Document reNNL3ZKBkdoXnvJGwGLD5J7a

480 CHAPTER 26 The temperatures in unheated spaces having large glass areas and two or more surfaces exposed to the outdoors (such as sleeping porches and sun parlors) are generally assumed to be the same as that of the outdoors. GROUND TEMPERATURES Ground temperatures to be assumed for estimating base ment heat losses qsually will differ for basement walls and floors, the temperatures under the floors generally being higher that those adjacent to walls.' Factors affecting these temperatures will be discussed. Temperatures Under Basement Floors The temperature of the ground under basement floors1* is affected by heat sources within the basement and is not in fluenced by atmospheric conditions. In computing losses through basement floors, the ground temperatures may be assumed to be the same as water temperatures at depths of 30 to 60 ft.17 Test observations indicate that heat losses through basement floors frequently are overestimated.1* Temperatures Adjacent to Basement Walls Ground temperatures near the surface and under open spaces vary with the climate, the season and the depth below the surface. The nearer the surface (during the cold weather) the lower will be the ground temperature. Frost will penetrate to a depth of over 4 ft in some localities, if the ground is not protected by snow. A thick blanket of snow will result in a higher ground temperature near the surface. Therefore, in localities where the ground is covered with a heavy hla.nk<t. of snow throughout the winter, the ground temperatures near the surface will be higher than when little or no snow is pres ent. The ground temperature adjacent to the walls of. a heated basement is greatly affected by the heat gain from the base ment. Unfortunately, complete data on ground temperatures adjacent to buildings are not available, but since the recom mended transmission coefficient for basement walls in contact with the soil is only 0.10, any reasonable, assumed ground temperature will not materially affect the calculated heat loss. BASEMENT TEMPERATURES AND HEAT LOSS The allowance to be made for basement heat loss depends on whether the basement is to be heated or not. If the basement is heated to a specified temperature, the heat loss should be calculated in the usual manner, based on the proper wall and floor coefficients (see Chapter 24) and the outdoor air and ground temperatures. Heat kiss through win dows and walls above grade should be based on outdoor tem peratures and the proper air-to-air - coefficients. Heat loss through basement walls below grade should be based on the floor and wall coefficients for.surfaces in contact with the the soil, and on the proper ground temperature. Table 4 .... Below Grade Heat Losses for Basement Wads and Boors Ground Wafer Tomporatm* kmoirt Floor ton* . Batov Grade Watt tot* ttv/Sq Ft . Bto/Sq Ft 40 3.0 6.0 50 2.0 4.0 60 1.0 2.0 * BmcJ os tenement tcmpeaUaro of 70 F and <7 of 0.10. Anmod twice tenement Soar loo.. 1965 Guide And Data Book The heat loss values for below-grade basement walls and floors given in Table 4 are sufficiently precise for general practice. If a basement is completely below grade and is not heated, the temperature in the basement normally will range be tween that in the rooms above and that of the ground. Base ment windows will, of course, lower the basement tempera ture when it is cold outdoors, and heat given off by the hating plant will increase the basement temperature. In any rA the exact basement temperature is indeterminate if the basement is not heated. In general, it is found that the transient heat from the heating plant warms the air near the basement' ceiling sufficiently to make it unnecessary to make an allow ance for floor heat loss from rooms located over the basement. The temperature in crawl spaces below floors will vary widely depending on the number and size of wall vents, tire amount of warm piping present and type of piping insulation. It is necessary, therefore, to evaluate the conditions and to select an appropriate temperature by judgment. HEAT LOSSES FROM FLOOR SLABS Two types of concrete floors used in basementless bouses are (o) the unheated floor, relying for warmth on heat de livered above floor level by the hating system, and (6) the heated floor containing heated pipes or ducts that constitute a radiant slab or portion thereof for complete or partial beating of the house. For type (a), the floor heat loss, economically considered, is of minor importance since it generally comprises about 10 percent of the total heat loss of the house. From the oomfort standpoint, however, it may be most important, nro honors with cold floors are not successfully heated. In thin connec tion; it should be remembered that a well insulated floor does not assure comfort if downdrafts from windows or walls create pools of chilly air over considerable areas of'the floor. For this reason, a floor of type (a) should not be used in a severe climate except with a Hp^ting system that delivers enough heat near the floor to counteract the downdrafts of the exterior walls and the heat transmission through the floor. The results of some experiments1*'1' with type (a) uriheated floor slabs indicate that the heat loss from a concrete slab floor on grade is more nearly proportional.to the perim eter than to the area of the floor, and that the heat loss can be estimated by means of the equation: . Ue - PP ft - C> (3) where Hr " heat loss of the floor, Btu per hour. P = perimeter or exposed edge of the floor, linear feet. P * heat loss coefficient, Btu per (hour) (linear foot of ex posed edge) (degree difference in temperature between the indoor air and the outdoor air). (F ranges between 0.81 for & floor with no edge insulation to 0.55 for a floor with edge insulation.) k - indoor air temperature, Fahrenheit. C -- outdoor air temperature, Fahrenheit. In most instances the values given in Table 5 for edge los are of sufficient precision for use.