Secondary Author(s)

Withers Jr., Charles

Report Number




Buildings; Ducts; HVAC


Field research performed in 70 central Florida homes found that return grills are almost always located in the central zone of the house, and that individual rooms rarely have ducted return air or return transfer pathways. When interior doors were closed, the closed rooms went to +0.0249 inches of water gauge (in w.g.) (+6.2 pascals (Pa)) wrt outdoors (wrt = with respect to), and the central zone went to -0.0116 in w.g. (-2.9 Pa) wrt outdoors. Room pressures as high as +0.150 in w.g. (+37.3 Pa) and central zone pressure as low as -0.059 in w.g. (-14.7 Pa) wrt outdoors were found. These pressures create exfiltration from the closed rooms and infiltration in the central zone. With the air handler operating, the house infiltration rates increased, on average, from 0.46 to 0.60 air changes per hour when all interior doors were closed. According to homeowner reports, interior doors are closed 11% of the time, on average. These pressures and increased infiltration rates impact indoor air quality, indoor relative humidity, energy use, peak electrical demand, comfort, and system sizing.

Date Published



Reference Publication: James B. Cummings and Charles R. Withers, Jr.. "Unbalanced Return Air in Residences: Causes, Consequences, and Solutions". ASHRAE Transactions Vol 112, Part 1, January 2006.

Local Subjects

Buildings - Ducts; Buildings - HVAC


FSEC Energy Research Center® Collection



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