Codes and standards are intended to provide a minimum level of protection for the public’s health, safety and welfare. And, generally, these documents accomplish that intended purpose in the urban/metropolitan areas of the United States. However, that may not be accurate when one gets into the rural or more isolated areas of the country. It also requires that design professionals have the appropriate knowledge of fittings and flow patterns to assure the system works as intended.

Recently there was a post on the ASPE Connect “Open Forum” regarding “Circuit Venting — Horizontal vs. Vertical Connections.” The correspondent was trying to get a better understanding of how the circuit or loop venting system operated. They had done some research in the IPC (International Plumbing Code), UPC (Uniform Plumbing Code) and the NSPS (National Standard Plumbing Code) to gain knowledge about how these venting methods correctly worked.

Circuit or Loop venting is a specific style of venting with strict code requirements. Circuit venting is found in Section 911 of the OPC, again based on the IPC. This method of vent was popular in larger restrooms, as it could minimize the piping material and the number of individual vents for certain type of fixtures. However, with the development of commercial fixture carrier fittings, its use has been greatly reduced. The use of the circuit vent system eliminated the need to individually vent each fixture that was allowed to waste into the system. And, yes, carrier fittings do involve the individual venting of each water closet.

However, in theory, one does not need to use those individual vent taps on the carrier fitting if one desires to utilize the circuit vent system. The horizontal waste line created by the carrier fittings provides the bottom half of the circuit vent system. The vent portion can be created by appropriately placing fitting into the waste system and routing the vent portion above the carrier fittings. But, in my judgment, this just complicates the installation, and to some extent, defeats the concept for using commercial carrier fittings.

As part of the inquiry, the following image was provided to show an existing installation of a circuit venting system.

The first thing that caught my eye was the “sanitary tees” being used incorrectly. As described in the discussion, the vertical risers served individual water closets with a vent connection shown between the last two water closets.

A “sanitary tee” is a fitting designed in such a manner that side inlet has a slight downward pitch to direct the flow. This type of fitting, according to code, may only be installed in the horizontal to vertical position in the direction of flow. In other words, the side inlet of the flow must be in the horizontal position with the flow through the main body of the fitting always in the vertical position. This style of fitting shall not be installed in the vertical to horizontal position or in the horizontal to horizontal position. As pictured in the image, the fitting is installed with the side inlet in the vertical position and the main flow through in the horizontal, bad flow pattern. According to Section 706.3 of the Ohio Plumbing Code (OPC; the code I am most familiar with), taken from the IPC, “Installation of fittings” states; fittings shall be installed to guide sewage and waste in the direction of flow. Changes in direction shall be made by fittings listed in Table 706.3 (not shown).

While a sanitary tee does have a slight directional pattern to direct flow, it is only intended to direct that flow from the horizontal to the vertical “stack.” The installation position shown in the image results in the flow “slamming” into the bottom of the horizontal pipe and flowing in both directions. Additionally, dumping the waste in from the top of the pipe disturbs and blocks the “vent air” that should occupy the top half of the horizontal pipe.

In the case of the “circuit vent” system, it is important to maintain the “vent air” in the top of the pipe; hence all connections should be from the side using a wye and 1/8-bend or combination fitting arrangement. The risers seen in the image are the extensions from the closet rings on which the water closets were set. Try to imagine yourself flowing head first down the waste piping, you do want to flow a smoothly as possible; dropping straight down into the horizontal pipe would not allow you that smooth flow and create turbulence in the vent air and waste stream.

Using a sanitary tee, as shown in the image, blocks the uniform flow of the waste and restricts the flow of vent air. It will result in “splash,” where the flow goes in both directions and blocks the vent air momentarily.

It all comes down to the appropriate selection and installation of the various fittings that can maintain a uniform flow with minimal disturbance to the flow patterns of the waste stream and vent air. Using a sanitary tee, as shown in the image, blocks the uniform flow of the waste and restricts the flow of vent air. It will result in “splash,” where the flow goes in both directions and blocks the vent air momentarily. It is similar to the “hydraulic jump” that occurs at the base of a stack when a branch is connected too close to the base of that stack. The “hydraulic jump” blocks the vent air from passing through the line momentarily, causing a “wave” to form along the waste stream in the direction of flow. While this condition may be short-lived, it is not desirable for good design and acceptable operation of the plumbing system.

Plumbing relies on maintaining vent and waste space within the piping. In the horizontal waste line, air occupies the top half of the pipe while wastes occupy the bottom half. Plumbing is a half-full, half-empty type of design. While in the vertical waste line, the waste adheres to the piping wall due to frictional forces and vent air occupies the “core” of the pipe with the waste swirling around that core of vent air.

The critical areas are where a change in direction occurs — the base of the stack, side inlets, offsets, etc. In these areas, the waste and vent air can become turbulent and intermixed. These areas cause violent pressure fluctuations to occur. And within a plumbing system, one needs to maintain those fluctuations within the 2” water column used for the design of a trap seal.

So we need to use the appropriate fitting and correctly locate vents along the piping where turbulent conditions might occur. This does not mean that the waste will not eventually flow through the system and discharge as desired. Nor does it mean that even under proper design and installation, there may be no momentary disruption of vent air within a drainage system. But it can result in permitting sewer gases to enter habitable spaces because of those pressure fluctuations. And keeping sewer gases within the system and out of the habitable spaces is the main purpose of the plumbing code.