Design Considerations for Pressure-Boosting Systems in Commercial Buildings

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.

Pressure Boosting Systems are typically comprised of two or more pumps, suction and discharge headers, control panel, and bladder tank. The pump style, size, and quantity and are all dependent on the Building Load Profile. For most commercial building applications, the small footprint and multi-stage design make the Grundfos CR Vertical Multi-Stage pump the best selection. The pump size and quantity will be determined by partial load performance and the redundancy desired. Typically, a higher quantity of smaller pumps is more efficient at partial load conditions without adding much to the initial system price when compared to a duplex (two-pump) system of larger pumps.

A properly sized and charged bladder tank is crucial to the overall performance of the Pressure Boosting System. Based on the load profile and building height, we can determine the size and location of the bladder tank. In low- to mid-rise buildings, the tanks will typically be installed at the pump system discharge. In high-rise buildings, the tanks will be installed on the upper floors, off the main riser.

The last consideration is the level of controls required for the building. Critical applications like hospitals, research facilities, and high-rise buildings will require control features like those on the Grundfos CU352 used on the Hydro MPC Booster System. A graphical user interface shows feedback of the system and any alarms, as well as advanced control features like Proportional Pressure Control, Reduced Operation for Emergency Power, Soft Pressure Buildup, and Communications for Building Management Systems.

To effectively design for today’s buildings, we must look beyond Hunter’s Curve. Cougar USA has made hundreds of Booster System selections for commercial buildings, and not once have we been wrong.