Supply & Wastewater Piping

Supply Piping 

Water enters the house through a single supply pipe (usually 1 or 3/4 inch inside diameter) that is buried deeply enough in the ground to prevent it from freezing.  The supply pipe is not reduced in size until it reaches the water heater, after which it may be divided into branch lines to supply plumbing features.  The water supply to individual fixtures consists of a pair of supply lines - one hot, one cold - usually of 1/2 inch diameter, depending on the number of fixtures that are fed by the same pair of pipes.  

Supply piping can consist of copper pipes with soldered fittings, polyvinyl chloride (PVC) plastic pipes with glued fittings, or flexible cross-linked polyethylene plastic (PEX) tubing with crimped fittings.  Copper has been the standard of quality since the 1950's, but PVC, introduced in the 1970's, is seen by some to be an improvement because it is faster and safer to install (no flame to melt solder is required) and it is not corroded by minerals in the water.  PEX, the most recent introduction, is even simpler to install than PVC because it can bend like a garden hose, it is quieter with water running through it, and it can withstand expansion when freezing.

Supply piping should be wrapped in thermal insulation.  This usually takes the form of either a plastic foam jacketing or slit tubes of closed cell rubber, which are slipped over the pipes, or a wrapping of glass fiber insulation encased in a vapor-resistant foil or plastic film.  This insulation helps keep cold water cold in the pipes and hot water hot.  It helps conserve the energy used to heat the water.  In humid conditions, it prevents condensation of moisture on cold water pipes.  In milder climates, it can help water from freezing in the pipes.  In cold climates, it is best to design every building so that no supply pipes are located in exterior walls or unheated spaces.  If this is not possible and piping must run in crawl spaces, exterior walls, attics, or other unheated locations, it is imperative that the pipes be located on the warm side of the building insulation, next to the finish material.  

Wastewater Piping

Lines connected to toilets are at least 3 inches in diameter and are called drain piping or soil lines.  Lines free of solids, called waste piping, can be smaller - usually 1  1/2  or 2 inches in diameter.  The line at the base of the system that is the receptor for all drain and waste piping is the building drain.  At a distance of 2 feet beyond the foundation wall, the building drain becomes the building sewer.

The wastewater system contains noxious, flammable sewer gases.  These gases are kept out of the house by means of a P-trap at each fixture that seals the end of the pipe with water.  When wastewater is released from the fixture, it displaces water held in the trap.  A vent on the sewer side of each trap supplies air so that draining water cannot create a suction to pull water out of the trap.  The vent piping extends through the roof of the house.   A relatively new device, the air admittance valve (AAV), admits air to the wastewater system without venting through the roof.  These valves are installed in the DWV system inside the building and admit air whenever negative pressure occurs in the system.  The valve remains open until the system returns to zero pressure.  The use of AAVs in new construction can substitute for a considerable amount of  vent piping.  Clean-outs are required at strategic points in the system to allow reasonable access for the cleaning of clogged lines.  

The entire system, consisting of drain pipes, waste pipes, and vent pipes, is called a drain-waste-vent (DWV) system.  Materials for residential DWV systems generally consist of black ABS pipe (composed of an acrylonitrile-butadiene-styrene copolymer) that is glued a the fittings.  This pipe is the least expensive of all approved alternatives, and its only significant drawbacks are that it transmits the sound of draining water and expands and contracts considerably with thermal changes.  Other waste pipe materials include PVC pipe, used because of its lighter weight, and cast iron pipe, used because its mass makes it quieter than plastic.  All DWV components, including vents, must be sloped to drain.  All plastic pipes require special attention to thermal expansion and contraction.