## Building Heat Transfer

The design of space heating or cooling systems for a building requires the determination of the building thermal resistance. Heat is transferred in building components by all modes: conduction, convection, and radiation. In an electrical analogy, the rate of heat transfer through each building component can be obtained from

"total where

ATtotal = total temperature difference between inside and outside air (K). "totai = total thermal resistance across the building element, = YRi (m2-K/W). A = area of the building element perpendicular to the heat flow direction (m2).

It is obvious from Eq. (6.28) that the overall heat transfer coefficient, U, is equal to

"total

As in collector heat transfer, described in Chapter 3, it is easier to apply an electrical analogy to evaluate the building thermal resistances. For conduction heat transfer through a wall element of thickness x (m) and thermal conductivity k (W/m-K), the thermal resistance, based on a unit area, is x

The thermal resistance per unit area for convection and radiation heat transfer, with a combined convection and radiation heat transfer coefficient h (W/m2-K), is

Figure 6.1 illustrates a single-element wall. The thermal resistance due to conduction through the wall is x/k, Eq. (6.30), and the thermal resistance at the Outside air

FIGURE 6.1 Heat transfer through a building element and equivalent electric circuit.

Outside air

FIGURE 6.1 Heat transfer through a building element and equivalent electric circuit. inside and outside boundaries of the wall are 1/h; and 1/ho, Eq. (6.31), respectively. Therefore, from the preceding discussion, the total thermal resistance based on the inside and outside temperature difference is the sum of the three resistances as

hi k ho

total