An HVAC system is designed to provide conditioned air to the occupied space, also called the “conditioned” space, to maintain the desired level of comfort. To begin to explain how an HVAC system works let’s set some design conditions.

First, we will need to determine the ventilation requirements. We know that in the respiratory process the contaminate carbon dioxide is exhaled.

In buildings with a large number of people, carbon dioxide and other contaminants such as smoke from cigarettes and odors from machinery must be continuously removed or unhealthy conditions will result.

The process of supplying “fresh” air (now most often called outside air) to buildings in the proper amount to offset the contaminants produced by people and equipment is known as “ventilation.”

Not only does the outside air that is introduced into the conditioned space offset the contaminants in the air but because of its larger ion content, outside air has a “fresh air” smell in contrast to the “stale” or “dead air” smell noticed in overcrowded rooms that do not have proper ventilation.

In many instances, local building codes stipulate the amount of ventilation required for buildings and work environments. Let’s say that an HVAC system supplies air to a suite in an office complex and the code requirement is for 20 cubic feet per minute (abbreviated cfm) of outside air for each building occupant.

If the suite is designed for 10 people then the total outside air requirement for the people in the suite is 200 cfm. An additional amount of outside ventilation air may be required if there are exhaust hoods such as laboratory fume hoods, kitchen hoods, and spray hoods or there are other areas where the air needs to be exhausted or vented to the outside such as bathrooms and restrooms.

This ventilation air is called make-up air. If more air is brought into a room (conditioned space) than is taken out of a room the room becomes positively pressurized. If more air is taken out of a room than is brought into a room the room becomes negatively pressurized.

These air pressures, whether positive or negative are measured in inches of water gage (in wg) or inches of water column (in wc).

Commercial office buildings are typically positively pressurized to about 0.03 inches of water pressure. This is done to keep outside air from “infiltrating” into the conditioned space through openings in or around doorways, windows, etc.

Other areas that need positive pressurization are hospital operating rooms and clean rooms. Examples of negative rooms are commercial kitchens, hospital intensive care units (ICU) and fume hood laboratories.

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