Document re7D8jV5eDRnoQxZbKve0EnGV

American Society of Heating and Ventilating Engineers Guide, 1929 If a roof contains two or more dormers and the attic is unheated, it is advisable to disregard the roof structure propter and consider only the top-floor ceiling in determining the radiation requirements of the building. In this case it will be necessary to assume the temperature in the attic, which can be taken to be the mean between the temperature under the top-floor ceiling (not the attic) and the outside temperature. AREAS WHERE HEAT LOSSES OCCUR Heat is lost from a building by transmission through all of those sur faces which separate heated spaces from the outside air or from unheated colder spaces within the building. In general, five kinds of surfaces are involved: (1) outside walls, (2) outside glass, (3) inside walls or parti tions next to unheated spaces, (4) ceilings of upper floors, either below a cold attic space or as the underside of a roof slab, and (5) floors of heated rooms above an unheated space. In most cases, only items (1) and (2), outside wall and glass surface, are considered. Failure to take account of the other heat losing surfaces, items (3), (4) and (5), when they exist in a building, has generally resulted in more or less dissatisfaction with the operation of the heating plant, as a result of failure to heat the rooms having such surfaces as indicated by items (3), (4) and (5). The net outside wall surface is usually determined by reference to the scale plans and elevations of the building concerned. In some cases of course, the actual building may have to be measured. The total area of all outside openings which are occupied by windows and doors is accurately measured and listed as glass. The glass, area is then deducted from the total outside wall area for each room and the difference is the net wall area. The outside wall areas for any floor should be based on the vertical floor to floor heights and the horizontal distance from center to center of partitions separating different rooms. If there are no partitions, measure from inside face of one wall to inside face of next wall. The areas of walls, ceilings and floors next to cold or unheated spaces are found, of course, by taking the inside .dimensions of such areas, measured on the heated side. CALCULATIONS FOR HEAT TRANSMISSION LOSSES The calculations for heat transmission losses are made by multiplying the area 5 in square feet of wall, glass, roof or floor through which the loss takes place, by the proper coefficient U for such construction (Tables 7 to 13, or by computation as described underTransmission Coefficients by Computation) and by the temperature difference between the inside air temperature t at the proper level (in many cases not the "breathing line") and the outside air temperature h- Therefore, where fit = SU (t-h) (9) Ht = B.t.u. per hr. transmitted through the material of the wall, glass, roof or floor. S = area in sq. ft. of wall, glass, roof or floor, taken from building plans or actually measured. (Use the net inside or heated surface dimensions in all cases.) 46 v Chapter I--Heat Losses from Buildings U - coefficient of heat transmission or B.t.u. per hr. per sq. ft. per I deg. fahr. difference between the inside and outside air temperature for air conditions , such as exist in the given locality in coldest weather. 1 _ k) = temperature difference between inside and outside air, in which f must always be taken at the proper level. Note that l may not be the " breathing line" temperature in many cases. For examples showing application of equation (9) to practical examples ee Applications at the end of this chapter, in which the heat require- nents are computed for typical cases. ViTnhdeMeofvfeecmteonft wind on the heating requirements of any building should ie given consideration under two heads: 1. Wind movement increases the heat transmission of walls, glass, and roof, affecting joo2.r Wwainllsdtomaovmemucehntgrmeaateterreiaxltleynitntchreaansegsootdhewainllsf.iltration (inleakage) of cold air . hrough the cracks around doors and windows, and even through the building materials themselves, if such materials are at all porous. It is entirely possible that a building may require more heat on a windy day with a moderately low outside tempierature, than on a quiet day with a much lower outside temperature. It will therefore be evident that the wind movement in any locality must be given careful considera tion in computing the probable heating requirements of a building, and for the purposes of calculation, not less than the average wind movement in any locality during December, January and February should always be provided for in computing (I) the heat transmission of a building, and (2) the heat required to take care of the infiltration of outside air. The first condition is readily taken care of as already explained, by using a surface coefficient K, for the outside wall surface which is based on the proper wind velocity (Table 6). In case specific data are lacking Table 13. Coefficients of Transmission (U) of Doors, Windows and Skylights Note.--These coefficients are based on a wind exposure of 15 miles per hour, and are expressed in B.tu. per hour per square foot per deg.-fahr. difference in temperature between the air inside and outside of the door, window or skylight. Single-----Double-- Triple___ A. Windows and Skylights a .... 1.13* t .. 0.45* .... 0.281* B. Solid Wood Doors** t Nominal Thickness Inches 1 Actual Thickness Inches V 0.563 0.485 m2233_'A_____________i2m2_&K%_s___________0000_....4332_3827_2217___________ -- See page 59. Volume l. "Mechanical Equipment of Buildings;" by Harding and Willard. . fiCtoims psuuteffdiciuesnitnlyg Caccur1a.t0e ftoor uwsoeotdh.eKsai m=>e 1c.o3e4ffaicniedntKo2f =tra4n.0s2m.ission for doors containing thin wood els, as that of single panes of glass,' namely, 1.13 B.t.u. per hr. per square foot per 1 deg. fahr. differ. - !--' '* - --a air temoerature.