D1 Plumbing Installations

Topic

Sizing a plumbing system begins with a simple observation: not all the taps, showers, and toilets in a building are used simultaneously. Therefore, the design flow rate is not the sum of the flow rates of all installed fixtures, but rather that sum adjusted by a simultaneity factor reflecting how many are actually in use at once—a factor that decreases as more fixtures are connected to the same pipe section.

The pressure available at the most critical point of use—typically the highest point and the one furthest from the supply connection—is the mains pressure minus all cumulative pressure losses along the route. These losses follow the principles described in the module on water physics in buildings: they increase with water velocity and the distance traveled, and decrease as pipe diameter increases. This final pressure must exceed the minimum pressure required for each fixture to operate; if it does not, the building's most critical fixture will lack service, even if the others function normally.

The flow regime within a pipe—whether the water moves in an orderly (laminar) or turbulent fashion—also influences these losses, depending on the fluid's velocity, viscosity, and the pipe diameter. When mains pressure is insufficient to overcome the elevation and losses reaching the upper floors of a multi-story building, a booster pump system is used to maintain service throughout the building's height, providing exactly the pressure the mains supply alone cannot deliver.

Thus, this module—which begins by considering fixtures that are rarely all used at once—concludes by showing that sizing a plumbing system is, fundamentally, a matter of determining how much excess pressure is available at the most favorable point, so that it can be dissipated along the way while still ensuring sufficient pressure reaches the most critical point.

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