps are made of
copper, plastic, steel, wrought iron, or brass, with plastic most
commonly used. The most acceptable type of trap is called a
P-trap (see Figure 14.3; see also Photos 14.1 and 14.2). Straps and U-traps can easily be siphoned, so they are
prohibited by the building code. An integral trap is built in as
part of the fixture. The integral trap in a vitreous china water
closet is cast as part of the fixture.
In most instances, a trap is installed immediately downstream
of the fixture, as close to the fixture as possible, usually within
2 ft (0.6 m) of it, unless the fixture is designed with an integral
trap (e.g., a water closet). Because the trap may occasionally
need to be cleaned, access is necessary. A removable plug
in the trap may provide access or the trap may have screwed
connections on each end for easy removal.
In locations where a fixture is infrequently used, water in the
trap may evaporate and, with the water seal not working,
gases may back up from the sewer and drainage pipes
through the fixture and into the building. Floor drains, which
are used to take away the water after washing floors or which
may be used only in case of equipment malfunctions or
repairs, present the most serious possibility of losing their
water seal. When floor drains are connected to the drainage
system, the possibility of a serious gas problem exists. The
designer of the system can avoid such a situation by not tying
the floor drain into the drainage system. Instead, the floor drains could be tied into a drywell, from which there
will be no gases.
The water seal in a trap may be broken if there is a great deal of vacuum pressure in the pipes. A vent system
is attached to the sanitary drainage system to reduce the vacuum and to equalize the pressure throughout the
system.
Historically, a building trap was located at the end of the building drain (inside the building and just before it
connected to the sewer line). It was theorized that this trap would act as a seal to keep gases from entering the
building’s sanitary drainage system from the sewer line. On the other hand, a building trap may impede the flow
of wastes in the system. For this reason, codes disallow use of a building trap except in special installations.
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Interceptors
Many substances (e.g., grease, fat, oil, hair, sand, clay, wax, or debris) are accidentally or intentionally placed
into a building drain, potentially creating blockages that can cause backups and overflows, or contaminating
wastewater, which makes treatment difficult and more costly. Interceptors are passive devices designed into a
plumbing system that trap, separate, and retain these toxic or undesirable substances from wastewater before
it is discharged into the sewer line.
Grease can solidify and coat the inner walls of pipes, creating a stoppage. Restaurants, cafeterias, and other
commercial food establishments with cooking facilities must have a grease interceptor or grease trap that
receives wastewater from sources such as sinks, dishwashers, floor drains, and washing area drains before
draining to the municipal sewer system. Manufacturing plants, vehicle service facilities, car washes, and other
similar establishments must have an oil-sand interceptor to separate and remove floatable material (oils) and
settleable materials (sands and metals) from wastewater before it is discharged to the municipal sewer system.
Barber shops, beauty salons, pet grooming facilities, and any other establishments that discharge hair and/or
other fibrous materials in wastewater must have a hair interceptor.
Typically, interceptors are sized for at least a 30-min peak wastewater flow detention time from all contributory
sources. A grease interceptor for a restaurant can have a capacity of 1000 gal (3800 L) or more. Interceptors
should be located as close to the discharging fixture as possible; they sometimes also serve as the trap, with
some exceptions. Grease traps must be located at least 10 ft (3 m) from hot water faucets. All hot water must
cool to 120°F before entering the grease trap.
An interceptor must be readily accessible for periodic cleaning, inspection, and testing. Wastes captured in an
interceptor must be disposed of following health standards. Precious metals (e.g., from polishing jewelry in
manufacturing plants) can be recovered.
Fixture Branches
Each plumbing fixture is connected horizontally to the sanitary drainage system by a drain line called a fixture
branch. Beginning with the fixture farthest from the stack, the branch must slope 1/8 to ½ in per ft (10.4 to 41.6
mm per meter) for proper flow of wastes through the branch. Branch piping, which serves urinals, water closets,
showers, or tubs, is usually run under the floor. When these fixtures are not on the branch, the piping may be
run in the floor or in the wall behind the fixtures. Branch piping may be copper, approved plastic, galvanized
steel, or cast iron. (See Photos 14.3 through 14.10.)
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Stacks
The fixture branches feed into a vertical pipe referred to as a stack. When the wastewater that the stack will
carry includes human waste from water closets (or from fixtures that have similar functions), the stack is referred
to as a soil stack. When the stack will carry all wastes except human waste, it is referred to as a waste stack.
