Document zbGZeLX2zBgMpvo71w5yxom2B

American Society of Heating and Ventilating Engineers Guide, 1936 be used. If weatherstripped, the values may be reduced one-half. A single door which is frequently opened, such as might be found in a store, should have a value applied which is three times that for a well fitted door. This extra allowance is for opening and closing losses and is kept from being greater by the fact that doors are not used as much in the coldest and windiest weather. CHOOSING WIND VELOCITY Although all authorities do not agree upon the value of the wind veloc ity that should be chosen for any given locality, it is common engineering practice to use the average wind velocity during the three coldest months of the year. Until this point is definitely established the practice of using average values will be followed. Average wind velocities for the months of December, January and February for various cities in the United States and Canada are given in Table 2, Chapter 7. 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 transmission 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 determine the 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 winds may usually be included within an angle of about 90 deg. The windows that are to be figured for prevailing 136 SO no 150 200 250 soo Infiltration CF.HPer roar Or Crack Fig. 4. Infiltration through Sash Perimeter of Window with and without ' Storm Sash--^IU-in. Crack and H2-IN. Clearance 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.) l? II 1.0 as P? S a`(at/ as 04 as oz Siat t 0 J 1 1 t 1 1t1 T1 t s' s 1 7 1 * y, s 7 y ,1r r 77 7t j r )7 7r r/ 7 r rJ~ // yf *7y y' v O' OlVKM OA5H OU5PZNDE.D C Storm Sash Fastened With Four Turn Buttons II II II 1 II 1 1 1 1 1 1 Infiltration CJF.H. Per Foot Of Crack 5003 4569 4033 35.40 2630 2042 0 rIG- 5- Infiltration through Sash Perimeter of Window with and without Storm Sash--J^-in. Crack and J^-in. Clearance 137