“An Optimized Sanitary Stack Configuration for Mid-Rise Multifamily Construction”

The design practices surrounding American sanitary drainage and vent systems closely reflect those in plumbing codes published in the first half of the 20th century. While venting methods such as circuit venting and horizontal wet venting were more recently introduced in some areas of the country, these configurations can be found in the National Bureau of Standards (NBS) publications, which are essentially the framework for the International Plumbing Code (IPC) and Uniform Plumbing Code (UPC).

As building drainage theory progressed in the second half of the 20th century, mostly led by the Building Research Station (BRS) and Heriot-Watt University in the United Kingdom, drainage design in American plumbing codes remained mostly the same. This is prominently demonstrated by the emphasis on fixture vent piping over the venting of sanitary stacks, particularly for stacks serving fixtures in mid-rise buildings without auxiliary vent stacks. A majority of national standards utilize stack venting as a primary means of venting fixtures, a concept more familiar to those in the U.S. as the Philadelphia stack or the single-stack vent system, which can be found in Chapter 9 of the IPC and Appendix C of the UPC.

The following discussion will focus on the development and varying requirements of the single-stack vent system and proposals for modifications to the American variation to increase the functionality of this method in mid-rise multifamily construction.

History

After the end of World War II, the UK launched efforts to rebuild the hundreds of thousands of homes that were destroyed, seeking new strategies to optimize labor and resources. This led Alfred Wise of the BRS to investigate a simplified approach to sanitary drainage stacks. With extensive experimental testing, Wise developed a drainage configuration that allowed the elimination of individual fixture vents, also known as anti-siphon vents, relying instead on the free passage of air through the top of the drainage stack to limit negative pressure development to protect the water seals in fixture traps. Wise found that if the length of the horizontal drain between the fixture trap and stack connection was limited, the stack configuration was suitable for serving up to 10 floors of fixtures (see Figure 1).

Variations of Wise’s single-stack configuration quickly spread throughout Europe, Asia, Latin America, Africa, and Oceana. Wise’s reports seem to indicate that he was unaware that a nearly identical configuration was already in use in Philadelphia. This American variation was initially developed by Boston architect J. Pickering Putnam and introduced at the 1911 American Institute of Architects (AIA) convention in San Francisco. Sometime after the development of the fixture unit method, a load sizing table was introduced for the Philadelphia stack configuration, along with stack height limits for each stack diameter. The Philadelphia stack was further developed by the American Society of Plumbing Engineers (ASPE) and eventually was featured in the appendix of the UPC in 2006 and Chapter 9 of the IPC in 2012 (see Figure 2).

Despite being used in Philadelphia for more than a century and being listed as an approved method in the IPC, the single-stack configuration has failed to gain widespread use in the U.S., likely due to some key design limitations.

Fundamental Design Requirements

Design requirements for the single stack vary in each country, along with the name of the configuration itself, with China and Japan describing the method as extension ventilation and Europe using the term primary ventilation. One common principle shared by almost all national variations is that the single stack allows the elimination of all vent piping anywhere along the stack or serving the fixtures connecting to the stack, with the one exception being the top of the stack itself, extending beyond the top fixture connection full size and terminating to the atmosphere. Most national and regional standards, with some exceptions, have a maximum allowable pressure differential of 1-inch (25-mm) of water column at fixture traps, allowing the performance between drainage systems to be more easily compared.

Figure 3

Fixtures on the Lowest Floor

The pressure surge problems associated with fixtures connecting directly above or downstream of a stack are almost universally recognized in plumbing codes and design guides. Most standards require fixtures on the lowest floor to connect downstream of the stack at a specified minimum distance and avoid connecting to the lowest floor above the stack base, unless the lowest floor connection can be made above 2.5 to 8 feet (0.75 m to 2.5 m) or so above the stack base. Standards in Germany and China suggest providing a vent from the lowest floor fixtures and terminating this vent into the sanitary stack with a wye fitting at the same floor. In the UK, fixtures on the lowest floors connect into a stub stack, which is essentially a capped stack roughly 6 feet (2 m) in height. The stub stack connects horizontally downstream of the stack, relying on the airflow within the upper portion of the horizontal piping to alleviate pressure differentials.

