Natural ventilation, therefore, is not generally approved except as applied via ordinary windows or with monitors and hoods in the roofs of certain shed-like industrial buildings.
To achieve natural ventilation for rooms it is important that separate openings for supply and escape of the ventilating air shall be provided. Thus a room with windows on two orientations will be benefited by wind pressure far more than one having exposure on one side only, and a room having a warm chimney for exhaust and a window for supply will be more easily ventilated than one with a single window. However, a room having a single window equipped with double hung sash which can be opened at both top and bottom can be ventilated, whether on the windward or leeward side of a building. The warmest air in the room will tend to escape through the upper opening and air will enter from outside through the lower opening. Natural ventilation of this type can be effected more satisfactorily with double hung sash than with casement sash, since with the latter the difference in temperature between the top and bottom of the opening is less sharply defined. It is highly desirable that insect screens shall cover the upper as well as the lower halves of windows so that the upper sash may be low ered, and that the curtains, shades, etc., shall be of such design as to permit free air passage.
In many hotels the summer comfort and ventilation are im proved by having chimney flues or exhaust.fans withdraw air from the interior corridors, so that when the occupant of a small room opens the window and the transom over the door he is assured of a cross draft which will ventilate the room adequately.
Building codes in many communities attempt to protect public health by requiring definite minimum standards of ventilation, especially in rooms frequented by the public, and require that the air introduced shall be at least as free from impurities as the average outside air, and shall have been warmed in cold weather. A practicable measurement-level used in some of these codes avoiding controversial intensities of occupation or varying cubic feet of space per occupant, is to require delivery or exhaust of a specific volume of air per sq.f t. of floor area per unit of time for
each sort of occupancy. Table I. gives some representative values.
the square of the speed.
The power required by any fan varies at the same rate as the cube of the speed.
The universal availability of electric energy has permitted the use of the far more dependable and desirable mechanical ventila tion, and the tendency is toward the use of both supply and ex haust fans in such systems. Since buildings are not air tight but have increasing leakage as they age, a supply fan alone tends to encourage wasteful outward leakage of conditioned air, and an ex haust fan alone encourages inward leakage of unconditioned air. The combination of both fans permits maintaining an indoor pres sure commensurate with that out of doors and permits exact ad justment of the proportions recirculated. Thus, with the combined fans all air may be recirculated when heating or cooling a room or a building prior to occupancy, and all air may be taken from out of doors when the outside air is at the right temperature to serve without heating or cooling, with many variations between these extremes.
It is clear that any proper ventilating system should have air ways, filters, heaters, etc., with air-passing clearances such as to permit full utilization of outside air during the times when the outside air is in acceptable condition.