** The insulation shown ex tending under the floor horizontally for 2 ft pan also be located along the foundation wall with equal effectiveness if the insulation extends 24 in. below the floor level. Example 3: Calculate the heat loss from the floor of a 12 ft X 15 ft room with two exposures. The floor.is an unheated'con crete slab which is insulated at the edge with 2 in. of insulation extending horizontally for a distance of 2 ft from the edge, and the house is located in an area with an outdoor design tempera ture of -- 15 F. Heating Load 481 Table 5 . Heat Loss, of Concrete Boors at or Near Grade Level per Foot of Exposed Edge:-..u mended.****3'1* 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 houses, and may exceed 20 percent if only one inch of insula tion is used at the edge of the floor. ' 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 water proof membrane should be installed over the gravel fill. Obviously, it is important that such floors be laid several inches above grade, and that effective subsoil drainage be provided to avoid glaha anafeod by rain or melting snow, and consequent excessive heat loss. Solution: From Table 5, the heat loss per foot of exposed edge b 45 Btu per hr. The length of exposed edge is 12 ft + 15 ft =* 27 ft, and the total edge loss 27 X 45 = 1215 Btu per hr.' - Floors of type (b), containing heating pipes or ducts, are now in common use. The beat loss downward into the ground and outward through the edges of the floor slab is called the' reverse loss. . The results11 of an investigation in which a warm air perimeter duct was embedded in four types of concrete floorslab 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 correspond' ing 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 water-resistant material is the minimum thickness of edge insulation that should be used, but a 2-in. thickness is recom- Table 6 .... Boor Heat Loss to be Used When Warm Air Perimeter-Heating Ducts Are Embedded in Slabs* /, . Bfi per (hoar) (finoar foot of htofod odgo) ,t i t Cdgo Insulation Qrfdoor Den'pa l-Ea. Vortical Extend* 14k t-Typ* Extend* 2-in. t-lfpo Extend lampendure, mg Down 18 in. . mg at loest 13 . ing at liast.12 in. F Below Floor in. Deep and 12, in. Down end 12 m., Sorfoco Undot Under ' --20 ' -10 0 10 20 105 100 85 95 ..............90 75 85 80 ''65 75 '70 " ' 65 62 ".57 * ' '45 - * Factor* iadoda loan doernward throath iaow are* of slab. , TRANSMISSION HEAT LOSS , The basic formula for the loss of heat by transmission through any surface is given in Equation 4: H, - AU &-Q . (4) where Hi -- heat loss transmitted through the wall, roof, ceiling, ' floor, or glass, Btu per hour. A -- area of wall, glass, roof, ceiling; floor, or other exposed surface, square feet. - U coefficient of transmission, air-to-air, Btu per (hour) (square foot) (Fahrenheit degree temperature differ- - ence). U = indoor temperature near surface involved (this - may not necessarily be the eo-called breathing-line tem perature), Fahrenheit degrees. C " outdoor temperature, or temperature of adjacent un' : heated space or of the ground, Fahrenheit degrees. Example 4: Calculate the transmission loss through an 8-in. brick wall having an area of 150 sq ft, if the indoor temperature U is 70 F, and the outdoor temperature f. is -- 10 F. Solution: The coefficient of transmission 17 of a plain 8-in. brick wall is 0.41 (Chapter 24, Table 6). The area A is 150 sq ft. Substituting in Equation 4: Hi = 150 X 0.41 X (70 - (- 10)] - 4,920 Btu per hour. 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 in door-outdoor temperature difference (t, -- ti), and the proper value of U: a. Plat roofs. Select the coefficient oftransmission of the cefl. ing and roof from Tables 11 to 14, Chapter 24, or use appro.. pnate coefficients in Equation 1 if side walls extend appro* ciably above the routing of the floor below. b. Pitched roofs. Select the combined roof and ceiling coeffi- .. dent from Table 15, Chapter 24, or calculate the combined .. . roof rolling coefficient by meaos of Equations 4 and 5. Chapter 24, where these formulas are applicable, as explained in Chapter 24. 2. By estimating the attic temperature (based on the indoor outdoor design temperatures) by means of Equation 1, and .substituting for C in Equation 4, the value of 1, thus obtained, 'together with.the rolling area..4 and the ceiling coefficient U. Tus applies to pitched roofs, In the case of flat roofs it is not 'necessary to calculate the attic:temperatures, as the ceiling-roof heat loss rotn be deter""""! ns suggested in preceding paragraph la. 1 INFILTRATION HEAT LOSS ' The infiltration heat loss'includes (1) the sensible heat loss, or.the heat required to warm the outdoor air entering by infiltration, and (2) the latent heal loss, or the heat equivalent 'of any moisture which must be added.