Written by IAPMO 3:41 pm Uniform Codes Spotlight


August 24

From the 2021 UPC Illustrated Training Manual, Chapter 5, WATER HEATERS

510.2.4 Vent Offsets. Where the common vertical vent is offset, the maximum capacity of the common vent shall be reduced in accordance with Section 510.2.5, and the horizontal length of the common vent offset shall not exceed 18 inches per inch (18 mm/mm) of common vent diameter (D). Where multiple offsets occur in a common vent, the total horizontal length of all offsets combined shall not exceed 18 inches per inch (18 mm/mm) of the common vent diameter. [NFPA 54:13.2.5]

Vent offsets, illustrated in Figure 510.2.4, typically occur high in the venting system and tend to reduce the draft produced because they slow the flow of vent gases. This reduction in draft is offset by the requirement for a 10-percent capacity reduction required in Section 510.2.5.

510.2.5 Elbows in Vents. For each elbow up to and including 45 degrees (0.79 rad) in the common vent, the maximum common vent capacity listed in the venting tables shall be reduced by 5 percent. For each elbow greater than 45 degrees (0.79 rad) up to and including 90 degrees (1.57 rad), the maximum common vent capacity listed in the venting tables shall be reduced by 10 percent. [NFPA 54:13.2.6]

The sizing requirements were designed with the assumption that up to two 90-degree turns were part of the venting system, except for the zero lateral length case. The zero lateral case was assumed to extend straight from the appliance outlet to the vent termination. Adding additional elbows to the system is possible.

The requirements for derating the table capacities for elbows were revised to recognize elbows of less than 90 degrees. The new derating factors are shown in Commentary Table – 4.

Previously, a 10-percent reduction was required for each elbow, regardless of its angle. This revision, which was based on input from the Gas Technology Institute’s contractor, Battelle, provides flexibility to installers forced to use vent offsets and encourages the use of elbows of less than 90 degrees. See Figure 510.2.5. When sizing a common vent with elbows, please note the following important factors:

  • The tables include two 90-degree elbows. Do not derate the common vent table values for venting systems with one or two elbows.
  • For each additional elbow over two in the common vent, derate the common vent table value by 5 or 10 percent, depending on the angle of the elbow.

The 2021 Uniform Plumbing Code Illustrated Training Manual is available for purchase here.

(This is not to be considered the official position of IAPMO, nor is it an official interpretation of the Codes.)


IAPMO develops and publishes the Uniform Plumbing Code®, the most widely recognized code of practice used by the plumbing industry worldwide; Uniform Mechanical Code®; Uniform Swimming Pool, Spa and Hot Tub Code®; and Uniform Solar Energy, Hydronics and Geothermal Code — the only plumbing, mechanical, solar energy and swimming pool codes designated by ANSI as American National Standards — and the Water Efficiency Standard (WE-Stand)™. IAPMO works with government, contractors, labor force, and manufacturers to produce product standards, technical manuals, personnel certification/educational programs and additional resources in order to meet the ever-evolving demands of the industry in protecting public health and safety.

Last modified: August 24, 2023