The point of operation of down stream pumps will affect the performance of the upstream pumps and vice versa. In a district-type system, the customer at the end of the distribution system can require pumps of significantly greater horsepower than those close to the plant. It is not recommended to mix constant speed and variable speed zones in the same system. If the system is not properly balanced, the return distribution pipe may be over-pressurized. Over-pressurization will compound the system problems.
Valves can lift off their seats, resulting in hunting and poor temperature control.
Secondary pumps distribute the chilled water from the central plant to the connected buildings or zones. Individual buildings or zones are decoupled from the distribution loop through a tertiary bridge. A major advantage of tertiary pumping is that the individual zones are hydraulically and thermally decoupled. This allows the designer some flexibility to control each zone independently, relative to differential pressure and temperature. Zones may be a collection of coils, air handlers, or entire buildings.