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American Society of Heating and Ventilating Engineers Guide, 1925-26 efficients are always based on the difference between the air temperatures on the inside and the outside of the wall. Transmission Coefficients by Computation If heat transmission coefficients are to be compjuted, and in many, if not most cases, they should be computed, the following analysis of the transmission of heat through a simple, solid wallds used as the basis for such computations. The diagram in Fig. 1 exhibits four important temperatures: First the air temperature t inside of the building; second, the temperature t, of the inside surface of the wall; third, the temperature t, of the outside surface of the wall, andfourth, the air temperature to outside of the build ing. Heat reaches or enters the inside surface of the wall by radiation and convection, since the air and objects A within the building are always ' American Society of Heating and Ventilating Engineers Guide, 1925-26 ' Now Ko may not equal Ki, in which case (to-to) will not equal (t-t,). Usually, in an actual wall exposed to wind on the outside, Ko (Table 5) is greater-than' Ki and (t,-t0) must be less.than (t-h). Moreover, the heat He passing through the wall by conduction is equal to H, and H,, and if C is the coefficient of conduction = B.t.u. transmitted per hour per. square foot of material per 1 in.- thickness per degree difference' between the surface temperatures, then 1 H1 = HI = HC = (. -t,)S (3) i Fig. 1. Temperature Curve or Gradient from Air Inside to and through Wall to Air Outside, Wall Material Assumed Air-Tight A represents warm surfaces at temperature t of inside air; B represents cold surfaces at temperature to of outside air. For an actual temperature gradient curve see Fig. 2. warmer than the inside surface of the wall, when the inside air tempera ture t is greater than the outside air temperature to. This heat must then pass through the material of the wall from inside to outside surface by conduction, and is finally given off from the outside surface by radiation and convection, provided, of course, that equilibrium has been established and all four temperatures are constant. The..amount of heat reaching or entering the wall per hour depends on t and h and a coefficient K, varying with the character of the wall material. Ki may be defined as the B.t.u. per hour entering each square foot of wall surface per degree difference between the inside air tempera ture t and the inside surface temperature Hence the heat received by inner surface of the wall per hour by both radiation and convection is Hy = KAt-tJS (1)_ where 5 is the inner wall surface area in square feet and the other terms are as heretofore indicated. Whatever amount of heat H, enters the inner wall surface must be given off from the outer wall surface, so that if Ho represents heat emitted from outer surface H, = Ho ~ Ho (to -- to) 5. (2) 8 Fig. 2. Temperature Gradient Curves for Glass (Taken from Bulletin No. 24, Engrg. Exp. Sta., Penna. State College) These equations (1), (2) and (3) are fundamental and are used for determining values for Ki, Ko and C-for actual wall materials by test. They cannot be used for computing heat losses in an actual building, since the surface temperatures ti and to are seldom known, although these surface temperatures can be determined in a test by means of thermo couples. Hence, for actual conditions where the only temperatures known are the inside and outside air temperatures t and to, it is necessary to use a transmission coefficient U = B.t.u. transmitted per hour per square foot of wall surface per degree difference between the inside and outside air temperatures. Values of U for a limited number of walls are given in Tables 6-12. The heat H transmitted per hour from air inside to air outside is then computed as follows: H = U (t -to)S (4)