Document rBEYRGRx77Z1Q0RobberNnb50

250 CHAPTER 12 1954 Guide -m at the floor heat loss, these differentials to be subtracted from tfye breathing level temperature. ATTIC TEMPERATURES m Frequently, it is necessary to estimate the attic temperature, and in such cases Equation I can be used for this purpose:: AJJcTi + lM,U' +_AwUw + A,!/,) h = Ac 4- AtUt + AWU,, 4- AgUg (1) where t,, = attic temperature, Fahrenheit degrees. fi = inside temperature near top floor ceiling, Fahrenheit degrees. ta = outside temperature, Fahrenheit degrees. Ac -- area of ceiling, square feet. A, = area of roof,.square feet. A* = area of net vertical attic wall surface, square feet. Ae = area of attic glass, square feet. TT = coefficient of transmission of ceiling, based on surface conductance of 2.20 Uc (upper surface, see Chapter 9). 2.20 = reciprocal of one-half the air space resistance. = coefficient of transmission of roof, based on surface conductance of 2.20 Ur (lower surface, see Chapter 9). Uw = coefficient of transmission of vertical wall surface. Ug = coefficient of transmission of glass. Example 1. ing conditions: Calculate the t, = 70; t,, ~ temperature in 10; A* = 1000; an A, unheated attic, assuming the follow = 1200; Aw = 100; A, = 10; Ur = tf, 18 0.50; Uc = 0.40; Uw = 0.30; U,, = 1.13. Solution: Substituting these values in Equation 1: (1000 X 0.40 X 70) 4- 10[(1200 X 0.50) + (100 X 0.30) 4- (10 X 1.13)] u. = (1000 X 0.40) + (1200 X 0.50) + (100 X 0.30) + (10 X 1.13) t = 34,413 1041 = 33.1 F. Equation 1 neglects the effect of any interchange of air such as would ; take place through attic vents or louvers intended to preclude attic con- j densation. However, according to tests,8 such venting of attics by means of small louvers or other small openings does not appreciably reduce the || attic temperature and may be neglected without serious error. Neither does this equation take into consideration such factors as heat:|?; exchange between chimney and attic or solar radiation to and from the' roof. Because of these latter effects, actual attic temperatures are fre-|~; quently higher than the calculated values using Equation 1. The atticfg temperature may be calculated in the usual manner by means of Equation-^'; 1, allowing the full value of the roof. The error resulting from this assumpi|| tion will generally be considerably less than if the roof were neglected (as is.-i,, sometimes the practice) and the attic temperature assumed to be the samesg as the outside temperature. When relatively large louvers are installed;^ as is customary in the southern states, the attic temperature is often sumed as the average between inside and outside. For a shorter, approximate method of calculating heat losses throughjiS i Heating Load 251 attics, the combined ceiling and roof coefficient may be used, as described in Chapter 9. TEMPERATURES IN UNHEATED SPACES The heat loss from heated rooms into unheated rooms or spaces must be based on the estimated or assumed temperature in such unheated spaces. This temperature will lie in the range between the inside and outside temperatures, depending on the relative areas of the surfaces ad jacent to the heated room and those exposed to the outside. If the re spective surface areas adjacent to the heated room and exposed to the outside are approximately the same, and: if the coefficients of transmission are approximately equal, the temperature in the unheated space may be assumed to be the mean of the inside and outside design temperatures. If, however, the surface areas and coefficients are unequal, the tempera ture in the unheated space should be estimated by means of Equation 2. l(AiU\ 4- AtUt 4- A,t/, 4- etc.) 4- fco(Aa(7s 4- A.,(/1, 4- AeUe 4- etc.) ' Ait/, 4- AiUi 4~ Ajt/, 4- etc. 4- AJJ* 4' Abt/b 4- A0t/C 4- etc. . where ta -- temperature in unheated space, Fahrenheit degrees. i = inside design temperature of heated room, Fahrenheit degrees. to = outside design temperature, Fahrenheit degrees. Ai, A2, A>, etc. = areas of surface of unheated space adjacent to heated space, square feet. A, Ab, Ac, etc. = areas of surface of unheated space exposed to outside, square feet. Ui, Ut, Ut, etc. = coefficients of transmission of surfaces of A,, A2, A,, etc. U,, t/b, Uc, etc. = coefficients of transmission of surfaces Aj, Ab, Ac, etc. Example 2: Calculate the temperature in an unheated space adjacent to a heated room having surface areas (A,, A., and A3) in contact therewith of 100, 120, and 140 sqft and coefficients (Ui, Ut, and Ua) of 0.15, 0.20, and 0.25, respectively. The surface areaB of the unheated space exposed to the outside (A and Ab) are respectively 100 and 140 sq ft, and the corresponding coefficients are 0.10 and 0.30. The sixth surface is on the ground and is neglected in this example. Assume t = 70 and 4 = -- 10. Solution: Substituting in Equation 2: t = 70[(100 x 0.15) 4- (120 X 0.20) 4- (140 X 0.25)] 4- --101(100 X 0.10) 4- (140 X 0.30)1 (100 X 0.15) 4-(120 X 0.20) 4-(140 X 0.25) 4-(100 X 0.10) 4-(140 X 0.30) t, 4660 126 37 F. The temperatures in unheated spaces having large glass areas and having two or more surfaces exposed to the outside (such as sleeping porches and sun parlors), generally are assumed to be the same as outside. GROUND TEMPERATURES Ground temperatures to be assumed for estimating basement heat losses usually will differ in the case of basement walls and floors, the tem peratures under the floors generally being higher than those adjacent to walls. Factors affecting these temperatures will be discussed. Temperatures Under Basement Floors The temperature of the ground under basement floors9 is affected by heat sources within the basement and is not influenced by atmospheric