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December 4, 2013 -
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Today's tip from Building Operating Management comes from Daniel H. Nall of Flack + Kurtz. With the HVAC system's air and water distribution systems optimized, the heating and cooling source equipment can be optimized. There is a basic conflict between optimizing the efficiency of the distribution systems and optimizing the efficiency of the heating and cooling source equipment. For the distribution system, hotter hot water and colder chilled water can result in a greater temperature differential across the system, resulting in lower required flow and lower transport energy consumption. For the sources of heating and cooling, however, cooler hot water and warmer chilled water result in more energy-efficient production of these resources. Optimization of these conflicts will result in the most energy-efficient systems.
For both boilers and heat-pump-cycle heat sources, lower hot water temperature results in greater efficiency. Lower hot water temperature allows the utilization of condensing boilers and even lower hot water temperature increases the efficiency of the condensing boiler. Maximizing the efficiency of the entire system relies upon maximizing the thermal coupling between the distribution medium, air or water and the end use, the conditioned space. The key to optimal efficiency heating is thus close approach heating coils, or extended surface area convectors or radiant panels. By reducing the temperature differential between space and distribution medium (air or water), temperature differential across the transport system can be maximized while maintaining a lower-temperature heating source.