The rate of heat loss through a unit surface area of a window per unit temperature difference between the indoors and the outdoors is called the U-factor. The value of the U-factor ranges from about 1.25?W/m2•K (or 0.22?Btu/h•ft2•°F) for low-e coated, argon-filled, quadruple-pane windows to 6.25?W/m2•K (or 1.1?Btu/h•ft2•°F) for a single-pane window with aluminum frames. Determine the range for the rate of heat loss through a 1.2-m × 1.8-m window of a house that is maintained at 20°C when the outdoor air temperature is ??8°C.

Question

What will be an ideal response?

Answer 1

The range of U-factors for windows are given. The range for the rate of heat loss through the window of a house is to be determined.

Assumptions 1 Steady operating conditions exist. 2 Heat losses associated with the infiltration of air through the cracks/openings are not considered.

Analysis The rate of heat transfer through the window can be determined from



where Ti and To are the indoor and outdoor air temperatures, respectively, Uoverall is the U-factor (the overall heat transfer coefficient) of the window, and Awindow is the window area. Substituting,





Discussion Note that the rate of heat loss through windows of identical size may differ by a factor of 5, depending on how the windows are constructed.