the conventional compressor. The ring is held in place at fullload so that the ports are closed. When demand declines the modulating ring opens the two ports and the suction gases enter the scrolls closer to the discharge part. Capacity is reduced to 67% of full-load. The added mechanical complexity of this approach is accompanied by an added control complexity and the need to locate and train technicians capable of understanding this more sophisticated technology. Conclusions It is suggested that ISO Standard 13256-1 be updated so that rated values more closely reflect field conditions and include fan power sufficient to provide air distribution through typical duct systems and high efficiency air filters. In the interim, engineers and owners should be provided with a clear explanation of all conditions used to compute capacity and efficiency along with simple procedures to correct rated values to field conditions. References 1. ISO Standard 13256-1-1998, Water-source heat pumps—Testing And Rating For Performance—Part 1: Water-to-Air and Brine-to-Air Heat Pumps. International Organization for Standardization. 2. Kavanaugh, S. 2008. “A 12-step method for closed loop groundsource heat pump design.” ASHRAE Transactions 114(2). 3. WFI. 2008. Envision Residential Specification Catalog. WaterFurnace International. The advantages made possible by more complex and costly technologies must outweigh the accompanying disadvantages (see sidebar, “Dual-Capacity Versus SingleCapacity Heat Pumps”). The simple single-capacity heat pump is shown to be more efficient for conditions that are encountered in most field applications. Efficiency improvements of dual-capacity units are restricted to rated values. Longevity of previous unitary dual- and multi- capacity product lines is limited. Steve Kavanaugh, Ph.D., is professor emeritus at the University of Alabama in Tuscaloosa, Ala. www.info.hotims.com/30303-24 www.info.hotims.com/30303-11 April 2010 ASHRAE Journal 69