DWV Fundamentals
Plumbing Systems
CNST 305
Environmental Systems 1
Dr. Berryman
4a
Drain, Waste, Vent
Chapter 2,5,6 Appendix C
(Wentz) and 8.3 - 8.5 (Toa)
Trap

Uses water seal to prevent backflow
of gasses into building
P Trap
Integral Traps
Venting
Removes
dangerous odors
and explosive
gasses
 Prevents

Positive pressure
 Negative pressure

If not properly vented, a waste slug could
either blow out or siphon the trap seal
Venting (cont.)

Drainage
systems flow 
by gravity

As such:
EVERY TRAP
or
PLUMBING
FIXTURE
MUST HAVE
A VENT

Isometric
Diagrams
Combining Vents
Loop Vent
Circuit Vent
Example
Isometric
Diagram
Design Guidelines for Drain
and Waste (for most codes)


All traps must be vented
The waste stack cannot be smaller the
the largest horizontal branch connecting
to the stack
• 4” branch cannot connect to a 3” stack




At least one waste stack same as building
drain
No branch pipe smaller than the one that
proceeds it
Above ground cannot be less that 1¼”
Below ground cannot be less than 2”
Design Guidelines for Drain
and Waste (cont.)
1 ¼” drain pipe can handle on 1 DFU
 A drain line cannot be smaller than
the trap arm to which it is connected
 Use 4” trap arm for floor mounted
water closets

• Some codes do allow 3” (however, this is
not generally recommended)
Design Guidelines for Vents
(for most codes)




Above ground venting cannot be less
than 1 ¼”
1 ¼” vent can handle on 1 DFU
Underground venting cannot be less
than 2” in diameter
Vents must be a certain distance from
the trap that it serves
• (tble 19 ANSI A40
1993 Standard)
Design Guidelines for Vents
(cont.)

An individual vent must not be less than
½ the size of the drain that it connects
• 4” drain cannot connect to any pipe less than 2” in
diameter


An individual vent should not be larger
than the drain that it connects
Downstream piping always bigger than
the upstream section
• Downstream, in its natural state, is going OUT of the
building
Design Guidelines for Vents
(cont.)



A vent line must be 6” above flood level
rim of the fixture before turning
horizontal
Branch vents must slope toward a drain
Some codes require that at least one of
the building’s VTR be 4” in diameter or
larger
Next Time

Sizing DWV
4bc
Drain, Waste, Vent
Chapter 2,5,6 Appendix C
(Wentz) and 8.3 - 8.5 (Toa)