Water-cooled and evaporatively-cooled commercial unitary air conditioners (CUACs) and heat pumps (HPs) are very similar to air-cooled rooftop units (RTUs). However, water-cooled units reject heat to water (rather than to air), and evaporatively-cooled units spray water on the condenser coil to help remove heat using the latent heat of evaporation. CUACs and HPs are typically used to cool small- to mid-sized commercial buildings such as schools, restaurants, big-box stores, and small office buildings. Water- and evaporatively-cooled CUACs and HPs are far less common than air-cooled units.
In 2012, DOE adopted the efficiency levels for water-cooled and evaporatively-cooled CUACs and HPs in the 2010 version of ASHRAE 90.1 as new national minimum standards. ASHRAE 90.1 is a commercial building energy code. The Energy Policy and Conservation Act (EPCA) requires that if the equipment efficiency levels in ASHRAE Standard 90.1 are amended, DOE must either establish amended standards for the products at the levels specified in ASHRAE 90.1 or establish more stringent standards if they would result in significant additional energy savings and are technologically feasible and economically justified.
The new standards took effect in 2013 or 2014 depending on the equipment’s cooling capacity. The cooling efficiency of commercial water- and evaporatively-cooled CACs and HPs is measured by the energy efficiency ratio (EER), which is the cooling capacity (in Btu/hr) divided by the power input (in watts).
Water- and evaporatively-cooled CUACs and HPs are generally more efficient than air-cooled equipment. Water-cooled air conditioners can be used effectively in any type of environment, while evaporatively-cooled air conditioners are most effective in hot-dry climate regions. CUACs and HPs are rated by their cooling capacity (in Btu/hr), which represents the amount of heat that an air conditioner or heat pump can remove from an enclosed space per hour. One “ton” of cooling capacity is equal to 12,000 Btu/hr.