In this study, researchers with the U.S. Department of Energy's Building America team IBACOS built on research previously done in two new-construction unoccupied test houses-one in Pittsburgh, Pennsylvania (Poerschke and Stecher 2014) and one in Fresno, California (Stecher and Poerschke 2013). Specific traditional central air distribution systems were installed in each of these low-load homes. For this study, the cold-climate new-construction unoccupied test house in Pittsburgh, Pennsylvania, was modified to test the performance of a heating, ventilating, and air-conditioning (HVAC) system with varied airflow and small-diameter ducts.

The main goal of the small-diameter duct system is to simplify the task of bringing ductwork inside conditioned space, particularly on the single-story slab-on-grade type of home that is prevalent in the South and Southeast. Guidance is provided here to homebuilders and HVAC contractors on cost and performance tradeoffs between the conventional duct system and the small-diameter duct system.

Comparisons were made between variable-capacity heat pump operation modes with three constant airflow rates to determine the ideal tradeoff between maximizing thermal comfort and minimizing fan energy consumption.

ASHRAE Standard 55 (ASHRAE 2010a) was used to set limit metrics for temporal temperature variation; Air Conditioning Contractors of America (ACCA) Manual RS (Rutkowski 1997) was used to set a limit on spatial temperature uniformity (room-to-thermostat uniformity). The small diameter duct system was able to meet the temporal temperature variation limits much better than the spatial temperature uniformity limits.

TO4 2.1.2: High Velocity Space Cond. Systems