Finding the Design FlowThe hard part with designing water pipe sizes is estimating the design flow, which varies all over the place, depending on the time of day, the type of building, occupancy rates etc.Fortunately each Plumbing Code has a formula to help in estimating the 'design' flow.This is done by allotting a "Fixture/Loading Unit" to each plumbing fixture. This number allows for the flow, and frequency of use.The numbers are added up progressively as more and more fixtures are connected. The higher the number the greater the flow, but the percentage of fixtures operating together drops off (to a certain minimum).

The formula equates Loading Units to flow, in litres/sec, (or gals/min)Basically, as the number of Loading Units increases, the percentage of all fixtures operating together decreases. (to a certain limit)This is called the Probable Simultaneous Flow or Demand. Water Pipes for plumbing purposes are designed by the Loading Unit (or fixture unit) method.The programs use the formula of the respective code.Strangely this formula varies significantly in each Plumbing Code.

Some really interesting StuffThe International Plumbing Code (and the British Plumbing Code) suggests we allow for almost twice the water flow as the Australian code. The Uniform plumbing Code as used in the United States recommends more again