Characterizing the performance of residential air distribution systems

Abstract

Approximately 35% of US single-family houses contain forced-air heating and cooling ducts that pass through unconditioned spaces. These duct systems have been shown to have a potentially large influence on energy use and ventilation rates. To investigate the parameters affecting the performance of these systems, a 31-house field study of distribution-system performance based on diagnostic measurements was performed in California, and an integrated airflow and thermal simulation tool was developed. The results of the field study, a brief description of the simulation tool, and the results of the first applications of the simulation tool are presented. The field-study measurements generally agreed with the findings in earlier studies, provided field experience with new diagnostic tools, and provided system/house characterization data for use in simulation codes and in the development of retrofit protocols. Some highlights of the field results include: (1) building envelopes appear to be approximately 30% tighter for California houses built after 1979; (2) duct system tightness showed no apparent improvement in the post-1979 houses; (3) distribution-fan operation added an average of 0.45 ACH to the average measured air change rate of 0.24 ACH, and (4) an average of 20% of the furnace heating effect was measured to be lost due to duct conduction losses alone. The stimulation tool developed is based upon DOE-2 for the thermal simulations, MOVECOMP, an airflow network simulation model, for the duct/house leakage and flow interactions, and a combined heat and mass transfer model of the duct performance. The first complete set of simulations performed for a new ranch house in Sacramento CA indicated that steady-state duct-system efficiencies vary over a large range with outside temperature, ranging from 50 to 95% (decreasing with increasing outside temperature for cooling and decreasing outside temperature for heating). The simulations also indicated that the location of the return duct can have a large influence on duct-system efficiency during the cooling season.