Sensitivity of forced air distribution system efficiency to climate, duct location, air leakage and insulation

This study was performed in order to find suitable efficiency and leakage specifications for Energy Star duct systems and provide recommendations on duct insulation specifications. This analysis looks at a typical house, with a selection of duct locations, climates, duct insulation (R-value), and duct leakage. A set of calculations were performed with reduced capacity and airflow to look at the effect of variable capacity systems. This was done to address concerns regarding the increased efficiency of multi-capacity equipment due to good part load performance and how these efficiency gains may be offset by increased duct losses. The duct system efficiencies were calculated using the procedures in proposed ASHRAE Standard 152P "Method of Test for Determining the Design and Seasonal Efficiencies of Residential Thermal Distribution Systems" (ASHRAE 1999). This proposed ASHRAE Standard can be used to calculate duct efficiency for both design and seasonal weather conditions. In this report, the seasonal efficiencies are used for most of the analysis because they are the most appropriate for estimating energy consumption in buildings. The effects at peak conditions are examined for changing duct insulation in order to provide preliminary estimates of the potential responses to time of use pricing.

The study was performed in two parts. The first part focused on duct leakage and the second part on duct insulation. The HVAC systems in the two parts share many attributes, however, they differ in detail and so are treated separately here. All the calculation results are summarized in tables in the Appendix, and specific results are given in the text.