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Indoor Environmental Quality

Heating, Ventilating & Air Conditioning (HVAC) Systems

HVAC systems should be designed to meet the needs of a specific building based on its design, use, and occupant activities. The HVAC system should filter the air, heat or cool as necessary, and control relative humidity during the cooling season. Some systems also introduce outdoor air to dilute building contaminants. Then, the tempered air is circulated throughout the structure. A poorly maintained HVAC system can allow water to build up in the unit, creating conditions where the system itself can become a reservoir for biological contaminants. Inadequate systems can also allow high moisture levels that foster the growth of mold and mildew. For these reasons, it is very important that HVAC systems be inspected on a regular basis to ensure that the systems are clean and functioning as designed. Temperature and humidity control are two of the most important indicators of a building's IEQ and determine how occupants perceive the environment. Radiant heat loads, thermal sinks, drafts, and variations in the temperature over time and space can also affect people's perception of the indoor environment.

Achieving a comfortable temperature range for all occupants is difficult, but the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) has published recommended standards for thermal comfort parameters for people performing light sedentary activity and wearing clothes appropriate for the season. Maintaining a building within the following ranges of temperature and relative humidity will satisfy the thermal comfort requirements of most occupants.

Acceptable Temperature and Humidity Ranges
Measurement Type Winter Summer
30% Relative humidity 68.5°F - 76.0°F 74.0°F - 80.0°F
50% Relative humidity 68.5°F - 74.5°F 73.0°F - 79.0°F
Relative humidity 30% - 60% 30% - 60%

Source: ASHRAE Standard 55-1992, Thermal Environmental Conditions for Human Occupancy

In addition to thermal comfort, the control of relative humidity is important to limit the growth of microorganisms such as mold and dust mites. High relative humidity can increase mold and dust mites. To control such microorganisms, it is best to keep relative humidity below 60% (to control mold) and 50% (to control dust mites) at all times, including unoccupied hours. The amount of moisture absorbed by materials depends on the relative humidity of the air, so keeping indoor relative humidity below 60% prevents hydrophilic (water-loving) materials from absorbing enough water from the air to support mold growth, and is also more comfortable for occupants.

Moisture in the form of condensation often accumulates where warm moist air contacts relatively cool surfaces. A small boundary layer of high relative humidity can also form where warm moist air contacts a cool surface. The general strategies for dealing with condensation are to either raise surface temperature or reduce the total amount of moisture in the air. Deciding which strategy will be more effective depends on whether the dominating factor for condensation is the surface temperature or the total amount of water vapor in the air.

Surface temperature condensation is similar to what happens when a cold beverage can is taken outside on a hot day. The solution is to use an insulated can sleeve or holder to create a barrier between the warm air and the cold can. Similarly, indoor surface condensation problems caused by temperature differences are solved by adding insulation, sealing cracks to reduce infiltration of cold air, and improving air circulation at or near the cool surface. Surface condensation problems generally do not respond well to increasing air temperature.

In the example of condensation on a cold beverage can, it is not feasible to lower the amount of moisture in the air surrounding the can. However, sometimes in buildings the absolute amount of moisture in the air is the dominating factor, and in these cases solutions will include controlling moisture sources by using exhaust fans in bathrooms and kitchens, diluting moisture-laden air with outdoor air that has a lower absolute humidity (best in the winter when outdoor air contains little total moisture), or dehumidifying the air in the building. Problems due to high moisture levels in the air do not respond well to increasing the surface temperature.

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