We all agree that we need to vent a fixture trap to protect the trap from siphoning and allowing sewer gas into the building. Well, maybe we don’t all agree that every trap needs to have a vent (I think they do). There is some room for argument here.
Several types of venting systems are used and accepted in most places. I know of several locations that use different types of venting systems, such as the Philadelphia Single Stack system. The venting systems you’re familiar with may or may not be readily accepted, depending on your geographic location.
One time I was designing a building and the contractor called me and said he didn’t use that type of venting system and it was not allowed by code in his area. That was interesting because I had designed around a common venting method, the individual dry vent system.
I generally provide a vent for each fixture or back-to-back fixtures (a common vent), which gives you the most protection for your traps. Anyway, the venting system being proposed by the caller was a wet venting system, which in certain instances makes good sense to use. It was not, however, appropriate in the type of building with which we were dealing.
Later I received a phone call from someone else in the contractor’s office and we agreed on the dry venting system as designed.
The interesting part of the first phone call was the claim about individual dry vents not being allowed by code. Sorry, but the individual dry venting system has been around for a very, very long time. As far as I know it is part of every plumbing code.
Most plumbing codes allow for special venting situations. Whenever you meet the criteria listed in the plumbing code for a special venting system, it can save some money for the owner.
Plumbing codes differ in what they allow, so before you set your mind to using one of the venting systems I’m about to describe, check your plumbing code to see what the limitations are in your area. Common venting, stack venting, combination waste and vent systems, circuit venting and loop venting are all different systems that have appropriate applications.
Common venting is the simplest and is used where a common vent protects two traps. Generally the fixtures are located either back-to-back or side-by-side. Waste connections need to be made at the same elevation and drain into a common vertical waste pipe unless the waste connections are different sizes.
Then other connections are necessary.
Stack venting is exactly that. A vent extends from the waste stack and is then vented to atmosphere. Connections from other vents can be, and usually are, made to the stack vent.
Circuit venting is used to vent a battery of fixtures. Some localities have stricter regulations on when and where circuit venting can be used, so make sure you check. The vent is connected in between the last two fixtures on the drainage branch and then connected to a vent stack.
A vent stack is a vertical vent pipe installed for the primary purpose of providing circulation air to and from any portion of the drainage system. It is common to have vent stacks (which do not carry any drainage) in multistory buildings. Don’t confuse it with a stack vent, which is the vent extension of the waste stack.
Other fixtures may be connected to the same drainage piping that is protected by the circuit vent, but those fixtures may need to be individually vented.
Loop venting is a form of circuit venting, except it’s installed on the topmost branch of the building drain and is connected to the stack vent.
Both circuit and loop venting, properly applied, can save money on projects.
Combination waste and vent systems are useful for floor, hub or shower drains, where the amount of drainage expected is not great and the ability to provide a conventional venting system is not practical.
Basically, the waste piping is oversized to allow more air to move through the upper portion of the pipe and is then vented on both ends to accommodate both positive and negative pressures in the system. Industrial facilities, grocery stores and similar occupancies can benefit from combination waste and vent systems.
Finally, some codes permit air admittance valves, which allow air to be drawn into the drainage piping and keep the traps from being siphoned. One word of caution: You must follow the manufacturer’s guidelines if you use these. There is an application for every product and this one is no exception. Don’t think you can place them indiscriminately.
Plumbing systems operate on both positive and negative pressures in the venting system. You should understand the benefits and limitations of all the products you install.