What are plumbing vents and why worry about plumbing venting?
When we use the plumbing in our homes we do not think about how it works, nor do we wonder if it is safe and sanitary.
From the beginning of indoor plumbing the various plumbing codes have been being formulated. Through training, enforcement, and continuing education the evolving codes have been administered. The plumbing profession has done such a fine job of keeping us safe we are free to be aloof. Free to use the plumbing in our homes without worry or consideration.
It is this fact above all else that makes the simple physics of plumbing vents seem so arcane. The understanding of internal combustion is common knowledge, as complex as it is, yet the awareness of plumbing venting and its importance is rare. Your plumbing system, like your engine, operates according to the physical laws. When there is something wrong with your engine it may not take you where you want to go. When there is something wrong with your plumbing you may become ill. You may not know right away that you are being made ill and you may not know why till you are quite ill.
With the advent of easy to purchase and install plumbing materials more and more people are engaged in DIY plumbing. Many people in recent times took advantage of the booming real estate market to purchase remodel and resell homes. Many came to the task unaware of the importance of plumbing code and the potential dire consequences of ignoring it. Municipalities were hard pressed to keep track of this process. Today there are many homes with substandard plumbing systems, I see aberrations regularly. The occupants of these homes often have no idea there is anything amiss.
The common opinion is that plumbing is all about the channeling of fluids, the supplying of clean water and the disposal of tainted water. For the most part no thought is given whatsoever to the danger of tainted air. Plumbing vents are specifically designed to channel air. Air must precede and follow water moving in a pipe. The plumber must always maintain the free movement of air within the plumbing vents. Never though must that air be allowed to invade the living and working spaces of buildings.
Even a system that is properly installed is not fool proof. The disuse of a fixture, allowing the "trap seal", water in a p-trap, to evaporate is as much a problem as a poor installation. The solution is knowledge. It is important for the DIY installer and home owner to understand how and why plumbing systems work.
The plumbing code is not easy to understand. It has a language of its own and that makes it difficult to access. Only recently has our Oregon UPC code book been graced with its first index page. It is not easy to know the reasons for the rules given in the code. They are stated, the reasons are the stuff of apprenticeship, code review, and continuing education. At this juncture I must refer you to my page, Mechanics rules, There I recommend that you never break a rule you do not fully understand. There is no better place than plumbing to adhere to this maxim.
Free movement of air
As I stated above, in order for fluid to travel in a pipe the air before it must give way and the air behind it must follow. In the strictest sense this is not true at all, but we will get to that momentarily. The plumbing vents we are discussing here are not the vents of a home or building but all of the vents in all of the structures on any branch of the sewer system.
Consider this, let us say that a large quantity of water is flowing down stream in an under street sewer pipe. Farther down stream is a residence with insufficient plumbing venting, and beyond that no other structure. The air before the water in the sewer will move easily toward the down stream residence, encouraged by the easily flowing air behind it. In the downstream residence that has insufficient venting the air being pushed by the large flow of water will be forced out of the sewer and into the rooms of the residence. This will happen because the water in the fixture traps of the residence will be pushed out of the way by the force of the sewer gasses that would normally exit the system at that homes roof vents.
I might add at this juncture that many plumbers do not like the new AAVs, (air admittance valves), if for no other reason that they only do that, admit air. One should make certain that there is enough actual roof venting to allow drain side pressures from overwhelming p-trap trap seal before employing AAVs instead of roof vents.
On my page, How Does A Toilet Work? I state,
The main function of the P-trap in the toilet bowl, or any other p-trap, is to hold back the admittance of sewer gas into the room the fixture is placed in. It does this by being full of fluid. Under normal pressure sewer gas will not push the water out of its way and escape the drain. The toilet bowl is sealed against it by virtue of the water in it. For this reason the water in the trap way is called a "trap seal".
When I referred to normal pressure there I was considering the issue described above on this page because the job of the venting system is to maintain that normal pressure. If it is inadequate to the task the water in the trap way of that toilet will indeed be pushed aside and sewer gas will indeed spill into the room.
Aggregate cross sectional area.
The way a venting system maintains normal pressure is by having the required Aggregate cross sectional area. That is, according to the venting chapter of the plumbing code, the area of the accumulated vents of a building must be equal to the required drain size for that building. That size is determined by the type and number of fixtures as per the plumbing code in the chapter on drainage. If these established rules are followed no incursion of sewer will occur as the vents will allow the free movement of air both up and down stream.
The proper term for the water in a p-trap is "Trap seal". The proper term for a fixture vent is "Trap seal protection".
Self scouring action
Now we come to the erroneous notion that air must move before water flowing in a pipe. Of course the water will move, even if it is in a sealed tube. The air before it will separate the water and end up behind it. It is that separation we want to prevent. The water we are dealing with is not simple water, but water that is baring away solids. When that water is dissipated by air trapped in its way the solids have an opportunity to adhere to the walls of the pipe where they may accumulate.
Self scouring action, water flowing down stream with integrity, is the goal of the plumber. It is reached by sizing pipes appropriately for the fixtures they serve and installing plumbing vents in a way that maintains self scouring action through free movement of air. This of course is all done by simply adhering to the rules set forth in the plumbing code.