Duct Efficiency under Full-Load or Modulating Conditions:Implications for Heat Pump Performance

摘要

Ductwork in unconditioned spaces results in annual energy losses of 15 to 30% for spaceheating in the Pacific Northwest. This paper presents a further development of the distributionefficiency equations in showing that under full-load (or design) conditions the distributionefficiency is nearly independent of outdoor temperature. However, when there are internal gains,the full-load efficiency tends toward zero as the outdoor temperature approaches the heatingbalance-point. The full-load efficiency also applies in the case where there is a constant airflowrate through the equipment, but the equipment capacity is modulated to just meet the spaceconditioningload.The full-load distribution efficiency is lower than that which applies to equipment oflarger capacity that is cycling off and on. Residential air-source heat pumps, when using both thecompressor and auxiliary strip heat, modulate in such a way that the distribution efficiencyapproximates the full-load efficiency. This leads to lower duct efficiencies relative to gas orelectric furnaces employed with the same duct system. The full-load efficiency also applies forthe case of continuous air handler fan operationThe theory is illustrated with an example of detailed performance calculations for a heatpump in a Pacific Northwest city. The possibility of using the full-load distribution efficiency asa universal (I.e., valid in all locations) rating method for duct efficiency is also mentioned.