How to use the International Plumbing Code

Learn about design criteria for the design of plumbing systems

By Paulina Diaz August 2, 2022
Courtesy: CFE Media and Technology

 

Learning Objectives

  • Review fixtures and faucets, water distribution systems, sanitary drainage and vents within the International Plumbing Code.
  • Learn about booster pumps and pressure zones, and how to incorporate the local water pressure into calculations.

 

A model plumbing code available for design engineers is the International Plumbing Code (IPC 2018 edition). It provides the minimum design criteria for code compliance for the design of plumbing systems. Furthermore, it is the basis for these systems to perform at a level to prevent water contamination and promote longevity.

Plumbing systems include sanitary waste, vents, storm drainage and potable water distribution among others. Sanitary waste systems are open gravity systems designed to flow half-full compared to what is designed in heating, ventilating and air conditioning systems, which are generally pressurized closed systems.

The IPC is a comprehensive code and it is composed of 15 chapters and five appendices.

Fixtures and faucets

Chapter 4 indicates the minimum number of fixtures and faucet that are to be provided in a certain building type. A plumbing fixture, as referenced under the https://www.iccsafe.org/content/international-plumbing-code-ipc-home-page/IPC is provided with supply cold water and waste discharge. Fixtures and faucets are to be provided with the necessary amount of potable water per fixture/ faucet and the required pressures when serving a building with human occupancy.

Even though this article does not include discussing building types or classifications, it is recommended that the plumbing engineer refer to the IPC and the International Building Code for these types and classifications to provide the minimum number of required fixtures.

Section 403: Minimum number of required fixtures: Table 403.1 indicates the minimum number of fixtures required for the specific building classification. Before using this table, the engineer must have calculated the number of occupants for the specific facility per the IBC. The code also provides the criteria depending on male and female and water closets versus urinals to be specified. As an example, the business classification under table 403.1 indicate 1/25, which means 1 water closet per 25 occupants for the first 50 occupants and 1 water closet per 50 for the remainder in excess of 50 occupants.

Section 405: Installation of fixtures: A primary safety aspect when designing a plumbing system is preventing cross-contamination of different systems. Therefore, water supply must be protected by a backflow preventer. Furthermore, accessibility to the valves or the backflow preventer is important, as these pieces of equipment must be regularly maintained. For this reason, the code indicates specific criteria to maintain cleanliness around the fixture.

Water distribution system

Any building used for the purpose of human occupancy must be supplied with potable water at specified rates and pressures. This is covered under Chapter 6 of the IPC. One important concept is regarding water supply fixture units, which is the load demand of a specific fixture.

The code presents detailed information under Appendix E for step-by-step sizing of the water distribution system. In addition, it provides two key tables: table E103.3(2) provides load values assigned to fixtures and table E103.3(3) provides information for estimating demand. Within the first table, specific water supply fixture unit load values are indicated for each fixture type for either public or private. In the second table, load for the number of water supply fixture units is indicated for either flush tanks or flush valves. It is important to mention that diversity is taken in the calculation as not all fixtures will be used at the same time.

Section 604

  • 2: Hot- and cold-water systems, when interconnected, must consider preventive criteria to avoid flow from one system to the other. For example: an emergency shower or eye-wash station provided with a mixing valve.
  • 3: The total water demand calculated for a building is dependent on the fixture unit load, the total gallons per minute must be calculate for a peak demand. The system distribution will take these into account to size the pipe mains and branches. Table 604.3 provides the required capacity, which must be maintained during peak demand, for the pipe outlet flow depending on the fixture type as well as the required outlet pressure. For example, from Table 604.3 a private lavatory is indicated with  0.8 gpm at 8  pounds per square inch flow pressure.
  • 4: This section indicates the maximum flow rate and consumption for each fixture. Table 604.4 provides the criteria mentioned above with a shower head typically having the highest flow at a pressure of 80 pounds per square inch, which is the maximum pressure available at a fixture per the code.

It is worth mentioning about other standards such as U.S. Green Building Council LEED, Green Globes and Environmental Protection Agency, which for some fixtures indicate dual flow options, high and low, for energy conservation purposes.

Facilities such as hospitals, medical office buildings or ambulatory surgical centers are required to be provided with two water sources. This is indicated under IPC sections 609, 609.1 and 609.2. The primary reason is to avoid any interruption of water service to these critical facilities.

Booster pump and pressure zones

When designing the cold-water supply for a hospital bed tower, to verify if a booster pump is required several items are worth considering:

  • City water pressure in pounds per square inch at the facility service
    • Static pressure (psi)
    • Residual pressure (psi)
    • Flow rate (gpm).
  • Building height in feet.
  • Most remote fixture demands (psi).
  • System losses (psi).
  • Booster pump (horsepower, gpm and psi).
  • Pressure zones.