Soil and waste stacks may be copper, plastic, galvanized steel, or cast iron. These stacks service the fixture
branches beginning at the top branch and go vertically downward to the building drain.
In larger buildings, the point where the stack ties into the building drain rests on a masonry pier or steel post so
that the downward pressure of the wastes will not cause the piping system to sag. In addition, the stack must be
supported at 10-ft (3-m) intervals to limit movement of the pipe. When a stack length is greater than 80 ft (24.4
m), horizontal offsets are used to reduce free-fall velocity and air turbulence. Connections to fixture branches
and the building drain should be angled 45° or more to allow the smooth flow of wastewater.
Most designers try to layout plumbing fixtures to line up vertically floor after floor so that a minimum number of
stacks will be required. Many times, a central core of a multistory building will be used as a plumbing core, and
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a pipe chase, a space that is left to put the pipes in, runs
from the first floor to the roof of the building. (See Figure
14.4.)
Building Drains
The soil or waste stacks feed into a main horizontal pipe
referred to as the building drain. By definition, the
building drain extends to a point 2 to 5 ft (0.6 to 1.5 m)
outside the foundation wall of the building. The building
drain slopes 1/16 to ½ in per foot (5.2 to 41.6 mm per
meter) as it feeds the wastewater into the building sewer
outside the building. Slopes of 1/8 to ¼ in per foot (10.4
to 20.8 mm per meter) are common in most buildings.
Location of the building drain in the building depends
primarily on the elevation of the community sewer line.
Ideally, all of the plumbing wastes of the building will flow
into the sewer (whether it is a community or a private
sanitary system) by gravity. The drain is typically placed
below the first floor or below the basement floor. If the
height of the sewer requires the drain to be placed above
the lowest fixtures, it will be necessary for the low fixtures
to drain into a sump pit. When the level in the sump pit
rises to a certain point, an automatic float or control will
activate a pump that raises the wastewater out of the pit
and into the building drain.
Building drains are usually made of approved plastics,
copper (for above the floor), or extra-heavy cast iron (for
below the floor) pipe.
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Building Sewer
The building sewer is an extension of the building drain that carries wastewater from the building drain to a
community sanitary sewer main or an individual on-site sewage treatment (OSST) system. In community sanitary
wastewater systems, the building sewer may also be known as a house or building connection, or sanitary
sewer lateral. The building sewer can have slopes that range from 1/16 to ½ in per foot (5.2 to 41.6 mm per
meter). The extremely shallow slope of 1/16 per foot (5.2 mm per meter) is only common in large buildings
serving hundreds of fixtures.
Sanitary Sewer Main
The sanitary sewer main is a pipe through which the wastewater flows as it is conveyed from a building to the
wastewater treatment plant. Typically, the minimum size of a community sanitary sewer main for a gravity-based
system should be 8 in (200) mm in diameter.
Cleanouts
Provisions must be made to allow cleaning of the sanitary drainage system. Cleanouts are screw-type fittings
with a cap that can unscrew to allow access to the inside of the sanitary drain pipes. A cleanout should not have
a plumbing fixture installed in it or be used as a floor drain. Floor cleanouts (FCO) are found in horizontally
positioned building drain or sewer lines that are installed in the floor or in the ground. Wall cleanouts (WCO)
are placed in vertically positioned stacks. All cleanouts in vertical stacks should be located no higher than 48 in
(1.2 m) above the floor.
Cleanouts are generally required:
•
•
•
•
At the base of soil and waste stacks
At the upper end of building drains
At each change of direction of the horizontal
building drainage system greater than 60°; the
total of the fittings between cleanouts shall not
exceed 120°
At the junction between the building drain and
building sewer (usually 2 to 4 ft away from the
building foundation)
In addition:
•
•
•
Cleanouts should be no more than 50 ft apart,
including the developed length of the cleanout
pipe, in horizontal drainage lines of 4 in or less
size.
Cleanouts should be no more than 100 ft apart,
including the developed length of the cleanout
pipe, in horizontal drainage lines of sizes over 4
to 10 in.
Cleanouts should not be more than 150 ft apart,
including the developed length of the cleanout
pipe, in horizontal drainage lines exceeding sizes
of 10 in.
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Cleanout size is related to pipe size: 1½ and 2 in diameter pipe have a 1½ in cleanout; 2½ and 3 in diameter
pipe have a 2½ in cleanout; and 4 in diameter and larger pipe have a 3½ in cleanout. (See Photos 14.11 and
14.12.)
