Designing a water distribution system and water pipe sizing is a crucial task in any construction or renovation project. The 2021 International Plumbing Code (IPC) is a vital resource that provides guidelines for ensuring the proper sizing and installation of water piping systems. This blog post will offer a detailed explanation of how to size water pipe and distribution system following the IPC, focusing on allowable pipe materials, fixture units, pressure requirements, water hammer arrestor placements, allowable velocities, minimum pipe sizes, and how to use the IPC’s Appendix E.

Understanding Water Distribution Systems

A water distribution system is a network of pipes, valves, and other components that transport water from a central supply point to various fixtures within a building, such as faucets, showers, toilets, and appliances. A well-designed system ensures that water is efficiently and safely delivered to all fixtures while maintaining adequate pressure and flow rates. Properly sizing the pipes in a water distribution system is crucial to achieve optimal performance and prevent issues like low water pressure, water hammer, or pipe erosion.

Allowable Pipe Materials

The 2021 IPC outlines various materials that can be used for water distribution systems, including:

a. Copper or copper-alloy tubing (Type K, Type L, Type M, and DWV)

c. Chlorinated polyvinyl chloride (CPVC)

b. Cross-linked polyethylene (PEX)

d. Galvanized steel

e. Polybutylene (PB) f. Polypropylene (PP) (Note: This material is no longer widely used due to concerns about its durability and has been replaced by PEX in many applications.)

g. Stainless steel

Each material has its advantages and limitations. When selecting a pipe material, consider factors like cost, ease of installation, corrosion resistance, and local code requirements.  The most common pipe type for commercial is Copper and the most common pipe type for residential is PEX or Copper.

Determining Water Supply Fixture Units

Water supply fixture units (WSFU) are a measurement of the load a fixture places on the water distribution system. The 2021 IPC Table E103.2(2) assigns a WSFU value to various fixture types based on their water demand. For example, a water closet with a flushometer valve has a WSFU value of 10, while a lavatory has a WSFU value of 1.5. By adding up the WSFU values of all the fixtures in the system, you can determine the total water demand.

Required Pressure for Different Fixture Types

The IPC sets minimum and maximum pressure requirements for different fixtures to ensure they function correctly. Generally, water pressure should be between 20 psi (pounds per square inch) and 80 psi. Some fixtures, like flushometer valves and certain types of faucets, may require higher pressure. Consult the IPC Table 604.3 and the fixture manufacturer’s guidelines to determine the appropriate pressure for each fixture type.

Water Hammer Arrestor Placement

Water hammer arrestors are devices installed in water distribution systems to prevent water hammer, a sudden increase in pressure that occurs when a valve or faucet is closed quickly. The 2018 IPC requires water hammer arrestors to be installed in specific locations, including:

a. Quick-closing valves (e.g., solenoid valves and flushometer valves)

b. Appliances that use quick-closing valves, like dishwashers and washing machines

c. Branch lines with multiple quick-closing valves

Consult the IPC for more information on the sizing and placement of water hammer arrestors.

Allowable Velocity

Pipe velocity is the speed at which water flows through a pipe. The IPC sets maximum velocity limits for different pipe materials to prevent issues like erosion, noise, and water hammer. The maximum velocity for copper, galvanized steel, and stainless steel pipes is 8 feet per second (fps), while the maximum velocity for PEX, CPVC, PB, and PP pipes is 5 fps.

Minimum Pipe Size

The 2021 IPC Table 604.5 establishes minimum pipe sizes for different fixtures and appliances to ensure adequate water flow and pressure. For example, a water closet with a flushometer valve requires a minimum pipe size of 1 inch, while a lavatory requires a minimum pipe size of 1/2 inch. The minimum pipe size for a urinal flush valve is also 3/4 inch. Keep in mind that these are minimum requirements, and larger pipe sizes may be necessary to maintain proper pressure and flow rates in larger or more complex systems.

Using Tables in Appendix E to Convert WSFU to GPM

Once you have determined the total WSFU for your system, you can use the tables in Appendix E of the IPC to convert WSFU to gallons per minute (GPM), a more practical measure of water flow. Appendix E contains several tables that correspond to different incoming water pressures. Locate the table that matches your incoming pressure, and find the row that contains your total WSFU value. The corresponding GPM value in the table is the minimum flow rate required for your system.

Selecting Pipe Size Using Figures in Appendix E

After determining the required GPM for your system, use the pipe-sizing figures in Appendix E of the IPC to select the appropriate pipe sizes. These figures are charts that indicate the maximum GPM a pipe can handle based on its size and material. Locate the figure for your chosen pipe material, and find the pipe size that can accommodate your required GPM while staying within the allowable velocity limits. Be sure to consider pressure losses due to friction and elevation changes when sizing your pipes.

Example: Sizing a Pipe for a Restroom

Let’s walk through an example of sizing a pipe for a restroom with four water closets with flushometer valves, two lavatories, and one urinal. Assume an incoming pressure of 60 psi.

a. Determine WSFU:

• 4 water closets x 10 WSFU each = 40 WSFU
• 2 lavatories x 1.5 WSFU each = 3 WSFU
• 1 urinal x 2 WSFU = 2 WSFU
• Total WSFU = 40 + 3 + 2 = 45 WSFU

b. Convert WSFU to GPM using Appendix E: Using the table for 60 psi incoming pressure, find the row with 45 WSFU. The corresponding GPM value is approximately 48 GPM.

c. Select the pipe size using the figures in Appendix E: Assuming you’re using copper pipes, find the pipe size in the copper pipe-sizing figure that can handle 22 GPM while staying within the allowable 8 fps velocity limit. In this case, a 2-inch pipe would be suitable.

In conclusion, a 2-inch copper pipe would be appropriate for this restroom example, considering the water demand, incoming pressure, and allowable velocity. Keep in mind that this example is simplified and does not take into account factors like pressure losses due to friction and elevation changes. Always consult the IPC and local code requirements when designing and sizing water distribution systems.

Example: Sizing a Pipe for a Residential Kitchen

Consider a residential kitchen with one sink and a dishwasher. Assume an incoming pressure of 60 psi.

a. Determine WSFU:

1 kitchen sink x 1 WSFU = 1 WSFU 1 dishwasher x 1.4 WSFU = 1.4 WSFU Total WSFU = 1 + 1.4 = 2.4 WSFU

b. Convert WSFU to GPM using Appendix E: Using the table for 60 psi incoming pressure, find the row with 2.4 WSFU. The corresponding GPM value is approximately 5 GPM.

c. Select the pipe size using the figures in Appendix E: Assuming you’re using PEX pipes, find the pipe size in the PEX pipe-sizing figure that can handle 5 GPM while staying within the allowable 5 fps velocity limit. In this case, a 3/4-inch pipe would be suitable.

Conclusion

By following the guidelines and recommendations in the 2021 International Plumbing Code, you can properly size water pipe and distribution systems for various applications, ensuring optimal performance and compliance with building codes. A well-designed system with water piping sizing according to the IPC will provide adequate water flow and pressure to all fixtures while minimizing the risk of issues like low water pressure, water hammer, and pipe erosion.

If you want to learn more about the plumbing code you can access it here: 2021 International Plumbing Code

Also, we have found ways to access most codes online for free. They are all summarized here: Free Building Codes!

Lastly, if you’re jurisdiction is still using the 2018 codes we have you covered. Here our guide to Water Pipe Sizing: A Guide Based on the 2018 International Plumbing Code