First, the city pressure must be known. This is obtained from a flow and pressure test, also known as hydrant flow test, which provides the available gallons per minute at static and residual pressure. The static pressure is understood as “no flow” pressure while the residual pressure is measured with water flowing, where several hydrants around the building site are sampled.

For detailed information on flow test, see NFPA 291: Recommended Practice for Water Flow Testing and Marking of Hydrants.

Second, with the known values above flow and pressure, the design engineer proceeds to evaluate the pressure losses in the system due to building height, often known as the static head. The conversion from pounds per square inch to feet of head is:

0.433 psi = 1 foot

0.433 psi = 1 foot

For example, a water column of 1 square inch and 2.31 feet in height weighs 1 pound. This definition is useful to calculate the head loss from floor to floor. Therefore, the total building height and the floor-to-floor height must be known to calculate the static pressure differential loss using the formula above.

Third, the most remote fixture and its required outlet pressure, such as indicated in tables 604.3, 604.4 and 604.5, as well as total supply water fixture units, must be known to size the pipe mains and branches. This is where detailed step by step from Appendix E is useful to the engineer.

Fourth, the engineer must calculate the system losses once the piping mains and branches are designed. In addition, to the losses from the water meter, backflow preventer and valves serving the system. For example, using as a reference Appendix E, cold water flowing through an 8-inch pipe must overcome the pressure drop and friction loss from equipment and valves. These values may be found under table E103.3(5) and (6) for example a butterfly valve represents a pressure loss of 12.5 feet for an 8-inch pipe size. This concept is key as it represent the equivalent length in feet for the valve.

Last, depending on the available city pressure, the building height and the static loss, the design may require a booster pump to deliver the required flow to the most remote fixture.

Booster pumps

At a 19-story pediatric hospital in Georgia, the flow test provided an available pressure of 140 psi at one of the tested locations. The floor-to-floor height for this design varies from level to level but assuming 17-foot floor-to-floor, that is a loss of 7.35 psi per level. This represents as loss in elevation of 140 psi to start. If the engineer needs 50 psi at the most remote fixture, a booster pump is required. Booster pumps are commonly specified in plumbing systems to meet the pressure demand of buildings when the city pressure is lower than required for the last fixtures.

Figure 1: Cold water riser diagram indicating four pressure zones. Each zone is treated as its own envelope. Color scheme depicts each zone. Booster pump is located in the concourse level. Courtesy: WSP USA Buildings

The booster pump selected for 1,200 gpm is a pump package system with six pumps with variable frequency drives, each one delivering 240 gpm at 138 psi. The booster pump is in the concourse level and delivers cold water to four different zones as shown in Figure 1. Each zone is considered an envelope or a control volume, which is independent of other zones. Each zone has a set pressure limit controlled by a dedicate pressure-reducing valve located in the cold-water inlet serving the domestic water storage tanks.

Focusing on zone one as an example, the pressure-reducing valve is set to maintain a pressure of 50 psi, where the incoming pressure without a pressure-reducing valve is higher for this level because it is the closest to the booster pump. IPC requires a maximum of 80 psi allowable to any fixture. The remainder zones are set in the same manner as zone one (see Figure 2).

Hot water systems are usually provided with balancing valves on the return side at each level and circulating pumps at the lower level near the equipment.

Figure 2: Cold water riser diagram indicating four pressure zones. This diagram focuses on zone 1, which is provided with a pressure reducing valve to maintain the required pressure at 50 psi for that zone or envelope. Courtesy: WSP USA Buildings

Chapter 7: Sanitary drainage

Every building, public or private, is designed with a plumbing system to direct sanitary waste from the fixture inside the building to the exterior. This is accomplished via an engineered piping system, which provides continuity of flow from the fixture to the mains. The discharge location, external to the building, is a location coordinated with the civil engineer and it occurs at various points around the building underground.

Some municipalities may require a single exit point; therefore, each project design is dependent on the location and the local codes. These exit locations are studied on a per project basis, as these will vary with the building layout and orientation. Typically referred to as inverts, the sanitary waste and the storm drain mains are provided with an invert with respect to the ground level for the civil engineer to provide a tie-in point to the city main.

Commonly used piping materials for sanitary sewer system are cast iron and polyvinyl chloride. However, under section 702 of the IPC, there are several different piping materials permitted under the code. Table 702.1 defines those permitted above ground and 702.2 shows below-ground options. It is important to verify material use by location and with the authority having jurisdiction when designing a new building because some states may have different requirements.

For example, under the Department of State Health Services Title 25 Texas Administrative Code (TAC), PVC is acceptable for use in underground piping. However, it is now allowed above ground. One reason is due to the low flame spread characteristics of this piping material. The American Society of Testing Materials Standard E84 25/50 tests materials for flame and smoke spread. This criterion is also important when designing systems with plenum spaces; piping and duct insulation must meet ASTM E84.

Sanitary piping systems that run horizontally across the building are sloped at different rates depending on pipe sizes as indicated under the IPC.

