More than 60 years ago, the late Dr. Roy B. Hunter developed a system for calculating water loads in commercial buildings. The estimated water demand of fixtures (water closets, sinks, etc.) is given a value called Fixture Units which have an equivalent demand load in Gallons Per Minute (GPM). The Fixture Units and Demand Load relationship is known as Hunter’s Curve and is still the basis for plumbing system design today.
Hunter’s Curve can be effectively used to calculate total system demand, but it has a glaring flaw. There is no consideration for diversity in the system demand. Using Hunter’s Curve for the basis of design of a Pressure Boosting System results in a pump system sized for all fixtures being used simultaneously, a scenario that will likely never happen. The pumps are grossly oversized for partial-demand conditions which make up 90% or more of total operation, causing poor system control and unnecessary wear on the pumps and piping system. In addition to Hunter’s Curve, Cougar USA uses field experience and data collection for system design.
To generate an accurate demand load profile, we gather as much information as possible about the building. The type of building has a huge impact on the load profile; even with similar fixture units, hospitals, hotels, schools, and office buildings will all have different load demands throughout the day and week. Special applications, the height of the building, locations of equipment, and potential future expansion are all factors in creating the right Building Load Profile. Once the system requirements are determined, we must make the right equipment selection.