July 2015—Good air quality is an important component of a healthy indoor environment. Poor air quality leads to sick building syndrome complaints, lost worker productivity, and tenant health complaints such as allergic reactions, nausea, chronic fatigue, and respiratory problems.
Good indoor air quality (IAQ) includes the following elements:
- Acceptable temperature and relative humidity (RH)
- Controlled airborne contaminants
- Adequate distribution of ventilation air
Acceptable Temperature and Relative Humidity
IAQ is affected by temperature and RH because thermal comfort concerns are the cause of many complaints about poor air. Temperature and humidity are also among the many factors that affect indoor contaminant levels. Bacterial growth and fungal growth are promoted at humidity levels above 60 percent.
Controlled Airborne Contaminants
The contaminants that produce indoor air pollution include carbon dioxide, smoke, dust, odors, and biological contaminants such as mold and bacteria. Some of the most dangerous pollutants result from the gas released by materials used in newly finished construction and space furnishings. Indoor air may contain tobacco smoke; gaseous emissions from fabrics, furniture, and adhesives; and offensive odors from equipment, solvents, and people.
The growth of biological contaminants, including bacteria, mold, and fungi, can be controlled by keeping systems clean and dry. Biological contaminants thrive in areas that are dark, moist, and laden with nutrients. Design characteristics that help keep ducts and equipment clean include sloped drain pans; easily cleaned, nonporous surfaces; and easy accessibility to the ducts. Sloped drain pans prevent water from building up and developing into nutrient-filled slime. Easily cleaned, nonporous surfaces will not hold moisture, eliminating the need for duct-lining materials. Easy accessibility to the ducts for cleaning is especially important in areas where dirt builds up because of the turbulence of direction change or the presence of some obstructing device such as a damper.
Adequate Distribution of Ventilation Air
The primary purpose of ventilation is to control IAQ by diluting polluted indoor air with less contaminated outside fresh air. In many instances, ventilation air alone will reduce IAQ problems. Improperly balanced HVAC systems can create stagnant areas in an otherwise properly ventilated building. In certain instances, additional air filtration and treatment may be necessary.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has established guidelines for determining ventilation rate and procedures to assure air quality. These guidelines are accepted across the United States. Building owners and managers should familiarize themselves with Standards 62 through 89 in particular, keeping in mind that local codes can amplify or supersede these ASHRAE guidelines. The ventilation required for any given building varies according to the number of people who occupy it and the size of the space. People give off body odors that require a minimum of 5 cubic feet per minute (cfm) of ventilation air per person, or 0.05 cfm per square foot for satisfactory dilution.
The amount of outside air considered adequate for proper ventilation has varied substantially over time. The standard in effect when the HVAC system for a particular building was designed may have had a different requirement than is currently recommended. The current standard is ASHRAE 62-1989, Ventilation for Acceptable Air Quality, which recommends 15 to 20 cfm per person, based on the type of occupancy. The previous standard of 5 cfm per person appeared in 1981 to supersede the 1973 pre-energy crunch standard of 15 cfm to 25 cfm per person. Always check the latest publications of ASHRAE, Building Officials and Code Administrators (BOCA), and EPA to learn the standards that are in effect.
New technologies are being used in the struggle to maintain acceptable IAQ levels while battling the high cost of cooling/heating the outside air needed for make-up air. Enthalpy wheels and makeup air units (MAUs) are gaining popularity with design engineers and are becoming commonplace in buildings where general exhaust air is dumped to the outside. The enthalpy wheel is a heat recovery unit that operates on the air-to-air principle. Many existing systems can be retrofitted with an enthalpy wheel. The wheel rotates very slowly and is cooled by the cool, dry exhaust air.
For example, the return air in the summer is approximately 70 degrees F with an RH (relative humidity) of 40 percent. The wheel passes through the exhaust air where it gives up heat to the cooler 70 degrees F return air. The cooler wheel continues around until it passes through the warmer (90 degrees F with 80 percent RH) outside airstream. The cooler enthalpy wheel passing through the warmer airstream absorbs the warm, humid air and continues around to the return air duct where it again gives up heat to the cooler exhaust air.
The enthalpy wheel can be used year-round and can reduce energy consumption by a considerable amount, making it more cost-effective to introduce fresh air into a building. In newer systems, an MAU is installed on a roof or in a mechanical space. The MAU can be designed to use an enthalpy wheel that is tied into the fresh air supply duct and the exhaust air discharge duct of the unit. Exhaust air temperatures can range from 70 degree F to 74 degrees F, with an RH level between 30 and 50 percent. Outside air temperatures fluctuate depending on location and season. Energy costs can be reduced by as much as 40 percent when an enthalpy wheel is used in conjunction with an MAU.
This article is adapted from BOMI International’s course Air Handling, Water Treatment, and Plumbing Systems, part of the SMA and SMT designation programs. More information regarding this course or the new High-Performance certificate courses is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.