In your post, you mention "two motive forces that produce the airstream rate...." What are these forces and how do they influence airflow?  What design and equipment considerations should be made to ensure these forces are present and sufficient?  Need References 

 

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Advantages of Natural Ventilation

It is hard to make certain explanations about the advantages and disadvantages of natural ventilation contrasted to mechanical systems, nearly because they are influenced by some external conditions such as the utilization of the building and the climate-changing. Nevertheless, this question cannot be passed in this post about natural ventilation, and some general explanations can be presented.

Comfortable Airflow in a Building by using Natural Ventilation

If well fixed and maintained, there are several benefits of a natural ventilation system, compared with mechanical ones. Natural ventilation can practically supply a high ventilation level more economically due to the employment of natural powers and large openings. They are more energy effective, especially if heating is not needed. The systems which are well-designed could be utilized to provide higher levels of daylight.

The other principal advantage contended for natural ventilation is that it helps keep a sustainable building environment. Considering thousands of years that natural ventilation has existed, this claim is hard to explain. One contributory element here is that natural system needs no electrical energy for fans, which can form 25% of the electrical energy usage in a mechanically ventilated structure.

There are also records that occupants of the house want to control their environment and prefer not to be entirely isolated from the external environment. Natural Ventilation can bring about both these requirements, while a conventional air-conditioning device does not.

In general, the advantage of natural ventilation is its capability to supply a very high air-change level at a low cost, with a very ordinary system. Although the air-change rate can change considerably, buildings with new ventilation systems (that are modeled and worked appropriately) can obtain very high air-change levels by natural power, which can considerably be greater in quantity than minimum ventilation needs.

Disadvantages of Natural Ventilation

There are some disadvantages to natural ventilation systems, which are discussed below.

Natural ventilation is changeable and based on outside climatic situations comparative to the inside environment. The two motive forces that produce the airstream rate, including temperature difference and wind, change accidentally. Natural ventilation may be hard to control, with airstream being uncomfortably heavy in some locations and static in others.

There can be some problems in monitoring the airstream direction due to the lack of a stable negative pressure; pollution of corridors and adjacent rooms is, therefore, a hazard.

Natural ventilation prevents the usage of particulate filters. Security, cultural, and climate standards may command that windows and vents remain closed; in these situations, ventilation level may be much lower.

It only operates when natural powers are available; when a high ventilation level is needed, the requirement for the accessibility of natural power is also high.

Natural ventilation systems usually do not operate as anticipated, and normal function may be interrupted for different reasons such as doors or windows not open, equipment destruction, utility service interruption (if it is a high-tech system), inferior modeling, incorrect management, or poor preservation.

Although the preservation cost of natural ventilation systems can be very low, if a ventilation system cannot be set properly or preserved due to a shortage of funds, its operation can be changed, causing an increase in the hazard of the transfer of airborne pathogens.

Another disadvantage of natural ventilation is that it is restricted in the extent to which it can supply cooling in hot climates and especially ones that are also moist. For this system to be allowable in some climates, it is necessary to join it with some form of tolerable (low-energy) cooling system.

Natural ventilation does not need extra space for plant rooms or networks of ducts, but some places are often needed for stacks (chimneys, atria). The specific disadvantage is that errors in modeling a natural system may be more difficult to revise.

Other feasible problems, such as air contamination, insect vectors, noise, and security, also require to be noticed. Nevertheless, these problems can be overcome, for instance, by using better modeling or hybrid (mixed-mode) ventilation.