Venting
Vents are pipes that introduce sufficient air into the
drainage system to reduce air turbulence (from siphoning
or back pressure) and to release sewer gases to the
outside. (See Photo 14.13.) The prime purpose of
venting is to protect the trap seal. If traps did not exist in
a drainage system, a venting could be eliminated.
Without a vent, as water drains from a fixture, the moving
wastewater tends to siphon water from the trap of
another fixture as it falls through the drain pipes. As a
result, vents must serve the various fixtures, or groups of
fixtures, as well as the rest of the drainage system. Vent
piping may be copper, plastic, cast iron, or steel.
Types of venting methods are as follows:
1. Individual Vents
The individual venting technique is defined as the installation of a vent pipe for every trap or trapped fixture. It
is the easiest method of ensuring the preservation of a trap seal but the most costly because of the number of
vent pipes required in the venting system. An individual vent must be located in close proximity to the trap to
properly vent it.
A more effective way of reducing the cost of venting has been in the combining of vents into a system. This
would include common venting, circuit venting, wet venting, combination drain and vent, waste stack venting,
and single stack systems.
2. Common Vents
The common venting method serves two fixtures located on the same floor; it is essentially an individual vent
that serves no more than two traps or trapped fixtures. This type of vent must be located close to the traps it
vents to properly vent it. When the fixture connects at different levels, the drainage pipe between the two traps
must be increased to compensate for the combined water and airflow.
3. Wet Vents
The wet venting method uses a single vent pipe to provide venting for all of the fixtures of one or two bathroom
groups (e.g., a water closet, lavatory, shower, bathtub, and bidet) that are located on the same floor. The vent
pipe for the lavatory typically serves as the vent for the other fixtures in the bathroom. Plumbing codes used to
require the water closet to be the last fixture in line on a wet vent system. However, recent tests provided
evidence that the order of the fixtures does not influence the overall performance of the wet vent system. The
most recent standard permits the fixtures to be located in any order when connecting to the system.
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4. Circuit Vents
A circuit venting system is a horizontal venting pipe
serving up to eight fixtures. Each fixture must be
connected to a single horizontal drain in this technique.
The vent connection is made between the two upstream
fixtures—that is, those fixtures connected to the
horizontal drain pipe that are the farthest away from the
vent stack. In this system, all of connections and the
main piping must remain in the horizontal orientation.
Vertical drops are generally not permitted.
5. Combination Drain and Vent
A combination drain and vent system allows the
distance from trap to vent to be extended infinitely,
provided the drain stays in the horizontal orientation and
there is a vent somewhere within the horizontal branch.
It is based on oversizing the horizontal drain, so there is
an increased likelihood of stoppage in the drain line. This
is the most popular method of venting a floor drain or
venting island fixtures. A combination drain and vent is
a marginally effective venting method.
6. Relief Vent
A relief vent is a continuous pipe of lesser or equal
diameter running parallel and alongside the soil and
waste stack in a multistory plumbing system. It is used
to equalize air pressure within the stack.
Vent stack configurations are shown in Figures 14.5
through 14.9. Codes limit the distance between the trap
outlet and the vent to ensure proper venting. These
distances depend on the venting technique and size of
the drain and lines.
A vent stack extends vertically through the building and
up through the roof to the exterior of the building. Vents
from a fixture or group of fixtures ties in with the main
vent stack, which extends to the exterior. It must extend
beyond the roof at least 6 in (152 mm) and terminate to
open air well beyond attic vents, windows, doors, or intake air vents. A vent stack is used in multistory buildings
where a pipe is required to provide the flow of air throughout the drainage system. The vent stack can also begin
at the soil or waste pipe, just below the lowest horizontal connection, and may go through the roof or connect
back into the soil or waste pipe not less than 6 in (150 mm) above the top of the highest fixture. See Figure
14.10.
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Air Admittance Valves
An air admittance valve (AAV) is a pressure-activated,
one-way mechanical venting port used to eliminate the
need for expensive venting and roof penetrations (See
Photo 14.14.) Wastewater discharges cause the AAV to
open, allowing air to circulate in the vent system. When
there is no discharge, the valve remains closed,
preventing the escape of sewer gas and maintaining the
trap seal. Individual or branch-type air admittance valves
may be used for venting individual, branch, and circuited
fixtures. AAVs are not permitted for venting combination
drain and vent systems and wet vented systems. AAVs
are typically made from polyvinyl chloride (PVC) plastic
materials with ethylene propylene diene monomer
(EPDM) rubber valve diaphragms. Valves come in two
sizes: one for fixture venting and a larger size for system
venting. The valves fit standard diameter pipes, ranging
from 1¼ to 4 in. Screening protects the valves from
foreign objects and vermin. Using AAVs can significantly
reduce the amount of venting materials needed for a
plumbing system, increase plumbing labor efficiency,
allow greater flexibility in the layout of fixtures, and
reduce long-term maintenance problems where conventional vents penetrate the roof surface.
