SECTION 914
SINGLE STACK VENT SYSTEM
914.1 Where permitted. A drainage stack shall be permitted to serve as a single stack vent system when sized and installed in accordance with Sections 914.2 through 914.9. The drainage stack and branch piping in a single stack vent system shall provide for the flow of liquids, solids, and air without the loss of fixture trap seals.
914.2 Stack Size. Drainage stacks shall be sized according to Table 914.2 A maximum of two water closets shall be permitted to discharge to a 3 inch stack. Stacks shall be uniformly sized based on the total connected drainage fixture unit load with no reductions in size.
Stack Size(inches) / Maximum Connected Drainage Fixture Units
Stacks Less than 75 Feet in Height / Stacks 75 Feet to Less than 160 Feet in Height / Stack 160 Feet or Greater in Height
3 / 24 / NP /
NP
4 / 225 / 24 /NP
5 / 480 / 225 / 246 / 1015 / 480 / 225
8 / 2320 / 1015 / 480
10 / 4500 / 2320 / 1015
12 / 8100 / 4500 / 2320
15 / 13,600 / 8100 / 4500
Table 914.2
Single Stack Size
914.3 Branch Size. Horizontal branches connecting to a single stack vent system shall be sized according to Table 710.1(2).
Exceptions:
1.No more than one water closet within 18" of the stack horizontally shall be permitted on a 3" horizontal branch.
2.A water closet within 18" of a stack horizontally and one other fixture with up to 1-1/2 inch fixture drain size shall be permitted on a 3" horizontal branch when connected to the stack through a sanitary tee.
914.4 Length of horizontal branches.
914.4.1 Water closets shall be no more than four (4) feet horizontally from the stack.
Exception
Water closets shall be permitted to be up to eight (8) feet horizontally from the stack when connected to the stack through a sanitary tee.
914.4.2 Fixtures other than water closets shall be no more than twelve (12) feet horizontally from the stack.
914.4.3 The length of any vertical piping from a fixture trap to a horizontal branch shall not be considered in computing the fixture's horizontal distance from the stack.
914.5 Maximum vertical drops from fixtures. Vertical drops from fixture traps to horizontal branch piping shall be one size larger than the trap size, but not less than two (2) inch in diameter. Vertical drops shall be four (4) feet maximum length. Fixture drains that are not increased in size, or have a vertical drop exceeding 4 feet shall be individually vented.
914.6 Additional venting required. Additional venting shall be provided when more than one water closet is on a horizontal branch and where the distance from a fixture trap to the stack exceeds the limits in Section 914.4. Where additional venting is required, the fixture(s) shall be vented by individual vents, common vents, wet vents, circuit vents, or a combination waste and vent pipe. The dry vent extensions for the additional venting shall connect to a branch vent, vent stack, stack vent, air admittance valve, or be extended outdoors and terminate to the open air.
914.7 Stack Offsets. Where there are no fixture drain connections below a horizontal offset in a stack, the offset does not need to be vented. When there are fixture drain connections below a horizontal offset in a stack, the offset shall be vented in accordance with Section 915. There shall be no fixture connections to a stack within 2 feet above and below a horizontal offset.
914.8 Separate Stacks Required. Where stacks are more than two stories high, a separate stack shall be provided for the fixtures on the lower two stories. The stack for the lower two stories may be connected to the branch of the building drain that serves the stack for the upper stories at a point that is at least 10 pipe diameters downstream from the base of the upper stack.
914.9 Sizing Building Drains and Sewers. The building drain and branches thereof, and the building sewer in a single stack vent system shall be sized in accordance with Table 710.1(1).
SUPPORTING INFORMATION This proposed change is not waste stack venting as described in IPC Section 910. This proposal is a single stack vent system for the entire drainage system, including water closets and urinals.
This proposed change is based on the Philadelphia Plumbing Code, which has used single stack vent systems successfully for over 100 years. The stacks are oversized to provide for the flow of liquid waste, solid waste, and air. The lengths of trap arms are limited and the vertical drops from fixture traps are oversized to prevent the trap from self-siphonage. Fixture drains that do not meet the requirements for a single stack venting system must be conventionally vented in accordance with the other Sections of IPC Chapter 9.
The Philadelphia System is described in Volume 2, Chapter 3 of the American Society of Plumbing Engineers (ASPE) Data Book as follows:
“Philadelphia or one pipe system refers to using one stack instead of having separate drainage and vent stacks. These systems depend on relieving the pressures by making the pipe larger than required for drainage pipe in a two-pipe system. These systems also use unvented traps (“S” traps) that depend on oversized traps and refill from flat bottom fixtures to maintain the trap seal.
This system limits the trap arm length to reduce suction buildup. The size of the main stack is increased to limit pressure and vacuum buildup. See Figure 3-8. Check with the local authorities to see if this system is allowed. Contact the City of Philadelphia for specific requirements.”
Note that the trap sizes in the one pipe vent system are comparable to those in IPC Table 709.1 and are not oversized as stated in the ASPE Data Book. What is oversized is any vertical drop that forms an “S” trap. See Section 914.5
The following is a comparison of the stack loading allowed by the various model plumbing codes for conventionally vented drainage systems, compared to the one stack vent system:
STACKSIZE / PROPOSED
SINGLE
STACK / PHILADELPHIA
PLUMBING
CODE / 2000
IPC
(a) / 2000
NSPC
(a) / 2000
UPC
(a)
3” / 24 (b) / 75 / 72 / 72 / 48
4” / 225 / 225 / 500 / 500 / 256
5” / 480 / 480 / 1100 / 1100 / 600
6” / 1015 / 1015 / 1900 / 1900 / 1380
8” / 2320 / 2320 / 3600 / 3600 / 3600
10” / 4500 / 4500 / 5600 / 5600 / 5600
12” / 8100 / 8100 / 8400 / 8400 / 8400
15” / 13,600 / 13,600
(a) Based on the total DFU allowed on a stack of more than 3 branch intervals.
(b) A 3” stack is limited to two water closets by Table 914.2.
The Philadelphia Plumbing Code allows 75 DFU and six (6) water closets on a 3” stack, which is comparable to a conventionally vented drainage system. This proposal limits a 3” stack to 24 DFU and two (2) water closets, which is comparable to an average dwelling unit.
In the Table above, the allowable DFU loading on 3”, 4”, 5” and 6” stacks in the proposed single stack vent system is 50% or less of that allowed in a conventionally vented system. The percentage of oversize diminishes in the 8”, 10” and 12” stacks. However, stacks that large will be taller than 75 feet or 160 feet and Section 914.2 will require that they be increased one or two pipe sizes, making the stack loading 50% or less for all stack sizes.
Although the pipe sizing is larger in a single stack vent system, it results in construction cost savings by reducing the amount of vent piping required.