In peak demand calculations, fixture units should be

In peak demand calculations, fixture units should indeed be added directly as gallons per minute (gpm). This approach recognizes that fixture units serve as a convenient way to quantify the probable demand that different sanitary fixtures contribute to the overall plumbing system. Each fixture is assigned a specific number of fixture units that reflect its expected usage, typically based on established plumbing codes.

The direct addition of fixture units in terms of gpm allows for a clearer understanding of total potential water flow needs during peak usage times, as it aggregates the flow requirements of all fixtures in a straightforward manner. This ensures that the plumbing system can be designed to accommodate the maximum anticipated load, thus preventing issues like insufficient water supply or pressure drops.

In contrast, the other methods of handling fixture units—such as converting them to different measurements before aggregation or estimating them separately—would introduce unnecessary complexity or inaccuracies into the demand calculations. Calculating based solely on the maximum flow neglects the variable usage patterns of different fixtures, which can lead to an overestimation of the system's requirements. Therefore, the practice of adding fixture units directly as gpm is essential for accurate peak demand assessments in plumbing design.