Document xzRV6jxggVN2ea3ODRmGpJ7O0

458 CHAPTER 25 1965 Guide And Data Book heigut. Fur tile cpeCiuC Case illustrated 12 Fig. 3(c), the pressure differences created by wind and those due to tem perature difference acting singly <yyn be added to determine the combined effect. Strictly speaking, however, this is not true in most instances since the effective areas of openings for inflow and outflow are not the same and further the ratio of these areas does not remain constant as the relative magni tudes of the pressure forces change. The inside pressure dis tribution must shift to maintain inflow equal to outflow and, the pressure differences depend on this equality. Information on tha relative importance of wind and tem perature difference forces based on actual observation is quite limited.* In studies on tall buildings14 exfiltration from windward facing rooms in upper stories was noted at low out door temperatures even with relatively high winds. Pressures due to temperature difference during the heating season dominate at entrances to tall buildings1 in cold climates. Thus temperature difference forces must be taken into ac count in estimating air flow into tall buildings at various elevations. In low buildings the nature of the terrain and shielding provided by adjacent structures will have a major effect on wind velocity and therefore on the relative impor tance of wind and temperature difference. Air leakage studies on two houses7 in an urban area indicated that temperature difference forces were at least as important as wind forces corresponding to weather station winds of 15 mph, when indoor-outdoor temperature differences were 70 F deg or greater in one instance, and 20 F deg or greater in another. CALCULATION OF INFILTRATION Air infiltration may account for a significant proportion of the heating or cooling requirements for buildings. It is there fore important to be able to make an adequate estimate of its contribution with respect to both design loads and energy requirements. Air infiltration is also an important factor in determining the relative humidity that will occur in buildings or, conversely, the amount of humidification or dehumidification required to maintain given humidities. There are two methods of estimating air infiltration in buildings. In one case the estimate is based on measured leakage characteristics of the building components and se lected pressure differences. This is known as the crack method, once cracks around windows and doors are usually the major, source of air leakage. The other method is known as the air change method and consists of resuming a certain number of air changes per hour for each room, tire number of changes assumed being dependent upon the type, use, and location of the room. The crack method is generally regarded as being more accurate, provided that leakage characteristics and pressure differences can be properly evaluated. Otherwise the sir change method may be justified. The air leakage due to opening and closing of doors in vestibules is sometimes based on the air change method, even though the air leakage estimates for other rooms are based on the crack method. Except for vestibules and reception halls, it is not advisable to attempt to apply the air change method to factories and industrial and commercial buildings, because of wide variations in the type and percentage of fenestration which is the principal source of leakage in such buddings. In refrigerated rooms or buildings, in which opening of doors provides the major source of air leakage, infiltration is usually' based on estimated air changes. The accuracy of estimating infiltration for design load cal culations by the crack or component method is restricted both by tire limitations in information on air leakage characteris tics of components and by the difficulty of estimating the pressure differences under appropriate design conditions of Table I .... Air Changes Taking Place under Average Conditions in Residences, Exclusive of Air Provided for Ventilation* Kjod of Rood or fioSdfag Nwfar of ASr Change* faking Plot* pot Hoot Rooms with windows or exterior doors on one ride. Rooms with windows or exterior doors on two H l Rooms with windows or grtorinr doors on three * Far rooms with wewtfeastcipped windows ar with storm ash. use temperature and wind. The calculation'of infiltration rates suitable for prediction of renn] energy requirements is more difficult, since ideally the product of infiltration indooroutdoor temperature difference integrated over the season, is required. The measurement of air leakage characteristics of building components, such as walls, windows and doors, b quite straightforward and the principal problem here lies in the variety of components used in buildings for which specific air leakage data are not available, and the differences that develop between components as tested and as installed or constructed in the field. The major limitation, however, is in estimating the appropriate pressure differences. Further stud ies are required to determine practical relationships between the various factors affecting air change. Air Change Method There are few published data on the air change rates for different buildings. Experience and judgement are required to obtain satisfactory estimates by this method. The values in Table 1 may be used with reasonable precision for residences, and are the requirements for each room. Some engineers as sume that the total infiltration allowance for the entire build ing is % the sum of the infiltration allowances of tire individual rooms, since whatever air enters on the windward side gener ally leaves the building on the leeward side, and the infiltra tion requirements do not exist simultaneously on all sides or in all rooms. For residences, it is common practice to assume that the total infiltration allowance for the residence is equal to the sum of the infiltration allowances of the individual rooms. Air Leakage Through Windows Air leakage associated with closed windows occurs through cracks between the frame and surrounding construction and between and frame, the latter usually predominating- Air flow through window cracks can be expressed by an equation of the form: where Q - CAp* (3) Q - flow rate. C = proportionality constant. n b exponent of flow, between H &nd 1. Ap = pressure difference across window. Air leakage characteristics of windows are usually expressed as flow rate per foot of sash crack. The assumption that all cracks in a window arrangement are the same is not strictly correct, but usually is close enough for design purposes. Infiltration rates for a number of window types at pressure ' differences corresponding to different wind velocities are given Infiltration and Ventilation 459 . Table 2 .... Infiltration Through Windows Exprotsod fa cubic foot pot tool of crock pot hoar* Typo of Window fieooriu 5 Double-Hung Wood Sash Windows (Unlocked) Around frame in masonry wall--not calked*........................ Around frame in masonry wall--calked*................................ Around frame in wood frame construction*.......................... Total for average window, non-weatherstripped, craek and %-in. clearance." Includes wood frame leakage4........................................................................................... Ditto, weatberstripped4............................................................ Total for poorly fitted window, non-weatherstripped, %ain. crack and Ha-In. clearance." Includes wood frame leakage4.................................................................................... Ditto, weatherstripped4............................................................ 3 1 2 7 4 27 6 Wind Velocity, Mite per Hear 10 15 20 25 30 8 14 20 27 35 2 3456 6 11 17 23 30 21 39 59 80 104 13 24 36 49 63 69 111 154 199 249 19 34 51 71 92 Double-Hung Metal Windows* Non-weatherstripped, locked................................................... Non-weatherstripped, unlocked........... ................................ Weatberstripped, unlocked....................................................... 20 20 6 45 47 19 70 96 125 154 74 104 137 170 32 46 60 76 Rolled Section Steel Sash Windows* Industrial pivoted, Ke-m- crack*.............. ............................. Architectural projected, Hs-in. crack*................................... Architectural projected, H*-in. crack*................................... Residential casement, %4-in. crack*....................................... Residential casement, Ha-ta- crack*....................................... Heavy casement section, projected, crack*.............. Heavy easement section, projected, Ha *n- crack'............ 52 15 20 6 14 3 8 108 36 52 18 32 10 24 176 244 304 372 62 86 112 139 88 116 152 182 33 47 60 74 52 76 100 128 18 26 36 48 38 54 72 92 Hollow Metal. Vertically Pivoted Window*. ........................................................................ 30 88 145 186 221 242 * Tt v*laa jives inthi*t&bU, withtheexteptico ai the** It* double-ban*nod hollow metal window* are J8 pcoent loo then feet vehjee to Uow lor bcildin* np of presun fa rooms, sad am based on test data, reported fa tbs papers listed fa p-- footnotes. * Tbs values fjvsn for frame leaksca are per foot of aasb perimeter, a* determined tot double-bung wood windows. Some of the frame leakage fa masoer? walls cri*fastes fa tbe briek wall itself, and cannot be prevented by reiving For tbs additional reesca that aalldng b not dons perfectly end dsterhntee with time. It b con sidered advisable to choose the masonry frame leakage values for ealked.frames as Uw average detenaiaoaby tbs calked and noo-calked tests. *lhe fit of the average double-hang wood window was determined as He-fa. uaok and )l-fa- clears new by measurements on approximately 800 windows under * The values given are tbe totsb for tbs window opening per foot of each perimeter, and include frame ImVagr and to-milod tUra&ert laohops. The frame v-f* vans included are for wood frame but apply as well to ------wmiw| a 00 Ft--1efficiency of frame relHwy. * A Hi-fa- sack and clearance represent s poorly fitted window, mnch poorer than avoage. 1 Windows tasted fa pises fa building, so that no reduction from tat values b uoessary, as mentioned fa footnote a. * Industrial pivoted window generally tsed fa fadustrbl buildings. Ventilators hcrbontaUy pivoted at centeror slightly above, lower part swinging out. , .Architecturally projected made of same sections as industrial pivoted, except that outside framing member b heavier, and it has refinements fa weathering and hardware. Used in semi-monumental buildings stteh as schools. Ventilators swing in or out and are haUwreJ on side arms. Hs-in. mack b obtainable fa the bat praotacs of manufacture and installation, Hv-in. erack considered to raproent average practice. `Of samedesign and section shapes as so-eallad heavy section cescswaf, but of lighter weight, `/win. ereck b obtainshie in the bat practice of manufacture and fa* rtalhtioa. Va-fa- crack considered to represent avenge practice. * Made cf heavy sections. Ventilators swing fa or out and stay set at any stallstmn, `/a-fa. ereck considered to represent average practice. Known as lot * With reasonable care fa installation, leakage at contacts where windows are attached to steel framework and at muUfans, b negligible. With Yw-fa. crack, repretetrtfagpoar installation, leakage at contact with steel framework b about one-third, and at mullioaa, about coe-aixtb of that gives tor industrial pivoted windows in in Table 2.1# The values given in the table are 20 percent less than test values, which implies that the actual pressure dif ference across windward walls in a building is approximately 0.64 p,, taking the exponent n in Equation 3 equal to The wind velocity in Table 2 corresponding to pressure differences due to chimney effect can be found from Equation 2. An alternative approach is to equate Equations 1 and 2, solve for V*, and increase the leakage values found in Table 2 at this wind velocity by 20 percent. There are a variety of designs of windows now in use in both residences and commercial buildings for which there are no published air leakage data. In some instances data may be available from the manufacturer. In large rnmynereial build ings it is quite common for architects to specify minimum in filtration rates based on.testa by. independent laboratories. The specification11 of the Aluminum Window Manufacturers Association includes m^rimnm air leakage rates for a variety of aluminum -window types.based on a pressure difference during test of 0.301 in. of water. The fit of double-hung wood windows is determined by crack and clearance. Crack thickness is equivalent to onehalf tiie difference between the inside window frame dimen sion and the outside sash width. Hie difference between the width of the window frame guide and the sash thickness is considered as the clearance. .The length of the perimeter opening or crack for a double-hung window is equal to three times the width, plus two times the height, or in other words, it is the outer sash perimeter length, plus the meeting rail length. All of the window crack in any given room is not necessarily used in estimating the infiltration heat loss by the crack method. The length of crack to be selected in any given case depends on the number of exposed rides, as explained in Chapter 26. Values of leakage shown in Table 2 for the average double-hung wood window were determined by using, on nine windows tested in the laboratory, the average measured crack and clearance of a large numberof windows found in a field survey. In addition the table gives figures for a