Down the rabbit hole: Small code provisions can lead to a much larger picture
Sometimes, little things carry really big implications. We have all probably heard the phrase “snowball effect” to describe a situation in which something small develops into something much larger through a series of events. Kind of like how that brownish-orange spot on the ceiling can end up with holes in drywall, new pipes, wet carpet and a much poorer homeowner. In the codes, there are sometimes sections that at first appear straightforward and relatively benign, but once you take some time to consider everything that comes along with the simple code section, you realize it really does open a sizable can of worms.
Section 901.2 of the 2015 UPC is one of those sections. Simply titled, “Vents Required,” this section states “Each plumbing fixture trap, except as otherwise provided in this code, shall be protected against siphonage and back pressure, and air circulation shall be ensured throughout all parts of the drainage system by means of vent pipes installed in accordance with the requirements of this chapter and as otherwise required by this code.” This seems like a fairly simple requirement, but when you consider everything that goes along with it, you realize meeting the requirement requires a lot more thought, planning and understanding than the words of the section would imply.
The principle of ventilating the drainage system to retain trap seals has been the basis of modern plumbing systems since the end of the 19th century. Prior to the 20th century, plumbing fixtures were rarely used inside the building. The protection of the trap and its trap seal by a vent has allowed the plumbing fixture to be placed safely inside the building. Without a vent, the trap seal is subject to siphonage or back pressure, which would allow sewer gas to enter the building or room. The consequences of sewer gas escaping the drainage system and entering the room can be lethal, as was proven in the Severe Acute Respiratory Syndrome epidemic reported in Hong Kong in 2003, where 65 deaths and 321 infected people resulted from the virus being transmitted, in part, through dry traps.
Siphonage can occur in two ways. It occurs when there is sufficient negative pressure (aspiration or negative pressure below atmospheric) to draw water out of the fixture trap. A negative pressure occurs in a drainage stack as water rapidly flows downward, drawing air into itself (entrainment). If the sheet of water flows past an unvented branch connecting to the stack, a negative pressure will also be produced in the branch, affecting any trap seals in the branch. If the negative pressure is great enough, then a siphon will occur. Also, a negative pressure occurs in the drain when a fixture is discharged. The discharging fixture may siphon an adjacent fixture trap that is not vented. For these reasons, venting is required to relieve any aspiration that may affect a trap seal.
Siphonage also may occur by a fixture siphoning itself at the end of its discharge. This is called self-siphonage. According to “Recommended Minimum Requirements for Plumbing” (BH13) published by the National Bureau of Standards, self-siphonage occurs when there is an unbalanced water column in an inverted U shape caused by the P-trap as seen in the common siphon.
Self-siphonage is also caused by the fluid velocity, or the momentum pull, exerted by the moving column of water, whether the pipe is horizontal, vertical or inclined. In a horizontal drain from the trap, the momentum pull in its full intensity is exerted at the end of the discharge. Therefore, such configurations are prohibited without the fixture traps properly vented to prevent self-siphonage.
Back pressure occurs when there is positive air pressure (an increase or positive pressure above atmospheric) in the drainage system. As water flows down a stack drawing air along with it, it also pushes air ahead of itself. The air volume will flow through the building drain and dissipate into the sewer drainage system unless there is a blockage in the air path, such as when the building drain is full or when there is a building trap or a backwater valve. Such blockages in the air path generate positive pressures and may have an adverse effect on trap seals. This back pressure on the outlet side of the trap could push air bubbles through the trap way releasing sewer gas into the living space and, if significant enough, could even blow the water out of the trap way.
A particular blockage or closure in the air path is the hydraulic jump. A hydraulic jump is a flow condition where the flow rate becomes greater than the drain capacity, causing the depth of the flow to be greater than the pipe diameter. This sudden rise is called a jump and may completely block air flow. Hydraulic jumps may occur at horizontal junctions where one branch flows into another and at the base of drainage stacks. When the hydraulic jump completely fills the drain, positive air pressure waves will be generated, and if they last long enough then trap seals in close proximity may be affected. Back pressures generated by hydraulic jumps must be relieved by the venting system to protect trap seals.
Section 901.2 also requires air circulation be ensured throughout all parts of the drainage system by means of vent pipes. This statement means air must circulate throughout the drainage system including the building drain, building sewer and public sewer. The basic principles of venting ensure the equalization of atmospheric pressure on both sides of the trap seal, within plus or minus a one-inch water column, will be provided at all times and in all parts of the drainage system.
As already mentioned, building traps block air circulation to the building drain and sewer, and therefore are prohibited in Section 1008.1 except where allowed by the authority having jurisdiction. An example of where building traps are used is the combined sewer and storm drainage. The reason the building trap is not used extensively in the Uniform Plumbing Code is because the storm system is intended to be separate from the sewer system. Sewer manholes are sealed — unlike storm system manholes — and do not provide for air circulation in the sewer system. Therefore, the circulation of air for the public sewer, the building sewer and the building drainage system must come from the vents in the building alone. In Section 1008.1, the building trap is allowed but will require a relieving vent to provide for the circulation of air upstream of the building trap, and in Section 904.1 it will require another vent downstream the building trap to allow circulation of air to the public sewer or private sewage disposal system.
So, what started out as a section that in its most simplistic interpretation said, “Traps must be vented,” ended up opening the door for quite a few requirements and a broader understanding of the basic hydrodynamics at play in venting systems. Whether you prefer the phrase, “mountain out of a molehill,” “the snowball effect,” or “opening Pandora’s Box,” things in the code — and in life — are not always as simple as they may appear at first glance.