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Mechanical ventilation options include unit ventilator fans on the outside wall of each room to circulate room air and replace a fraction of it with outdoor air. Window or through-wall air-conditioning units can also be run as fans. A central heating and cooling system with coils of hot or chilled water will temper the air in room ventilation units. Fixed location fans can provide a reliable, positive airflow to an interior space.

Some residences have a principal exhaust fan designed for quiet, continuous use in a central location. This whole-house ventilator (Fig. 21-3) has a motor-driven fan for pulling stale air from living areas of the

house and exhausting it through attic vents. Without an adequate exhaust fan, the building may not have enough air for combustion equipment, such as furnaces and stovetop barbecues, to function correctly, and fumes may not be exhausted properly. Equipment that demands a large amount of exhaust should have another fan supplying makeup air running at the same time.

Bathrooms and kitchens have exhaust fans (Fig. 21-4) to control odors and humidity. By creating negative pressures, exhaust fans help contain odors within the space where they originate. In radiant heated buildings, exhaust fans are sometimes the only source of air movement. The air that residential kitchen and bathroom fans dump outdoors is replaced by air leaking into various parts of the house. The result is a loss of heating or cooling energy.

Codes prohibit discharging exhaust fans into attics, basements, or crawlspaces. The American National Standards Institute (ANSI) and ASHRAE have jointly published ANSI/ASHRAE 90.2-1993, Energy-Efficient Design of New Low-Rise Residential Buildings, which requires user-controlled exhaust fans of at least 23.6 L/s (50 cfm) capacity for bathrooms, and 47.2 L/s (100 cfm) for kitchens. The intake should be as close as possible to the source of the polluted air, and the air path should avoid crossing other spaces. Kitchen fans can exhaust grease, odors, and water vapor directly above the range, with a duct vertically through the roof, directly through an exterior wall, or horizontally to the outside through a soffit above wall cabinets. Self-ventilating cooktops

may exhaust directly to the outside or, when located in an interior location, through a duct in the floor.

In bathrooms, the exhaust fan (Fig. 21-5) should be in the ceiling above the toilet and shower or high on the exterior wall opposite the door. It should discharge directly to the outside, at a point a minimum of 91 cm (3 ft) away from any opening that allows outside air to enter the building. Residential exhaust fans are often combined with a lighting fixture, a fan-forced heater, or a radiant heat lamp.

Residential fans are often very noisy, which can be K an advantage when masking toilet sounds, but may be annoying at other times. Models are available with a high-efficiency centrifugal blower that provides virtually silent performance, and a lighted switch that indicates when the fan is on. Highly energy-efficient motors are available that use about a third of the electricity of standard versions, and which may qualify for local utility rebates. Some designs allow easy installation in new construction as well as retrofit applications. Models are available that activate automatically to remove excess humidity. Fluorescent or incandescent lighting fixtures, and even night-lights, are included in some designs. Fans for use over bathtubs and showers should be Underwriters Laboratories (UL) listed and connected to ground fault circuit interrupter (GFCI) protected branch circuits. Larger multiport exhaust fans are designed for larger master bathroom suites, where they can vent the toilet area, the shower, and a walk-in closet with one quiet unit. The acoustically insulated motor is mounted in a remote location, and flexible ducts are run to unobtrusive grilles at three separate areas.

Fan models are available for use in business or small offices that offer computerized operating programs to ensure regular exchanges of air. Again, quiet operation and high energy efficiency are available. In addition to ceiling mounts, exhaust fans come in models for mounting through the wall without ducting, with a concealed intake behind a central panel that can be decorated to match the room, and for moving air from one room to another through the intervening wall via grilles on both sides. Blower fans that use an activated charcoal filter to remove odors are offered in unducted models, which filter and recirculate air but do not remove the air from the room. In-line fan systems for residential and light commercial applications locate fans in flexible round ducts or rigid square and rectangular ducts to exhaust air from several rooms.

Operable exterior openings (windows or skylights) are permitted instead of mechanical fans, but must have an area of not less than one-twentieth of the floor area, and a minimal size of 0.14 square meters (1.5 square ft). If natural ventilation is used for kitchen ventilation, openings must be a minimum of 0.46 square meters (5 square ft).

Public toilet room plumbing facilities must be coordinated with the ventilation system to keep odors away from other building spaces while providing fresh air. The toilet room should be downstream in the airflow from other spaces. The air from toilet rooms should not be vented into other spaces, but exhausted outdoors. By keeping slightly lower air pressure in the toilet rooms than in adjacent spaces, air flows into the toilet room from the other spaces, containing toilet room odors. This is accomplished by supplying more air to surrounding spaces than is returned. The surplus is drawn into the toilet rooms and then exhausted. Exhaust vents should be located close to toilets and above them.

Overall room exhaust fans are also used in storage rooms, janitor's closets, and darkrooms. The amount of outdoor air supplied is slightly less than the amount exhausted, resulting in negative air pressure within the room. This draws air in from surrounding areas, preventing odors and contamination from migrating to other areas.

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