Residential Air Cleaners

Until recently, small, inexpensive, tabletop appliance-type air cleaners have been quite popular for residential use. They generally contain small panels of dry, loosely packed, low-density fiber filters upstream of a highvelocity fan. Tabletop units may also consist of a fan and an electronic or other type of filter. Small tabletop units generally have limited airflow and inefficient panel filters. Most tests have shown these tabletop units to be relatively ineffective. The combination of low filter efficiency and low airflow in these units causes them to provide essentially no cleaning when assessed for impact on the air of the entire room. Some of the units produce harmful levels of ozone and do not have automatic controls to limit ozone output.

Another major type of residential air cleaner is the larger but still portable device designed to clean the air in a specific size room (Fig. 20-3). Due to their larger and more effective filters or collecting plates, these portable room air cleaners are considerably more effective in cleaning the air in a room than the tabletop units and have become increasingly popular in the past several years. Room-size air cleaners are generally utilized when continuous, localized air cleaning is necessary. Most units may be moved from room to room to reduce pollutant concentration levels as needed. As with tabletop units, room units incorporate a variety of air-cleaning technologies.

Air-cleaning systems can also be installed in the central heating or air-conditioning systems of a residence or in an HVAC system. These units are commonly re-

Figure 20-3 Portable air cleaner.

the air-conditioning system that would allow the dust to spread throughout the area.

If the sources of allergy problems are present in a residence, air cleaning alone has not been proven effective at reducing airborne allergen-containing particles to levels at which no adverse effects are anticipated. Cats, for example, generally shed allergen at a much greater rate than air cleaners can effect removal. Dust mites excrete allergens in fecal particles within the carpet or the bedding, where air cleaners are ineffective. For individuals sensitive to dust mite allergen, the use of impermeable mattress coverings appears to be as effective as the use of an air-cleaning unit above the bed. Source control should always be the first choice for allergen control in residences.

If the choice is made to use an air cleaner, choose one that ensures high efficiency over an extended period of time and does not produce ozone levels above 0.05 parts per million (ppm).

ferred to as in-duct units, although they are not actually located in the distribution ductwork, but rather in un-ducted return air grilles or ducted return air plenums. These central filtration systems provide building-wide air cleaning and, by continuously recirculating building air through the unit, can potentially clean the air throughout the entire air-handling system, ductwork, and rooms. However, with these types of units, the HVAC fan must be in constant operation for air cleaning to occur, since the airborne contaminants must be captured and carried back to the centralized filter for capture and retention. Thus central filtration systems must be operated with the fan on for constant air movement through the HVAC system. Generally, residential HVAC systems run their fans only intermittently to maintain a comfortable indoor temperature. Research indicates that a highly efficient room unit will be more effective at removing pollutants in the room where it is located than a central filtration system.

Both outside air and recycled air must be filtered. Inadequate filtration is a result of low-efficiency filters, improper installation, or torn, clogged, or otherwise ineffective filters. Ductwork is often installed without any provision for access or cleaning, leading to a massive buildup of contamination that can spread to building occupants. Poor maintenance in the ducts puts even more demands on the filters. It is best to remove pollutants at the source, and therefore ASHRAE recommends dust collectors at the source rather than filters for dusty areas. For example, the maintenance workshop in a hotel would have a vacuum that removed sawdust immediately from the worktable, rather than a filter in

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