The Philadelphia stack, however, requires separating out the two lowest floors for stacks greater than three floors in height. The stack serving the lower two floors may also use the Philadelphia configuration or may use another venting option, such as horizontal wet venting, both of which require extending a vent running from the lowest floor to an atmospheric termination point. Providing vent piping along the entire stack length undermines the advantages of the configuration over the typical horizontal wet vent stack (see Figure 3) and is unnecessary in this application for the protection of water seals.

Height Limitations

As drainage stacks increase in height, the amount of airflow required to alleviate negative pressure from drainage increases. Depending on the design standard, the single stack is limited to a maximum of 10 to 20 floors to avoid airflows that would cause water seals in traps to fail. Once a stack height is considered too great, most design standards recommend providing an auxiliary vent stack connecting above the sanitary stack base, along with intermediate relief connections along the length of the stack. This configuration, widely known as the secondary ventilated stack, still allows fixtures to connect into the stack without a vent connection between the trap and stack connection.

For the 4-inch stack, a typical size for stacks serving bathrooms in most countries, the Philadelphia configuration is effectively limited to 75 feet (23 m),* a height equivalent to about seven or eight floors. This height may be exceeded by increasing the stack diameter by one size. The 5-inch stack, typically considered a special-order size, is limited to 160 feet (49 m). The IPC also requires an auxiliary vent stack for the single-stack system if the height exceeds more than five floors, though this is a standard requirement for all sanitary stacks in the IPC and it may have been the intention to add the single stack to the list of exempted configurations.

Horizontal Distance from Stack Limitations

To avoid issues of self-siphonage, design standards limit horizontal drainage branch lengths between the trap and the branch connection to the stack. In Europe, this is generally limited to somewhere between 13 feet (4 m) and 33 feet (10 m). The UK has no limits for water closets or bathtubs but limits lavatories to a maximum of 10 feet (3 m). For the Philadelphia variation, water closets may be up to 8 feet (2.5 m) from the stack, while other fixtures may be up to 12 feet (3.7 m). Most configurations allow an S trap from the lavatory, including  the Philadelphia stack, though the Philadelphia stack requires increasing the fixture drain from a 1½-inch drain to a 2-inch drain prior to transitioning vertically to provide additional protection against self-siphonage.

Proposed Modifications to the Single Stack

Figure 4

Thanks to a research grant provided by ASPE’s Portland Chapter and assistance from Dr. Michael Gormley of Heriot-Watt University, a new variation of the single stack is being proposed in the upcoming 2024 UPC. This variation addresses the issues with the impractical venting requirements of the Philadelphia stack for fixtures on the lower floors by utilizing the approach used in Germany and China (see Figure 4). The proposal also only requires separating the first floor from the stack for stacks less than 75 feet (23 m). These minor modifications to the single-stack method will significantly increase the practicality of the configuration, making it a viable alternative to conventional sanitary stacks for mid-rise multifamily construction.

*4-inch stacks greater than 75 feet in height may only be used if the load does not exceed 24 drainage fixture units (DFU), equivalent to six floors of bathrooms.

References

  1. 2021 International Plumbing Code. (2020). Country Club Hills: International Code Council.
  2. 2021 Uniform Plumbing Code. (2020). Ontario: International Association of Plumbing and Mechanical Officials.
  3. BS EN 12056-2 – Gravity Drainage systems inside buildings. (2000). British Standards Institute.
  4. Design Manual for Building Water Supply and Drainage. (2019). Beijing: China Architecture Design & Building Press.
  5. DIN 1986-100 Drainage systems on private ground. (2016). German Institute for Standardisation.
  6. Lansing, J. (2020). A Comparison of British and American Plumbing Engineering Standards and Practices. Zurich: World Plumbing Council.
  7. Swaffield, J. (2010). Transient Airflow in Building Drainage Systems. London: Spon Press.

About the Author

John Lansing, CPD, LEED Green Associate, is a Certified Plumbing Designer in Portland, Oregon, working with PAE Consulting Engineers. John was the recipient of the World Plumbing Council’s 2018 Education and Training Scholarship, which gave him the opportunity to author a report on differences between British and American plumbing engineering. He also authored Maintaining Domestic Hot Water Return Temperatures Above the Growth Range of Legionella, published by ASPE. John is passionate about sustainable and regenerative plumbing strategies as well as studying plumbing engineering standards and design guides from other countries.

Any opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.

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