A cure for hyperventilation
By R. David Eagan, CPD, CPE
Chapter 9 of the 2015 International Plumbing Code (IPC-2015) offers us a wealth of ways to vent a fixture or combination of fixtures. Among the venting methods available are: individual, common, wet, waste stack, circuit, island fixture vent, and one other approach which is the topic of this article, the Combination Waste and Vent System.
First, here is a little background information about me, and why this subject is of special interest to me. In addition to my design work, I am an estimator for a mechanical contractor. In this capacity, I have seen the work of countless plumbing engineers and designers over the decades. But of the thousands of plumbing drawings I’ve pored over, rarely have I encountered what I consider to be the simplest and most efficient method available to the plumbing design discipline — the Combination Waste and Vent System.
Granted, it can only be used to vent floor drains, floor sinks, sinks (including service sinks and mop basins), lavatories and drinking fountains, but that’s a large variety of fixtures. The only specific types of those fixtures that can’t be included in a Combination Waste and Vent System are sinks with food waste disposers and clinical service sinks — i.e., those that might introduce solids into the piping and thus disrupt the relatively smooth flow of water and air within the piping. But in those cases where this method can be employed, the venting is greatly simplified. Oftentimes, hundreds or even thousands of feet of pipe are eliminated, roof penetrations are minimized and valuable above-ceiling space is opened up for ductwork and other building services.
So, what venting method is the most common? Individual venting, almost exclusively. Water closet and lavatory side-by-side? Two vents. Add a floor drain? We’ll need another vent. Four water closets, two urinals and four lavatories? Ten vents. You get the picture.
I rarely run across even the simple circuit vent or waste stack vent (i.e., a vertical Combination Waste and Vent). Compare the Combination Caste and Vent, which always just has one vent. Why are these less efficient venting methods so common? Truth is it’s quicker to just draw a vent on everything than it is to spend the time to design a system. But, it goes deeper than that. My pet theory is that the engineer or designer thinks, consciously or subconsciously, “The more vents, the better the system will work,” and “Cost is not really my concern.”
Both thoughts are misguided. Adding an unneeded vent (or vents) does nothing but complicate design, waste resources, create one more potential maintenance issue and drive up the cost of the project. It is a misuse of money that could be spent on amenities. In an industry that is increasingly conscious of sustainability, there are no surer means of conservation than the unmined ore and uncast pipe that go unused in the making of uninstalled pipe. Perhaps the code should include a requirement in Chapter 3, “General Regulations,” that systems be designed and installed in the simplest manner possible. That may be difficult to enforce, but such a provision would at least bring the issue to the forefront of every design.
How does the Combination Waste and Vent work? The only limitations on the designer, other than the types of fixtures mentioned above, are:
- The piping must be sized per Table 915.2.2, (or oversized, at least in theory, but in practice we designers tend to oversize piping anyway).
- The horizontal piping within the system is limited to a maximum slope of 4 percent.
- The vertical piping within the system is limited to a height of no greater than eight feet (to prevent hydraulic jump at the base of the vertical pipe).
These constraints conspire to limit the flow volume, velocity and turbulence within the system so that an airway above the flow line is unobstructed throughout. When properly sized and installed, the horizontal length of any segment within the system is “unlimited” (IPC-2015, ¶915.2.2). This is because each segment serves as both waste and vent for any fixtures connected to that segment.
Why is the Combination Waste and Vent not used more often? This method can be used on one or 100 fixtures, as long as they are the proper type. But, as I mentioned, the mindset of designers is: one fixture, one vent. This thinking has to change. There is also the dogged adherence to the maximum fixture-to-vent distances in Table 909.1. With the Combination Waste and Vent, those distances don’t apply. And then, there are the code officials — the plans examiners and inspectors. They are a conservative bunch, and anything out of the ordinary raises a red flag. As one of my fellow designers said of an article I wrote on this subject in our ASPE chapter newsletter, “That’s great, but I’ll never get it past the plans examiner!”
What’s the solution to that hurdle? Education and exposure to the method. The code officials often graciously assist us in understanding and applying the code correctly, and this is a chance for us to return the favor. Don’t shy away from an out-of-the-ordinary design; approach it as a challenge and an opportunity.
There are a few caveats that I would like to see addressed in future code requirements for the Combination Waste and Vent. Although the Code stipulates that the horizontal length of such a system “shall be unlimited” (IPC-2015, ¶915.2.2), the Code elsewhere acknowledges that the lengths of vents affect their performance, and thus require that longer vents be enlarged accordingly (IPC-2015, ¶906.2) to overcome the effects of friction loss on the flow of air (i.e., a fluid) within the system. Consideration should be given to the same effect in Combination Waste and Vent systems.
Numerous graphic examples of the allowable vertical segments of combination waste and vents show piping connecting to the top of the horizontal portion of the system. This disrupts the airway, and could be eliminated if all connections were required to be in the horizontal as is the case with the circuit vent (IPC-2015, ¶914.1.1).
Connecting “to a horizontal drain that serves vented fixtures” (IPC-2015, ¶915.2.3) as an alternative to the dry vent is too vague to be of much guidance to the designer. What if the horizontal drain is a circuit vented branch that already has eight fixtures connected? It is already at its maximum capacity past the eighth fixture — how can it then also vent a combination waste and vent system that could include dozens of fixtures? What if the vented drain serves water closets? The vented drain becomes part of the Combination Waste and Vent and would include prohibited fixtures.
Another point of caution is that if fixtures are added in the future to your system, it may no longer comply with code (e.g., if in a future renovation, a water closet is connected within your system). Look ahead!
By way of example, such a venting method proved to be the perfect scheme for a design-build project our company recently completed in Tennessee. It was an expansion to a food processing facility, and our work consisted primarily of adding hand sinks and floor drain serving mixers. This was a large, open floor space without walls that were nearby to house or conceal vents. Those of you who have experience with food processing plants understand that any exposed piping above slab must be easily and frequently cleaned. The less vent piping above slab to clean, the better. And, no vent piping above slab is best of all. Alas, even this system has to have at least one “dry vent connected at any point within the system” (IPC-2015 ¶915.2.3). But, we were able to locate that one vent in an adjacent mechanical room that is not part of the processing area and thus not subject to strict cleanliness procedures.
I have included a floor plan and an isometric drawing that illustrate the project and the simplicity of the Combination Waste and Vent System solution. For the sake of clarity, hand sinks are sized as lavatories and have a Drainage Fixture Unit (DFU) value of one each (IPC-2015, Table 709.1). All floor drains and floor sinks are three inches, and have a DFU value of five each (IPC-2015, Table 709.2). The vent is sized in accordance with IPC-2015, ¶906.2.
Examine the drawings and think for a moment how many vents you would normally expect to find on an arrangement of this type and where you would place them. Then, see what the code says is adequate. It may surprise you!