Reducing annual energy consumption by proper selection of HVAC equipment

Many of the procedures used by HVAC designers were intended primarily at providing a workable rather than optimum energy systems. It was a common practice to include large safety factors to ensure proper operation under all conditions. To the designer, the extra cost of the oversized equipment was justified. What was not understood was the heavy penalty on annual energy consumption this procedure produces. The purpose of this paper is to present quantitative examples of how over sizing augments annual energy consumption. As an example, a simple heating system employing a boiler is examined. The boiler is quoted as having an efficiency of 80%. What is often not understood is that this efficiency only applies to the steady-state operation of the boiler. The annual efficiency in actual usage will be considerably less. This annual efficiency can be found by using the part-load efficiency of the boiler and the Bin method of annual energy estimation. For the above boiler, the annual efficiency was found to be 44%. If the boiler was oversized by 100%, the annual efficiency drops to 38%. The above results can be extended to effect substantial improvements in energy consumption. A single large boiler can be replaced by two smaller boilers whose combined capacity meet the design loads. By using this arrangement, the annual system efficiency can be increased to 64%. The added cost of the extra equipment can be quickly recovered by the large savings in operating costs.