Ventilation is the process of moving air from one space to an entirely separate space and is primarily a matter of air volume. It should not be confused with circulation, which is the moving of air around and within a confined space. In contrast to ventilation, circulation is a matter of air velocity. Ventilated air may or may not have been conditioned, and it may be supplied to or removed from the spaces by either natural or mechanical means.
The ventilation of a structure is important not only for the health of its occupants but for their comfort as well. The proper ventilation of a structure will replace stale, warm air in the interior with fresh cooler air from the outdoors. It will reduce or eliminate odors, remove excess moisture, and lower humidity levels, especially in the basement, attic, and crawl spaces.
For many years, proper ventilation was not considered as important as it is today. Houses were not very well insulated, and outside air could be easily drawn into the structure not only through open windows and doors but also through the walls themselves. Older houses were said to breathe because air could move in and out of them without much difficulty.
The fuel crisis of the 1970s and the rapid increase in energy costs made the public aware that we had to have more efficient combustion appliances (for example, furnaces, boilers, water heaters) and that structures had to be more tightly constructed and insulated. Although the latter move did result in a reduction in heat loss and gain, it also produced new problems such as trapped moisture, stale air, and even health problems. For example, in tightly constructed and insulated houses, vent fans, clothes dryers, and kitchen exhaust fans can create a negative pressure, drawing air into the house through holes in the framing, chimneys, and even exhaust flues. This can cause backdrafting in combustion appliances, which can be a serious health hazard.
If ventilation is inadequate and more air is exhausted from the house than can be drawn in through natural ventilation, the result is depressurization, which causes toxic combustion gases to be released through cracks or poorly connected ducts in the heating and cooling system. It is important that a balance be maintained between air being exhausted and air being drawn into the house.
There are two types of ventilation: passive ventilation and mechanical ventilation. The former only uses openings (vents) in the roof and walls to allow heat to escape. The latter uses mechanical ventilators, such as exhaust fans, attic fans, and whole-house fans in combination with vents to remove the air from the house. Exhaust and attic fans are covered in Chapter 7 (“Ventilation and Exhaust Fans”).
The Motive Force
The force that moves the air in a room or building may be due to natural causes or mechanical means. In the first case, the ventilation is called natural ventilation. This kind of ventilation finds application in industrial plants, public buildings, schools, garages, dwellings, and farm buildings. The two natural forces available for moving air into, through, and out of buildings are (1) induction and (2) thermal effect. The inductive action is due to the wind force, whereas the thermal effect is due to the difference in temperature inside and outside a building (this being in fact the same as the chimney effect). The air movement may be caused by either of these forces acting alone or by a combination of the two, depending on atmospheric conditions, building design, and location.
The nature of the ventilating results obtained by natural means will vary from time to time because of variation in the velocity and direction of the wind and the temperature difference.
The wind ventilating effect depends on its velocity. In almost all localities, summer wind velocities are lower than those in the winter. There are relatively few places where the wind velocity falls below one-half of the average for many hours per month. Accordingly, if a natural ventilating system is proportioned for wind velocities of one- half the average seasonal velocity, it should prove satisfactory in almost every case.
When considering the use of natural wind forces for producing ventilation, three conditions must be considered:
1. The average wind velocity.
2. The prevailing wind direction.
3. Local wind interference by buildings, halls, or other obstructions.