Heating, Ventilating, and Air-Conditioning Systems

Sources of Indoor Air Quality Problems

Heating, ventilating, and air-conditioning (HVAC) systems constitute one of the four factors that can affect the indoor air quality of a building. If the HVAC systems are not able to control existing air contaminants and ensure thermal comfort (temperature and humidity conditions that are comfortable for most occupants) then indoor air quality complaints can occur.

HVAC System Operation and Design

The HVAC system includes all heating, cooling, and ventilation equipment serving a building: furnaces or boilers, chillers, cooling towers, air handling units, exhaust fans, ductwork, filters, steam (or heating water) piping.

A properly designed and functioning HVAC system:

  • provides thermal comfort
  • provides outdoor air to meet the ventilation needs of all occupants
  • isolates and removes odors and contaminants

Thermal Comfort

A number of variables interact to determine whether people are comfortable with the temperature of the indoor air. The activity level, age, and physiology of each person affect the thermal comfort requirements of that individual. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 55-1981 describes the temperature and humidity ranges that are comfortable for most people engaged in largely sedentary activities. The ASHRAE standard assumes “normal” indoor clothing.

Uniformity of temperature is important to comfort. When the heating and cooling needs of a room within a single zone change at different rates, rooms that are served by a single thermostat may be at different temperatures.

Humidity is a factor in thermal comfort. Raising relative humidity reduces the ability to lose heat through perspiration and evaporation, so that the effect is similar to raising the temperature. Humidity extremes can also create other IAQ problems. Excessively high or low relative humidity can produce discomfort, while high relative humidity can promote the growth of mold and mildew.

Ventilation to Meet Occupant Needs

Most air-handling units distribute a blend of outdoor air and recirculated indoor air. HVAC designs may also include units that introduce 100% outdoor air or that simply transfer air within the building. Thermal comfort and ventilation needs are met by supplying “conditioned” air (a blend of outdoor and recirculated air that has been filtered, heated or cooled, and sometimes humidified or dehumidified).

The amount of outdoor air considered adequate for proper ventilation has varied substantially over time. The current guideline issued by ASHRAE is ASHRAE Standard 62-2000. (ASHRAE committees are currently discussing and debating a revision to the 62 standard.) The building code that was in force when your buildings HVAC system was designed may well have established a lower amount of ventilation (in cubic feet of outdoor air per minute per person) than is currently recommended.

Control of Odors and Contaminants

One technique for controlling odors and contaminants is to dilute them with outdoor air. Dilution can work only if there is a consistent and appropriate flow of supply air that mixes effectively with room air. The term “ventilation efficiency” is used to describe the ability of the ventilation system to distribute supply air and remove internally generated pollutants.

Another technique for isolating odors and contaminants is to design and operate the HVAC system so that pressure relationships between rooms are controlled. This control is accomplished by adjusting the air quantities that are supplied to and removed from each room. If more air is supplied to a room than is exhausted, the excess air leaks out of the space and the room is said to be under a positive pressure. If less air is supplied than is exhausted, air is pulled into the space and the room is said to be under a negative pressure.

Control of pressure relationships is critically important in mixed use buildings or buildings with special use areas. Lobbies and buildings in general are often designed to operate under positive pressure to prevent or minimize the infiltration of unconditioned air, with its potential to cause drafts and introduce dust, dirt, and thermal discomfort.

A third method is to use local exhaust systems (sometimes known as dedicated exhaust ventilation systems) to isolate and remove contaminants by maintaining negative pressure in the area around the contaminants source. Local exhaust can be linked to the operation of a particular piece of equipment (such as a kitchen range) or used to treat an entire room (such as a smoking lounge, testing laboratory, or custodial closet).

Air cleaning and filtration devices designed to control contaminants are found as components of HVAC systems and can also be installed as independent units. The effectiveness of air cleaning depends upon proper equipment selection, installation, operation, and maintenance.