Comparison of Measured and Calculated Residential Cooling Loads PDF

Sensible cooling extraction rates measured at four houses in Ft. Wayne, Indiana are compared to loads calculated with Manual J Eighth Edition (MJ8) and the ASHRAE Residential Heat Balance (RHB) methods. The houses have identical basic construction and are fitted with windows that allow glazing to be changed. Two of the houses have south-facing primary fenestration and the other two west-facing. This setup permits simultaneous measurements of four combinations of glazing and orientation. Data for July, 2005 are presented and related to calculated loads. There is acceptable correspondence for south-facing cases. For west-facing, both MJ8 and RHB produce conservative estimates of cooling requirements. Incident solar intensities derived with the ASHRAE clear sky model are shown to exceed virtually all observed values, causing high calculated solar gains for the west-facing cases that experience near-normal sun angles. An additional source of difference may be the steady-periodic assumption used in load calculation that masks any load reduction due to actual day-to-day variability. Comparison of RHBcalculated hourly loads to measured values for representative days shows only approximate correspondence, indicating the need for further model improvements.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings X – December 2007

Units: Dual