Commercial unitary air conditioners (CUACs) and heat pumps (HPs), often called roof-top units (RTUs), are typically used to cool small- to mid-sized commercial buildings such as schools, restaurants, big-box stores, and small office buildings. They reject heat to the atmosphere by blowing outside air over the condenser coil. CUACs may also contain a gas heating section.
The Energy Policy Act (EPAct) of 2005 updated the original 1992 federal efficiency standards for CUACs and HPs with cooling capacities from 65,000-240,000 Btu/hr, and established new standards for equipment with capacities from 240,000-760,000 Btu/hr. The EPAct standards took effect on January 1, 2010. Cooling capacity represents the amount of heat that an air conditioner or heat pump can remove from an enclosed space per hour. The cooling efficiency of CUACs and HPs is currently measured by the energy efficiency ratio (EER), which is the cooling capacity (in Btu/hr) divided by the power input (in watts). The heating efficiency of HPs is measured by the coefficient of performance (COP), which is the heat delivered (in Btu) divided by the energy input (in Btu). These standard levels are the same as those published in the commercial building energy code, ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) 90.1 in November 2010.
In April 2015, DOE formed a working group to negotiate potential new energy conservation standards for commercial CUACs and HPs with cooling capacity of 65,000 to 760,000 Btu/hr. The working group recommendations were ultimately adopted by DOE in a December 2015 final rule. The new two-stage standard is based on a metric called IEER (integrated energy efficiency ratio), which captures efficiency at 25, 50, 75, and 100% of full capacity, better reflecting the range of operating conditions. The first stage would require that new rooftop air conditioners meet the efficiency levels in ASHRAE 90.1-2013, about 10% more efficient than current standards. The second stage would requite minimum IEER levels of 12.3 to 14.8 (depending on equipment type and capacity), about 25-30% more efficient than current minimum standards. DOE estimates that over the lifetime of units sold over thirty years, the proposed standards would save businesses nearly $50 billion (at 3% discount rate) and reduce electricity consumption by about 15 quadrillion Btu, more than any other efficiency standard ever adopted by DOE. The first stage would go into effect on January 1, 2018 and the second on January 1, 2023.
Technology options for improving the efficiency of CUACs and HPs include higher efficiency compressors, compressor staging, larger and better heat exchangers, higher efficiency fan motors and fan blades, and improved expansion valves. DOE’s High Performance Rooftop Unit Challenge has helped spur several manufacturers to develop and bring to market high-efficiency rooftop air conditioners. Equipment on the market today achieves IEER levels as high as 21.