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Positive Air Pressure Attenuator
A positive air pressure attenuator (PAPA) is a product
developed to protect buildings of 10 or more stories
against the unwanted positive pressures (i.e., back
pressure/positive transients) generated in the DWV
system. PAPAs are installed at the base of the soil and
waste stack and at various floor intervals, depending on
the height of the building (See Photo 14.15.) The unsteady
nature of the water flows cause pressure fluctuations
(known as pressure transients), which can compromise
water trap seals and provide a path for sewer gases to
enter the habitable space. A PAPA/AAV system counters
the tendency for the loss of trap water seals resulting from
positive pressure pulses in a soil and waste stack. The
PAPA/AAV system may be used in sanitary plumbing
systems as an alternative to relief venting, eliminating the
need for a continuous parallel relief vent pipe. It is a viable
option to the Sovent® system.
Sovent® Drain and Vent System
Multistory buildings traditionally rely on a complex drain and vent system with two stacks that run vertically from
floor to floor and vents and branches to every fixture. In high-rise buildings, if it works successfully, a drain/vent
scheme with a single stack and branches without vents is an effective substitute for the traditional two-pipe, drain
and vent system.
The Sovent® system is a system that combines the drain stack, branches, and vents into one pipe system by
using patented Sovent® fittings. Fritz Sommer of Switzerland, whose work was mainly driven by a need for
resource-conserving construction techniques, developed and patented the Sovent® fittings in the 1950s.
The system consists of four components:
1.
2.
3.
4.
vertical stack piping
horizontal branches to the fixtures
aerator fitting
de-aerator fittings
These components work together to collect wastes from the plumbing fixtures and transport them down a stack
to the building drain.
The system works on the principle that wastewater flowing down a vertical pipe tends to cling to the interior wall
surface and continue downward in a swirling motion. As the wastewater travels down the walls of the pipe, the
pipe center remains open and serves as an airway. The airway provides venting so there is a balance of
pressures within the drainage system. It eliminates the need for a separate venting system. However, if the fall
rate of wastewater is uncontrolled, the falling water will increase speed and meet air resistance, which will flatten
out the falling waste until it blocks the stack. This downward moving blockage can throw off the pressure balance
in the system and suck water out of fixture traps. Specially designed fittings are placed in the vertical stack at
each floor to eliminate speed buildup and blockage, thereby maintaining the airway and allowing for good
drainage.
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Horizontal branches and branch runouts connect to the plumbing fixture and transport the wastes to a specially
designed stack. Generally, vents to individual fixtures are not required if fixture placement is near the stack; for
example, a 4-in soil/ waste line may be run horizontally out to 27 ft from the Sovent® stack without the use of
traditional venting methods.
The Sovent® stack is a vertical pipe that conveys wastes from the upper levels of a building to the base of the
stack. The stack begins just above the bottom-most de-aerator fitting (to be described later) and continues to
just above the highest fixture connection. The main difference between the specially designed stack and the
traditional waste and vent stack is the Sovent® stack will remain one size throughout its entire length. It is not
permitted to change diameter because it functions for both drainage and venting purposes. Sovent® stack size
is based on the total number of drainage fixture units that connect to that stack. The stack will penetrate the roof
to the atmosphere much like traditional vent systems.
The Sovent® aerator fitting is made of two separate chambers. The first chamber, called an offset chamber,
allows falling waste from the upper floors to enter the chamber and pass around the horizontal branch inlets.
This offset reduces the falling waste’s velocity, eliminating blockage before it is allowed to form. The second
chamber, named the mixing chamber, is fully separated from vertical stack flow with an internal separation
baffle. As horizontal branch flows enter the aerator fitting, it must transition to a vertical flow, smoothly uniting
with any vertical stack flow that may exist. Aerator fittings can have several branch inlets. The mixing chamber
provides the branches with sufficient air circulation to balance any pressure fluctuations that may occur. A second
internal baffle in the mixing chamber is located perpendicular to the separation baffle to prevent crossflow from
opposing branch inlets on that floor.
