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When you are dealing with large buildings, there will usually be detailed riser diagrams, blueprints, and specifications available to outline your work. House plans rarely show much more than fixture placement for plumbing. They frequently have wiring diagrams, but most don’t show a piping schematic. Long story short, commercial buildings are usually laid out for the master plumber by a designer. |
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water service or a 1-inch water service into the building. Primary piping would be three-quarters of an inch in diameter, with branch piping having a diameter of one — half an inch. A rule of thumb is that not more than two fixtures should be served off of a single half-inch branch. This is a simple system without much of a load. But, what would happen if the building you were working with was an office building with four stories and a basement? There would be much more to consider, and I will provide you with a sizing example for this type of building in a few moments. Many factors can come into play when sizing a water distribution system. The type of pipe or tubing being used is one factor. The friction loss among various |
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► sensible shortcut |
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Residential sizing is simple. Figure a 4-inch sewer pipe, plan on no more than two toilets on a 3-inch drain, and run three-quarter-inch water mains with no more than two water branches on a half-inch pipe. If you do this, you are unlikely to go wrong. |
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types of piping varies. Even the types of valves installed on the piping will make a difference in friction loss. Most important to sizing is the fixture-unit load on the system and the components of the system. In most cases, a job will start with one size of pipe and the pipes will grow increasingly smaller in diameter as they serve the various plumbing fixtures. Also, the rise of piping and the length of pipe runs will affect the sizing of a system.
Plumbing systems for most large jobs are designed by professionals who don’t work as plumbers. However, when you are remodeling a building,
adding onto an existing system, or working without detailed blueprints, the need for knowledge about pipe sizing may become very important. Sizing is also an element of most licensing exams for plumbers, so this is another good reason to learn and understand the principles used in sizing systems.
Two of the major plumbing codes have graciously agreed to allow me the use of excerpts from their codebooks to better show you rule and regulations pertaining to pipe sizing. Both of the codes offer sizing examples in their codebooks. Your local code may also offer similar sizing data. Once you have established numbers to work with, such as fixture-unit ratings, sizing a water system is a manageable task.