Section 704: Drainage piping installation

  • 1 provides uniform slope and pipe size. The acceptable or recommended slopes for sanitary drainage are 1/4, 1/ 8 or 1/16 inch, based on pipe size as listed in Table 704.1 of the IPC. These slopes are indicative of the pipe’s drop in elevation per linear foot of length of pipe run. During the schematic design of a new project, the sanitary piping size, specifically the larger mains, are coordinated with other disciplines and trades to verify pipe slope in ceiling spaces. The smaller the pipe, the higher the recommended slope (drop in elevation) per linear foot and vice versa.
  • 2: Designers need to keep in mind the importance of pipe size and direction of flow; this code section is specifically to avoid a reduction in pipe size in the direction of flow. This will cause turbulence and delay the gravity flow.

Section 709: Fixture units: A key concept to understand when designing a sanitary drainage system is the drainage fixture units, or dfu. The definition of dfu indicated in 709.3 is often used to convert from a known gallons per minute to dfu. This conversion is used in discharge waste to a drainage pump system, where 1 gpm = 2 dfu.

Section 710: Drainage system sizing

  • To size the horizontal pipe branches and vertical building mains the total fixture count for the building must be known. Two tables are important in this section. Table 710.1(1) provides the maximum allowable dfu for the main at a given pipe size and slope. While Table 710.1(2) provides the maximum allowable number of dfu for the horizontal branches for waste and stacks.
  • For example, from Table 710.1(1) a building drain or sewer of 3-inch pipe size sloped at ¼ inch carries a maximum load of 42 dfu. If the slope is increased to ½ inch, the capacity increases slightly to 50 dfu. The reader may refer to these tables under the code for additional values.

Chapter 9: Vents

Waste or drainage systems require vents to allow air to flow throughout the drainage system and sewer gases to exit the system and prevent accumulation of unpleasant odors. There are several options when designing a vent system to a sanitary system. A few will be explained here; for more detail, review the IPC.

  • Section 911: Common vent: This is the most common design for a vent serving a plumbing system consisting of a shared vertical vent riser for one or two fixtures in the same level. This is typically seen in small single-level buildings where it is more cost-effective to run through the roof in lieu of across the structure to combine the vents with branches (see Figure 2).
  • Section 913: Waste stack vent: This venting method is considered when all fixtures are discharging into a waste stack. This is a common and practical design when the building is a high-rise, in this scenario the restrooms are stacked one above the other with no offsets or shifted slightly. The waste stack is vertical, with prohibited tie-ins between the highest or lowest drain connection from other fixtures on the horizontal.

Figure 3: At left, a common vent is represented with two fixtures at the same level. Riser at right represents a waste stack vent, typically useful for a high-rise with stacked restrooms. Courtesy: WSP USA Buildings

Chapter 11: Stormwater plumbing

Storm drainage systems are designed to capture rainwater and dispose of it through main pipes. These mains run from the roof vertically down to an approved location outside the building, similar to the sanitary drains. The connection locations are coordinated with the civil engineer for tie-in to the city main storm system.

To calculate the size of the storm mains, the engineer must first calculate the area of the roof in question and the vertical wall area of any adjacent building, if applicable. In this case 50% of the wall area will splash water to the lower roof drain; therefore, it must be considered. The IPC tables are based on rainfall per 100-year event during a one-hour rainfall in inches for each location. Sections under Chapter 11 worth highlighting:

  • Section 1101.7: Indicates the amount of water to be collected within a roof deck considering the various levels. For this reason, there is a requirement for a primary drain and a secondary drain, refer as the overflow.
  • Section 1102.4: Building storm sewer pipe: Indicates a table with the approved piping materials for this scope. The most common in piping material for plumbing systems is hubless cast-iron, which conform with Cast Iron Soil Pipe Institute.
  • Section 1106.4 Vertical walls. For example, when calculating the main storm riser of a roof with vertical wall adjacency, if the design includes a lower roof of areas of 30 feet x 30 feet plus a vertical wall of area 15 feet x 15 feet, then the total area for stormwater collection would be:(30 feet x 30 feet) + [(15 feet x 15 feet) / 2] = 1,012.5 square feet

Appendix B of the code lists several rainfall rates in inches per hours for multiple states. For example, in Dallas the rainfall is 4 inches per hour. However, a local amendment for Dallas uses a rate of 6 inches per hour; therefore, verify the city requirements that provide criteria beyond the code.

With the area to be drained known, Table 1106.2 and 1106.3 provide pipe sizes for roof main depending on the desired slope   indicating the maximum gpm for a given pipe size.

Many municipalities have more stringent design storm requirements, it is recommended that the engineer research local code amendments adopted by the state in question prior to starting the design.


Author Bio: Paulina Diaz is a senior associate at WSP USA Buildings. She is a mechanical engineer with more than 10 years of experience and is focused on health care design.