Horizontal Drainage Is Wimpy
If you review this section in the IPC, you will find it is the most liberal venting method of any plumbing code. Having worked with the committee that wrote this section, I can tell you it means what it says and is as liberal as it appears. The basic premise of a combination drain and vent system is to oversize the drain and forget about the vent. Well, almost forget about the vent.
The IPC places many restrictions on a combination drain and vent system. All of the restrictions are necessary for the system to operate correctly. The most important restriction is that the drain be horizontal throughout its length. No offsets in the piping are permitted. The pitch is limited to a maximum of 1/2-inch per foot. The only exception to this requirement is the vertical drop from the fixture. For example, a lavatory can have a vertical drop from the trap arm to the drain below the floor. The vertical drop is limited to 8 feet to accommodate a standpipe from an elevated water heater.
The reason for this limitation is that horizontal flow in a drain is wimpy. There is no great demand for air to enter the system. Pressure does not build up in the system. Heck, if all of the flow were horizontal, it would be easy to vent a drainage system. The major pressure changes created in a drain are a result of vertical sections of drain pipes, especially multifloor stacks.
I try to relate the flow in a horizontal drain to that of a river. Pitch a river 1/2-inch per foot, and you have a gently flowing, romantic river. Put a few vertical drops and some rocks in the river, and you have some wild rapids. Put a major vertical drop in and you have a waterfall. While air moves all around the rapids and waterfalls, there is virtually no air movement caused by the gentle river.
Oversizing The Drain: The second most important requirement for a combination drain and vent system is oversizing the drain. I, for one, hate to have drains oversized. To me, oversizing a drain can be as damaging as undersizing. You are just asking for future stoppages. An oversized drain slows the flow of the drain. So why is this an important requirement? A combination drain and vent system relies on the slow flow in the drainage pipe. To assure this slower flow, the drain is oversized.
The system is also restricted to floor drains, sinks, lavatories and standpipes. I am always asked if the system could be used for showers and bathtubs. I always say the drain for showers and bathtubs is nothing more than a glorified floor drain. (A bathtub is also nothing more than an oversized lavatory.) Most plumbing inspectors interpret the plumbing code in the same way. But find out from the inspector before you proceed.
The allowance of a sink raises the question of, “Can a dishwasher or a garbage disposal connect to the sink?” The answer is, “Yes, of course they can.” There is no restriction against such connections, nor will they create a problem with the system.
The other concern with the fixture list regards standpipes. The question is always, “Can a clothes washer discharge into the standpipe?” Again, yes it can, because such an installation has no restriction. However, I should caution I would never have a combination waste and vent for a standpipe unless the drain was 3 inches in diameter. (Technically, a 2-inch drain could be installed.) I say that because you never know what kind of washing machine will be installed. Many manufacturers are using high-speed pumps on their clothes washers. These high-speed pumps can overcome the system if a 2-inch combination drain and vent is installed.
One More Limitation: The final limitation is to connect the system to a horizontally vented drain. That simply means the system cannot connect to a stack. Whatever drain it connects to must have a vent somewhere in the piping on the same floor level.
Hence, the combination drain and vent must be horizontal, oversized, limited to certain fixtures and connect to a horizontally vented drain. This conjures up questions of, “How far can I go?” “What length am I limited to?” “Can I go across the entire floor without a vent?” Of course, the beauty of a combination drain and vent is there is no limitation to the length of the system. The IPC will allow you to go forever if you follow the limitations. The laws of physics recognize the system will work regardless of its length.
Many plumbing codes with similar combination drain (waste) and vent systems have limitations on the lengths of the system. Every limitation in every one of these plumbing codes is arbitrary, without any technical basis.
One of the nice aspects of this system is it allows you to easily vent a floor drain in the middle of a floor. Many of my engineering colleagues show vents for floor drains running horizontally below the floor. Yet, just about every plumbing code prohibits a dry vent from offsetting horizontally below the floor. What they should be drawing on their plans is a combination drain and vent system for the floor drain.
I should point out while a combination drain and vent appears to be a Godsend, it is a marginal venting system. The system is prone to drain line stoppages. Another problem is the system can be very noisy. You will hear gurgling in the drain, but you will not lose the trap seal.
In areas of the country familiar with the unlimited combination drain and vent system, I have noticed a switch to air admittance valves. The air admittance valve allows the drain to stay properly sized. The noise in the drain is eliminated. Plus, the installation of the piping system is more flexible. You can have horizontal and vertical offsets wherever you like when using air admittance valves.
Plumbing contractors who used to swear combination drain and vents were the ultimate in venting systems are now swearing air admittance valves are the way to go. Of course, I still think a combination drain and vent beats an air admittance valve for venting a floor drain. By the way, the IPC permits both.