Water Demand and Fixture Units

For each fixture in a building, a maximum requirement for water flow, gal /min, can be estimated. Table 14.1 indicates the minimum flow rate and pressure required by code. The maximum flow may be considerably larger. Branch pipes to each fixture should be sized to accommodate the maximum flow and minimum pressure the fixture will require. Mains serving these branches, however, need not be sized to handle the sum of the maximum flows for all branches served. It is generally unlikely that all fixtures would be supplying maximum flow simultaneously or even that all the fixtures would be operating at the same time. Consequently, the diameters of the mains need be sized only for the probable maximum water demand.
In practice, the probable flow is estimated by weighting the maximum flow in accordance with the probability of fixtures being in use. The estimate is based on the concept of fixture units.
Fixture unit is the average discharge, during use, of an arbitrarily selected fixture, such as a lavatory or water closet. Once this value is established, the discharge rates of other types of fixtures are stated in terms of the basic fixture. For example, when the basic fixture is a lavatory served by as 11⁄4-in trap, the average flow during discharge is 7.5 gal /min. So a bathtub that discharges 15 gal /min is rated as two fixture units (2 x 7.5). Thus, a tabulation of fixture units can be set up, based on an assumed basic unit.

A specific number of fixture units, as listed in Table 14.4, is assigned to each type of plumbing fixture. These values take into account:
Anticipated rate of water flow from the fixture outlet, gal /min  Average duration of flow, min, when the fixture is used  Frequency with which the fixture is likely to be used The ratings in fixture units listed in Table 14.4 represent the relative loading of a water-distribution system by the different types of plumbing fixtures. The sum of the ratings for any part or all of a system is a measure of the load the combination of fixtures would impose if all were operating. The probable maximum water demand, gal /min, can be determined from the total number of fixture units served by any part of a system by use of graphs shown in Fig. 14.4.

The demand obtained from these curves applies to fixtures that are used intermittently.
If the system serves fixtures, such as air-conditioning units, lawn sprinklers, or hose bibs, that are used continuously, the demand of these fixtures should be added to the intermittent demand. For a continuous or semicontinuous flow into a drainage system, such as from a pump, pump ejector, air-conditioning system, or similar device, two fixture units should be used for each gallon per minute of flow.
When additional fixtures are to be installed in the future, pipe and drain sizes should be based on the ultimate load, not on the present load.

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