Document vVOB83DdkZn7jKG3gNw8vRv99

254 CHAPTER 12 1954 Guide ^ use. The heat loss downward into the ground and outward through the jj; edges of the floor slab is called the reverse loss. The results13 of an investigation in which a warm-air perimeter duct was embedded in four types of concrete floor slab and foundation constructions has verified the indication that Equation 3 can be used to calculate the reverse loss when warm-air perimeter heating ducts are used. To make the results of this application more usable, values corresponding to those shown in Table 5 for unheated' floors are given in Table 6 for concrete floors with a warm-air perimeter duct. The desirability of edge insulation is apparent. One inch of waterresistant material is the minimum thickness of edge insulation that should be used, but a 2-in. thickness is recommended.12-14 The values of edge loss in Table 6 indicate that the reverse heat loss of heated slabs is likely to be about 20 percent of the total heat loss of many types of present day Table 5. Heat Loss of Concrete Floors at or Near Grade Level per Foot op Exposed Edge houses, and may exceed 20 per cent if only one inch of insulation is used -v at the edge of the floor. a The concrete floor slab is usually placed on a gravel fill 4 in. thick or more, both to insulate the floor from the earth and to retard the rise of;-, ground water by capillarity. A waterproof membrane should be in- - stalled over the gravel fill. Obviously, it is important that such floors be laid several inches above grade, and that effective subsoil drainage be pro- j vided to avoid slabs soaked by rain or melting snow, and consequents;- excessive heat loss. TRANSMISSION HEAT LOSS The basic formula for the loss of heat by transmission through surface is given in Equation 4: H, = AU (t - f,,) where v! H, = heat loss transmitted through the wall, roof, ceiling, floor, or glass, tsiuj-- per hour. 'y[ A = area of wall, glass, roof, ceiling, floor, or other exposed surface, square feet>g :V'- | ."2 Heating Load ,, --___ _ u, ,,iausmission, air to air, Btu per (hour) (square foot) (Fahr enheit degree temperature difference) (Chapter 9). t = inside temperature near surface involved (this may not necessarily be the so-called breathing line temperature), Fahrenheit degrees. t,, = outside temperature, or temperature of adjacent unheated space or of the ground, Fahrenheit degrees. Example 4 Calculate the transmission loss through an 8 in. brick wall having an iasre-a o10f 1F5.0 sq ft, if the inside temperature f is 70 F and the outside temperature (,, Solution: The coefficient of transmission (I/) of a plain 8 in. brick wall is 0.50 (Chapter 9, Table 8). The area (A) is 150 sq ft. Substituting in Equation 4: Hi = 150 X 0.50.X [70 - (-10)] = 6000 Btu per hour. Table 6. Floor Heat Loss to be Used When Warm-Air Perimeter Heating Ducts Are Embedded in Slab* Btu per (hour) (linear fool of heated edge) Outdoor Design Edge Insulation Temperature, F Lin. Vertical Extend- ingDown 18 in. Below Floor Sureace Lin. L-Type Extend ing at Least 12 in. Deep and 12 in. Under 2-in. i me JDown L-Type Extend- at Least 12 in. and 12 in. Under -20 -10 0 10 20 105 95 85 75 62 . * Factors include loss downward throQgh inner area of slab 100 90 80 70 57 .i 85 75 65 55 45 Transmission Loss Through Ceilings and Roofs The transmission heat loss through top floor ceilings, attics, and roofs may be estimated by either of two methods: 1. By substituting in Equation 4 the ceiling area A, the inside-outside tempera ture difference (t -- t0) and the proper value of U: a. Flat roofs. Select the coefficient of transmission of the ceiling and roof from Tables 15 or 16, Chapter 9, or use appropriate coefficients in Equation 1 if side walls extend appreciably above the ceiling of the floor below. b. Pitched roofs. Select the combined roof and ceiling coefficient from Table 18, Chapter 9 or calculate the combined roof and ceiling coefficient by means of Equations 4 and 5, Chapter 9, where these formulas are applicable as explained m Chapter 9. 2. By estimating the attic temperature (based on the inside and outside design ^emperatures) by means of Equation 1, and substituting for t in Equation 4, the [jn%** ^us obtained, together with the ceiling area A and the ceiling coefficient ajiculate tahpeplaiettsictotempitpcehreadturroeosf,s.as Itnhethceeilcinasge-rooof ffihaet artoloofsssictains bneotdenteecremsisnaeryd atos ggested in paragraph la. INFILTRATION HEAT LOSS The infiltration heat loss includes (1) the sensible heal loss or . the heat e<u loss or twhaerhmeat eqouuitvsaidleenat ior feannteyrimngoibsytuirnefiwltrhaictihonm,uasntdbe(2a)dtdheed.latent