A Sovent® de-aerator fitting must be located at the base of each Sovent® stack and at any horizontal stack
offset. This fitting is designed to effectively deal with pressure fluctuations that occur when vertical falling wastes
suddenly turn horizontal.
Sewage Ejection
For the most part, sanitary drainage systems rely on the
force of gravity to create flow to discharge wastewater.
In some building installations, however, a fixture or
group of fixtures must to be installed below the level of
the nearest available sewer line. In these cases,
wastewater must be lifted to the level of the main drain
or sewer by a pumping system called a sewage ejector.
Typically, a sewage ejector can pump solids from 2 to 4
in (50 to 100 mm) in size or grinds solid wastes before
passing them through the ejector. Photo 14.16 shows
an installed sewage ejector.
A sewage ejector system consists of the sump basin, a motor-pump assembly, and a system of automatic
electrical controls. Wastewater from the sanitary pipes flow by gravity into the sump basin, a pit that collects
wastewater. As the wastewater level rises, it triggers a float switch that activates the pump. The pump then lifts
the wastewater through a check valve and discharge line into a typical building drain line, where it gravity flows
into the building sewer. It operates much like a sump pump.
The check valve in the discharge line prevents backflow. Without it, the pump will cycle continuously. A vent pipe
connects to the sump basin to relieve the suction created by the pump. A high water alarm is generally added to
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the system, to warn of pump failure or backup to prevent flooding. Basins are typically fabricated of fiberglass,
cast iron, or high-density polyethylene thermoplastic; they are typically set in a hole in a concrete floor slab.
A single ejector pump is installed in a small system, such as a single-family residence or small commercial
building. Larger commercial and industrial installations require two pumps to ensure continued operation if one
pump fails. The additional pump also provides extra capacity in times of extra heavy loads.
The size and capacity of a sewage ejector system is determined by the application. The manufacturers’ literature
specifies the capacity of the pump and the maximum size of wastewater solids that can be handled by a particular
pump. Attempting to eject solid matter that exceeds this rated size or materials that expand in water have the
potential to clog the system.
Typically, residential ejector systems must have the capacity of ejecting solids up to 2 in (50 mm) in size.
Depending on pump impeller design, a 4-in pump will normally handle spherical solids from 2 to 3 in and typically
range in motor size from 1/3 to 2 horsepower. Additionally, these systems are generally rated to a maximum
temperature of 180°F (82°F).
Sizing commercial or industrial installations involves application of complex formulas. In each case, the design
must consider total dynamic head, the highest vertical point, and the size of the basin provided. The farther the
distance the waste must be lifted, the more powerful the pump must be to do the job. Regardless of peak flow
requirement for a given application, the pump must always be able to provide a minimum velocity of 2 ft per
second through the line. Typically, in residential and small commercial applications, the water supply fixture unit
(WSFU) load can be used to estimate usage demands of plumbing fixtures served. Larger capacity systems are
required in motels, apartment complexes, and large office buildings because of higher peak demands. The
installation must conform to the local building code.
B. DRAIN AND VENT PIPE DESIGN
Drainage Fixture Units
The draining rate for plumbing fixtures is based upon the drainage fixture unit (DFU). Refer to Table 14.1.
Similar to the water supply fixture unit introduced in Lesson 6, the DFU is an arbitrarily chosen measure that
allows all of types of plumbing fixtures to be expressed in common terms; that is, a fixture having twice the
instantaneous drainage flow rate of a second fixture would have a fixture unit value twice as large. The WSFU
and DFU may differ slightly for a single fixture, because the rates of filling and draining are different.
Design Approach
The approach used to size drain and vent lines relies on tabular information found in code. Table 14.2 indicates
the maximum load in DFU and maximum pipe length for a given pipe diameter. The minimum pipe diameter is
based on the total connected DFU. In the case of vent lines, maximum developed length for a given pipe is also
a criterion. Developed length is the “centerline” length of the lines, excluding traps and trap arms. It is important
to ensure that a larger pipe diameter does not flow into a pipe having a smaller diameter.
Traps and trap arms are sized based on a specific type of fixture. Refer to Table 14.1 for minimum trap sizes.
Some fixtures such as urinals and water closets have integral traps built into the fixture so trap size does not
need to be specified.
