Residential air-distribution sysstems: Interactions with the building envelope

摘要

Residential air distribution systems, used both for heating and cooling and less commonly for ventilation, have important interactions with the building envelope. These systems can either be enclosed within the envelope or pass outside the envelope (in which case they represent an extension of the envelope). This paper addresses the three major types of interaction between air distribution systems that pass outside the envelope and single-family buildings: (1) duct leakage and duct conduction when the distribution fan is off, which act like a thermal bridge in the envelope; (2) duct leakage during system operation, which creates large changes in the quantity and location of air infiltration and exfiltration through the envelope; and (3) supply /return flow imbalances within individual zones during fan operation, which create elevated envelope pressure differentials, infiltration rates, and exfiltration rates. A simulation tool that was developed to take into account all of these interactions is presented and applied. The simulation tool, based upon the DOE-2 thermal simulation model, a multi-zone airflow network model (COMIS), and an equipment model for the ducts, is used to examine the magnitude of all three interactions. The interaction issues examined include air infiltration/exfiltration magnitude and location, overall thermal exchange when the system is off, and air exchange when the system is operating, with and without internal doors closed. The most surprising result of the analyses presented was that the thermal siphon effect for perfectly sealed ducts was shown to have an impact on the heat exchange between the house and unconditioned spaces that can be more than four times larger than that due to typical duct leakage when the fan is not in operation. This result suggests that this issue merits more careful examination than it has received in the past.