A Forensic Engineer's Guide to the Pitfalls of Residential Plumbing
Russell A. Zeckner, PE
The term plumbing is derived from the Latin word “plumbum,” the name of the metal lead that was the preferred material used by the Romans to construct their empire’s water supply systems. This bit of history is even relevant today as the continued use of this potentially toxic metal has made headlines because of recently exposed problems in the Flint, Mich., water supply.
Residential plumbing can be divided into two broad categories: water supply and waste removal.
Water supply plumbing is pressurized, has completely filled piping and is used to carry the potable liquid to all water consuming fixtures in a house. The water originates typically from a river, reservoir, or a well, and is then often treated to ensure its safety for human consumption.
Commonly, these water supply and treatment facilities are municipally owned in urban areas but in rural locales they are often privately held. Water pressure is produced with the use of electrically operated pumps or by the elevation of the ubiquitous water storage towers and then is transported to the residence through piping that could be made from a host of materials with polyvinyl chloride (PVC) and ductile iron as most common in current use.
Depending upon the locale’s climate, the water supply travels to a residence via piping buried deeply enough to prevent freezing. Just prior to entering the house the water often will pass through a meter that records the household’s water usage for billing purposes. At its entrance into the house, the water supply line is usually ¾-inch diameter and commonly made of copper. Almost immediately upon entering the house, basement or crawlspace is the system’s main shutoff valve. Just beyond the water main valve, a backflow preventer and/or a pressure-reducing valve may be present. Backflow preventers eliminate the possible contamination of the public water supply if the water line fails and pressure-reducing valves are needed if water pressures have the potential to exceed 90 pounds per square inch (psi). Inside the house, water is transported to appliances and fixtures though steel, copper, chlorinated polyvinyl chloride (CPVC), or cross-linked polyethylene (PEX) tubing or pipe usually ½-inch in diameter. Because it is pressurized, the pitch of water supply piping is not important and vertical piping runs are not a problem. One of the first appliances encountered in a water supply system is the water heater and afterward, the system is divided into both hot and cold water components. Water heaters most commonly are of the storage tank variety using natural gas, propane or electricity as their fuel. Less common, but offering greater energy efficiency, are solar, tankless and heat-pump models.
Water hammer is a condition often experienced in water supply systems and it is not only distressing because of its signature banging sound, but also because of the damage that it can cause to the associated pipe and fixtures. Water hammer occurs when the inertial energy of flowing water is suddenly stopped when a valve quickly closes. The collision of the moving water with closed valve can cause the pipe to move and bang against adjacent pipes or structure. Solenoid controlled valves, such as those found in ice-makers, dishwashers and washing machines are the most common culprits.
Unlike water supply plumbing, waste plumbing is not pressurized, flows via gravity and its pipes are normally only partially filled. Waste plumbing is larger than water supply piping, ranging from a minimum of 1 ¼-inches to 4 inches in diameter in a residence to accommodate both solid and liquid wastes. Pipes can include those made of cast iron, PVC, brass, lead or steel and most systems contain piping made from some combination of these materials. Slopes of waste pipes are critical and sags in the line and too low slopes can result in backups and/or clogs.
Waste plumbing also contains vent lines that generally rise vertically and terminate with an open end just above the house’s roof slope. These vent lines allow air and other gases to enter and exit the waste systems to enhance the flow by preventing vacuum lock while providing odorous gasses an easy pathway to the outdoors. Back-flow preventers are often installed in between a house’s main waste line and a municipal system to prevent sewage from backing up into the house if the municipal system becomes overwhelmed due to a storm or similar event.
Leaks in waste plumbing have more diverse causes than those in water supply systems. Failed connections, corrosion, worn seals, and mechanical damage are common defects leading to the loss of the waste system’s contents. Because waste systems are not pressurized and their flows are unmetered, undetected leaks can occur and persist for long periods of time. Given enough time, such leaks can result in the failure of large sections of a floor structure before they are detected.
It is not an overstatement that the creation of safe and reliable water supply and waste water removal systems remain critical for the creation and maintenance of our modern society.
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