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EXAMPLE 1:
The following number and type of plumbing fixtures serve two apartment units: two bathtubs, two water closets,
two lavatories, and two kitchen sinks. Assume the horizontal fixture branch serving these fixtures flows into the
waste stack. Assume the vent stack extends through the roof and is 22 ft long. Determine the minimum pipe
diameter required for the horizontal fixture branch, waste stack, and vent stack.
SOLUTION:
For the horizontal fixture branch, from Table 14.2, a 3-in diameter pipe is selected. A 3-in diameter pipe
used as a horizontal fixture branch can serve up to 20 DFU.
For the waste stack, from Table 14.2, a 2½ in diameter pipe can be selected but the 3 in diameter
horizontal fixture branch would then flow into a smaller pipe. A 3 in diameter waste stack is a prudent
choice.
For the vent stack, from Table 14.4a, a 2 in diameter pipe is selected, based on a 3 in diameter soil and
waste stack, a capacity of up to 30 DFU and a developed length of 22 ft.
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EXAMPLE 2:
The following number and type of plumbing fixtures serve six apartment units with two apartments on each floor:
six bathtubs, six water closets, six lavatories, and six kitchen sinks. Assume horizontal fixture branches serving
these fixtures flow into the waste stack at three locations (three intervals), two apartments per interval. Assume
the building drain is sloped at ¼ in per ft and the vent stack extends through the roof and is 42 ft long. Determine
the minimum pipe diameter required for the horizontal fixture branches, waste stack, building drain, and main
vent stack.
SOLUTION:
For the horizontal fixture branch, from Table 14.2, a 3 in diameter pipe is selected. A 3 in diameter pipe
used as a horizontal fixture branch can serve up to 20 DFU. Two apartment units have two bathtubs, two
water closets, two lavatories, and two kitchen sinks—a total of 18 DFU (see Example 1).
For the waste stack, from Table 14.2, a 4 in diameter waste stack is selected. A 4 in diameter pipe used
as a waste stack can serve up to 240 DFU.
For the building drain, from Table 14.3, a 4 in diameter pipe is required. A 4 in diameter pipe used as a
building drain can serve up to 216 DFU at a slope of ¼ in/ft.
For the vent stack, from Table 14.4a, a 2½ in diameter pipe is selected, based on a 4 in diameter soil and
waste stack, a capacity of up to 100 DFU, and a developed length of 42 ft.
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C. SYSTEM INSTALLATION
On a small project, the drainage piping typically varies in size
from 1½ to 4 in. It can be much larger in large hotels,
apartments, and office buildings. This larger size of pipe often
requires special provisions in wall width or furred-out areas.
Poured concrete slabs will require that the plumbing layout be
carefully considered. The pipes need to be placed in the
ground before the slab is poured, so accurate placement is
crucial. Typically, both the water supply and drainage pipes
are laid out next to each other, as they go to the same areas
of the building. String is usually stretched over the slab area
to mark where the pipes should be located. Many times, they
are planned so they will come up in a wall. However, the tub,
shower, and water closet piping will need to be placed in the
exact location where the fixture is to go. All piping must be
carefully located and the system checked for leaks before the
concrete is poured because any relocation or repairs of pipes
would be costly.
On larger projects with concrete walls and ceilings, it is usually
necessary to provide sleeves (holes) in the concrete for the
pipes to pass through to get from space to space. It will also
be necessary to provide inserts and hangers to support the
pipes.
The open spaces provided in truss-type construction make it
easier to run piping through to the desired location. The only
points of difficulty would be where it needs to pass by
ductwork or some other large pipe that is going in the opposite
direction. This will require coordination with the contractor
installing any heating, air conditioning, or ventilating ductwork.
In wood frame construction, the holes are sometimes drilled
to allow the passage of the pipes. These should be at the
middle of any load-bearing wood members so that a minimum
of structural damage is done. There are times when the width
of a wall needs to be increased to allow for pipes running
horizontally to pass by drainage pipes (or other pipes) running
vertically.
Pipe chases run from floor to floor to allow stacks and vents
to pass vertically between floors. A view of the interior of a
plumbing (wall) chase is shown in Photo 14.17. Chases are
typically located alongside elevator hoist ways and common plumbing walls. A drain stack is shown in Photo
14.18. Pipe tunnels (Figure 14.11) may be used on large projects to provide concealed space for the passage
of mechanicals at ground level and from building to building. Hangers from the top or side of the tunnel are used
to support the pipes. Access may be from either end of the tunnel or access floors may be provided.
CVIL 1083 – ENGINEERING UTILITIES 2 | 20