DRAIN, WASTE AND VENT PLUMBING

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1 Flow of waste in a drain system Solid and liquid waste flows in vertical pipes much differently than it does in horizontal systems. In near horizontal pipes we talked about maintaining the slope of the pipe typically between 1 4 inch and 1 2 inch per foot to have enough velocity to move the liquids and solids along the pipe, but not so much velocity that the liquids would pass the solids by and let the solids get hung up. With a vertical pipe, the water tends to attach to the walls of the pipe, creating a cylinder of water around the perimeter. The center of the pipe often remains filled with air. Most vertical pipes are designed to have no more than about a third of the pipe carrying waste. Turbulence At Bottom Of Stack Changes In Direction At the bottom of the pipe, there is a point of considerable turbulence and high pressure where the waste stops falling and starts to move horizontally. These pressures can push sewer gases back up through traps ahead of it in the system. The goal is to minimize the turbulence and pressure here with fittings that make gentle turns, rather than sharp turns. When we change from a horizontal to vertical direction or vice versa, we want to maintain the momentum of the flow and not allow material to get hung up at intersections. It goes beyond our scope to explore all of the fittings that are appropriate. As a general principle, we want to make the change in direction gradual, and encourage the material to flow in the intended direction. 12

2 Drain, Waste & Vent Pipes There are different names for pipes on the waste side of plumbing systems, based on their functions. Soil pipes carry liquids and solids. The pipe from a toilet is a soil pipe. Waste pipes carry only liquids according to some authorities and liquids and solids according to others. The terms drain and waste are often used interchangeably. Drain pipes are intended to carry liquids and solids (some say liquids only). Vent pipes carry air only (no liquids or solids). Note: Most pipe materials are described as DWV (Drain, Waste and Vent), which means they are suitable for all three functions. S1 Must Deal With Vapors Too Function Traps Seal Odors Out Of Home Definitions TRAPS Let s make it even more complicated. If we attach a pipe to the bottom of all the plumbing fixtures that allows the solid and liquid waste to run out of the house without clogging, it sounds like we have accomplished our goal. However, there are vapors (gases) that pose odor and health problems. We need to keep the sewer gases from backing out of the sewer pipes into the house. You are all probably a step ahead, and have already designed a trap. The main function of traps is not to catch rings that you have dropped down the drain, although that can be an important side benefit. The main purpose of a trap is to prevent sewer odors from coming back into the house. The trap under the fixture creates a low spot that is kept filled with water. This water gets pushed out of the trap and down the drain every time more water flows through the fixture, and as water flows out of the fixture, the last bit of water is left in the trap, forming a new seal to prevent odors from coming back up. We told you we don t like definitions, but we d better give you a few more. trap a fitting that provides a water seal against sewer gases trap weir (crown weir) the inside bottom of the highest part of the trap (trap outlet) trap dip (upper dip) the inside of the lowest part of the trap trap seal vertical distance from trap dip to weir tail piece the pipe from the fixture to the trap; also called fixture outlet pipe trap arm the pipe from the trap to the vent; also called fixture drain 13

3 Clogging Traps Self-Scouring Traps It s strange that a couple of minutes ago we were talking about having exactly the right slope for waste piping, and now we put a low spot in it. Won t this collect junk? Traps may accumulate sludge and can clog up. Over the years, there have been several design considerations to minimize the problem. Traps are engineered systems, and their shape is not an accident. Traps are designed to be self-scouring, so the waste that flows through them keeps moving. There are also rules as to how close the trap must be to the fixture and what size the trap is. As a last resort, traps have either a cleanout or some easy way to remove the trap for cleaning, such as slip joints. 14

4 S1 Water Shouldn t The velocity of water moving through a trap is important to its function. This is Move Too Fast why they have decided that putting two traps in series (double trapping) is not a Or Too Slowly good idea. The water velocity to the second trap is so low that clogs are likely. It s also why there are limits to how far below the fixture the trap can be placed. We don t want the water moving too fast when it reaches the trap. It may siphon out the water and then we d have no seal. The pipe coming out of the trap has to be horizontal so we don t speed up the water and siphon the trap. That s why we like P-traps and S-traps are not allowed. No Traps For Toilets Good home inspectors check under each fixture to make sure there is a trap. They also notice that toilets never have traps. You would think that such a smelly plumbing fixture definitely needs a trap. However, the toilet bowl itself is a trap. Water flows out of the toilet by spilling over a high point, down into the drain. Water coming from the toilet tank flushes the water in the bowl over the crown of the bowl (trap) and into the drainage system. If the toilet is well designed, the water takes whatever solids are in there with it. Toilets use siphons to help get rid of their solid and liquid waste. Toilets are the only self-trapped fixtures in the house and are the only traps designed to siphon as they flush. 15

5 Slope, Traps, Leaks And Freezing Things look pretty easy so far. We should be looking for waste pipes that are nearly horizontal, or are vertical (45 degrees or more off the horizontal). We look for traps under every fixture (including floor drains) except toilets. We check for leaks. In cold climates, we check to see if traps are subject to freezing. Check For Look into floor drains. If you see water in the trap and do not smell anything, that s Water In Floor good. If there is no water in the trap, or there is an odor, the water may have Drain Traps evaporated or leaked out of the trap. Water evaporates from floor drains because they hardly ever get used. Traps may be cracked by building settlement or heaving, tree roots or freezing for example. Water Pouring a bucket of water down the drain usually tells you if the trap is cracked, Evaporates Or unless it s a small crack. If the water stays in the trap, it probably just evaporated. If Leaks Out of the water disappears, the trap is probably cracked or broken and will have to be Drains replaced (this can be several hundred dollars to replace). Some people recommend using a little mineral oil on top of the trap water to keep it from evaporating so quickly. Trap Primers In new houses, they have a solution. They use a trap primer. This is usually a 3 8 -inch diameter plastic tube attached to a fixture that is used regularly. Every time the fixture is used, a little water flows through this primer and into the floor drain. This replaces any water in the trap that may have evaporated. Obviously, you can t overfill a trap, because the water just flows out over the crown weir (bottom of the trap outlet) of the trap and down the drain. 16