Heat Flow

The rate of heat flow (BTU/h) through a building component depends upon the thermal resistance of the wall, the temperature difference on both sides of the wall, and the area of the component. Expressed as an equation:

where Q = Heat flow rate (BTU/h); A = Area of building component (ft2); AT = Environmental temperature difference on each side of structure (°F); R = Total thermal resistance of the building component (°F bAUx h ).

Heat flows from the area of higher temperature to the area of lower temperature. With the inside temperature greater than the outside temperature, the heat flow will be from inside to outside.

Problem: Determine the heat flow for an 8.0 x 10.0 ft wall with a total R-value of 1.22 and is subject to an inside temperature of 60°F and an outside temperature of 10°F.

Solution:

Heat will move through this wall at the rate of 3,300 BTU/h.

The heat flow rate for all building components except floors can be calculated by using this method. If the floor is a concrete slab, the difference in temperature (AT) across the floor will change as one moves in from the outside edge of the building. The effective R-values for this type of floor or a concrete slab floor with insulation beneath it can be found in Appendix IX. When either of these two values is used, the perimeter of the building (ft) is substituted for the area (ft2) in the heat flow equation. If a floor has a crawl space underneath it, the total R-value can be calculated by using the same procedures used for a ceiling. Floors over basements are much more complex and will not be discussed in this text.

With the information presented in the previous sections the total conduction heat flow for a building can be determined. The process for completing a total heat flow calculation is included in a later chapter because total building heat flow must also include heat lost or gained through ventilation.

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