Finding solutions to indoor air problems

Out of sight, out of mind. If conditioned air seems to be making it to all the zones of a building and there are no more than the usual occupant complaints, a building owner is likely to forget about the HVAC system until it breaks down. Meanwhile the system may be consuming more energy dollars than it needs to and circulating unacceptable air quality to occupants. Appropriate, cost-effective retrofits and an active maintenance program can improve both indoor air quality and the efficiency of the HVAC system as well as increase employee satisfaction.

Intakes and Filtration

Critical to the amount of outdoor air being drawn into the air handling units is proper operation of the HVAC system. Blended with recirculated air, outdoor air dilutes contaminants generated in work areas. That's why accurate calibration and mechanical maintenance of outdoor air intake and return air louvers are extremely important.

Effective particulate filtration improves air quality and HVAC efficiency - dust build-up on the heat exchanger reduces its efficiency and it can raise energy bills. To remove solid contaminants, most air handling units incorporate particulate filters. These vary in efficiency from 20 percent to 99.997 percent. The selection should be professionally evaluated based on the uses and activities of the building, which may range from a general office environment to a computer room to a scientific research facility.

Gaseous contaminants can be removed with an activated carbon filter, an oxidizing filter - carbon plus potassium permanganate - or an electrostatic precipitator. Oxidizing filters chemically alter gaseous contaminants so that their odor or effects are eliminated; whereas activated carbon adsorbs and retains contaminants until the adsorptive capacity is reached. At this point the carbon must be removed and reactivated. Electrostatic precipitators generate electrons that attach to the airborne particles. Once charged, the particles are attracted to oppositely charged steel plates. These plates must be periodically cleaned to maintain removal efficiency.

The first two may be used where there are gaseous contaminants, such as in laboratories, outdoor sources, smoking areas and wherever perceivable odors are present. (The precipitator removes only gaseous contaminants that are "attached" to solids.) Not all gaseous contaminants are detectable as an odor. Therefore, even though there are no odors it does not mean there are no contaminants. Any one of the three types may also be recommended where there is unacceptable outdoor air quality, such as in some urban areas, in facilities with loading docks or those near factories.

In buildings not designed for a specific use, the HVAC system is often a "one size fits all" system, with minimal filtration designed for general office conditions. It can be retrofitted with additional or more efficient filters if there is enough space on the air handling unit to accommodate them and if the motor and fan are able to overcome the additional resistance. Selection is based on knowing what contaminants are being generated indoors and outdoor and at what rate they are generated; it may vary by building zone. Provisions to include and outdoor air source with adequate building relief air can also be designed into a retrofit.

Cost of such a retrofit varies widely, anywhere from $500 to $10,000 per unit depending on average size, efficiency level, filter type, extent of retrofit and ease of installation. For example, retrofit costs for a 2,500 cpm unit for activated charcoal may cost $500, while an oxidizing filter would cost about $1500. The benefit of the oxidizing filter is that it has a longer life, is more effective and removes a wider range of contaminants.

Filters must be replaced or regenerated when odors become noticeable. Particulate filters are replaced an average of twice a year, but experience based on filter breakthrough will dictate actual frequency. Follow a six-month filter change schedule for gaseous contaminants too, and adjust with experience.

The heat exchanger should be kept free of debris, and the condensate pans should be draining effectively to reduce the potential for growth of molds and fungus, significant contributors to poor air quality. There may be a need for occasional steam cleaning.

Air Distribution

Adequate air distribution volume is essential to indoor air quality. There should be six complete air changes per hour, at a minimum, and air change should be complete from ceiling to floor.

If ventilation systems are shut down during off-hours to save energy, they should be turned on at least two hours before occupancy to allow for stabilization of temperature and homogeneity of the air by eliminating stratification. If the motor/blower is a direct drive, little maintenance is necessary. But most are belt-driven, and these belts can become loose, reducing the efficiency of circulation.

Ductwork connections often allow for significant leakage at the seams, reducing air volume. Even if rigid ductwork is connected properly at installation, vibrations may loosen connections over time. Flexible ductwork is often crimped by hangers or where it is snaked over girders.

Ductwork connections should be properly installed and maintained, but cleaning is generally pointless and expensive - little of the dust that gets by the filters settles out in ductwork due to high air velocities within the ducts.

Upgrade ductwork whenever renovations dictate a redistribution of air, such as when series of window offices, is created in a formerly open area. In addition, variable air volume boxes and induction units should be properly maintained. Exhaust systems associated with specific areas, such as restrooms and food preparation areas, should be properly placed to ensure that air sweeps completely across the room from intake louvers to exhaust.

Building occupants, whether they be residents, employers or workers, have come to view acceptable indoor air quality as vital to their well-being and productivity - and they are demanding it. Building owners are learning that an investment in proper HVAC design, operation and maintenance and, in certain cases, in appropriate retrofits can pay off in stable occupancy rates and more efficient operations.

COPYRIGHT 1992 Hagedorn Publication
COPYRIGHT 2004 Gale Group