Simulations of sizing and comfort improvements for residential forced-air heating and cooling systems

Abstract

In many parts of North America residential HVAC systems are installed outside conditioned space. This leads to significant energy losses and poor occupant comfort due to conduction and air leakage losses from the air distribution ducts. In addition, cooling equipment performance is sensitive to air flow and refrigerant charge that have been found to be far from manufacturers specifications in most systems. The simulation techniques discussed in this report were developed in an effort to provide guidance on the savings potentials and comfort gains that can be achieved by improving ducts (sealing air leaks) and equipment (correct air-flow and refrigerant charge). The simulations include the complex air flow and thermal interactions between duct systems, their surroundings and the conditioned space. They also include cooling equipment response to air flow and refrigerant charge effects. Another key aspect of the simulations is that they are dynamic to account for cyclic losses from the HVAC system and the effect of cycle length on energy and comfort performance. To field test the effect of changes to residential HVAC systems requires extensive measurements to be made for several months for each condition tested. This level of testing is often impractical due to cost and time limitations. Therefore the Energy Performance of Buildings Group at LBNL developed a computer simulation tool that models residential HVAC system performance. This simulation tool has been used to answer questions about equipment downsizing, duct improvements, control strategies and climate variation so that recommendations can be made for changes in residential construction and HVAC installation techniques that would save energy, reduce peak demand and result in more comfortable homes. Although this study focuses on California climates, the simulation tool could easily be applied to other climates. This report summarizes the simulation tool and discusses the significant developments that allow the use of this tool to perform detailed residential HVAC system simulations. The simulations have been verified by comparison to measured results in several houses over a wide range of weather conditions and HVAC system performance. After the verification was completed, more than 350 cooling and 450 heating simulations were performed. These simulations covered a range of HVAC system performance parameters and California climate conditions (that range from hot dry deserts to cold mountain regions). The results of the simulations were used to show the large increases in HVAC system performance that can be attained by improving the HVAC duct distribution systems and by better sizing of residential HVAC equipment. The simulations demonstrated that improved systems can deliver improved heating or cooling to the conditioned space, maintain equal or better comfort while reducing peak demand and the installed equipment capacity (and therefore capital costs).