Document RjvObKzozrMY9X23kpEMeby3E

American Society of Heating and Ventilating Engineers Guide, 1935 be used. If weatherstripped, the values, may be reduced one-half single door which is frequently opened, such as might be found in a sto should have a value applied, which is three times that for a well door. This extra allowance is for opening and closing losses and is kfrom being greater by the fact that doors are not used as much i coldest and windiest weather. CHOOSING WIND VELOCITY Although all authorities do not agree upon the value of the wind vel^ ity that should be chosen for any given locality, it is common engineer^ practice to use the average wind velocity during the three coldest mont^ of the year. Until this point is definitely established the practice o( using average values will be followed. Average wind velocities for tb months of December, January arid February for various cities in the United States and Canada are given in Table 2, Chapter 7. 50.03 50 W 150 ZOO 250 300 Infiltration Cfifi Pae roar Or Crack Infiltration through Sash Perimeter of Window with and without- Fig. 4. Storm Sash--%4-in. Crack and JS2-IN. Clearance Fig. 3. Diagram Illustrating Crack and Clearance In considering both the transmission and infiltration losses, , the more exact procedure would be to select the outside temperature and the wind velocity corresponding thereto, based on Weather Bureau records, which would result in the maximum heat demand. Since the proportion of transrnission and infiltration losses varies with the construction and is different for every building,' the proper combination of temperature and wind velocity to.be selected would be different for every type of building, even in the same locality. Furthermore, such a procedure would neces sitate a laborious;cut-and-try process in every case in order to determine the worst combination of conditions for the building under consideration. It would also be necessary to consider heat lag due to heat capacity in the case of heavy masonry walls, and other factors, to arrive at the most accurate solution of the problem. Although Heat capacity should be con sidered wherever possible, it is seldom/possible to accurately determinethe worst combination of outside temperature and wind velocity for a given building and locality. The usual procedure, as already explained, is to select an outside temperature based on the lowest on record and the average wind velocity during the months of December, January and February. The direction of. prevailing wirids riiay usually be included within anarigle of about 90 deg. The windows that are to be figured for. prevailing 124 and non-prevailing winds will ordinarily each occupy about one-half the perimeter of the structure, the proportion varying to a considerable extent with the plan of the structure. (See discussion of wind movement in Chapter 4.) Infiltration C.F.H. Per Foot Of Crack Fig. 5. Infiltration through Sash Perimeter of Window with and without Storm Sash--J^j-in. Crack and J^-in. Clearance 125 99