In-Line Fans

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Summary

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In-line fans can offer an alternative--remote, in-line units are quieter, ventilate better, use less energy, and often require less maintenance than traditional fans.

In-line fans are duct-mounted blowers. The fan has both an inlet and an outlet duct, which originate at the interior and on the exterior of the house, respectively. If the unit is located in the attic, for instance, it can be further defined as remote-mounted. In-line fans are used for ventilating wet spaces such as bathrooms, kitchens, laundries, as whole house air exchange systems, and as radon mitigation systems. Various special controls may be used to operate these and other types of fans, such as humidistats or variable-speed controllers.

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Details

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Too much moisture is the primary cause of decay within buildings, and control of airborne concentrations of indoor pollutants is an increasing concern. To help solve these problems, fans can be used to move, dilute, or otherwise condition the indoor environment. However, few fans will be used consistently if they are noisy.

Remote-located fans provide quiet operation. Little noise transfers out of the exhaust duct due to sound-attenuating design features in in-line units, and even less can be distinguished through the intake ducts. Noise that is generated occurs outside of conventional living spaces, within attics or other buffered spaces. When fan operation is no longer an irritating factor, its potential uses flourish.

In-line fans are offered in many different configurations, sizes, and capabilities to meet air quality needs. They are available as simple single-port versions or versatile multi-port versions. Single-port units have one incoming supply duct and one outgoing exhaust duct. One recognizable single-port residential in-line fan utilizes a centrifugal blower wrapped in a straight-through cover. This configuration combines the superior airflow and pressure characteristics of centrifugal blowers within the general compactness of a common axial fan. Such simple versions are suitable for most single-family residential applications.

The multi-port versions have multiple ducts but always at least one inlet and one exhaust. These fans are housed within either a box-like shell or a molded bladder-like casing. The single fan can then orchestrate air movement to or from multiple locations (e.g., two bathrooms). By varying duct sizes, inserting "Y" fittings to branch duct runs, employing adjustable grill openings, and varying the fan speeds, different ventilation rates may be achieved for each location served by the fans ducts. Multi-port versions offer ventilation capacity appropriate for larger scale single-family, multifamily, and commercial buildings.

Fans may be used for intermittent, spot ventilation or for continuous ventilation. Bathrooms and kitchens commonly use spot ventilation to control odors and moisture. Continuous ventilators are commonly used to exhaust stale indoor air and bring in fresh outside air in an attempt to improve the quality of the indoor air supply.

Remote, in-line fans can be controlled in a variety of fashions. Common cutoff switches are always a good idea to include, but more complicated controllers such as variable-speed controls, mechanical timers, or sensors are becoming increasingly used. Motion, humidity, carbon dioxide (CO2), and air pressure sensors can improve the efficacy of any ventilation system when properly integrated.

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Installation

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Manufacturers usually provide installation instructions to help minimize operational or acoustic problems. Generally, however, setting up the unit is not too difficult. The fans may be placed almost anywhere. Closets, chases, wall cavities (if wide enough), basements, spaces between floors, and attics are common locations. In-line fans should be located in the duct run near the primary exhaust port yet still have not less than three times duct diameter on the exhaust side, and one time duct diameter on the inlet side. This improves performance and reduces perceived noise. Suspension attachments and other approaches can be used to reduce vibrational, structure-borne noise. Externally insulated ductwork muffles airborne noise and diminishes the likelihood of condensation within the duct. Condensation can damage ductwork, fan motor, and indoor air quality. Some fans installed on a horizontal axis require additional fitting with a condensate drain line to remove the water before it can harm the motor.

Ductwork is a key element in proper delivery of the air supply. It must be appropriately sized. It is harder to move air effectively through long, rough surfaced ducts (e.g., flex duct) than short, smooth ducts. For multi-port systems, equivalent grille pressures may be difficult to obtain from ducts that are not all the same length. Remote, in-line ventilation systems therefore commonly use flex-duct despite its high airflow resistance because it can be easily routed to intake and exhaust grilles. Droops, kinks, and tight turns in flex duct create still more problems and should be avoided. Manufacturer guidelines on installation and duct sizing should be followed carefully. All duct joints, fittings, and adjustable elbows should be sealed with duct mastic. Exhaust ducts must always terminate outside of the building shell, never in a crawlspace, attic, garage, or otherwise protected space.

Backdraft dampers prevent airflow from moving the wrong way in ducts when the fan is not operating. These can be important and should be installed judiciously in the duct system if the fan is not already equipped with one.

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Benefits/Costs

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Conventional ceiling-mounted exhaust fans are annoyingly loud, ineffective, offer little energy savings, and are short-lived. Remote, in-line units sound quiet, ventilate well, use little energy, and tend to be maintenance-free if properly installed. The Home Ventilating Institute (HVI) publishes acoustic ratings for ceiling-mounted fans. Similar ratings for remote-mounted units are not currently available.

Material pricing for in-line fans range from $140 to $700.

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Limitations

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Exhaust-only fans can depressurize a building. Air that would normally flow out chimney flues, water heaters, or furnaces can become sources of supply air to replace the fan-exhausted air. Dangerous concentrations of lethal exhaust gasses can then be drawn into the home. Providing an alternative source of make-up air using passive air inlets or balanced exhaust systems can alleviate this threat. Manufacturers offer ventilation systems and control devices to monitor and initiate corrective response to such air pressure changes. Carbon monoxide detectors serve as early warning devices for dangerous backdrafting episodes. Use of sealed-combustion appliances eliminates associated backdrafting concerns.

Although remote located fans sound quiet inside the home, they can be disturbing outside the structure. Be sure to carefully check the acoustic ratings of the equipment and locate the fan away from outdoor recreation areas. Also be careful about placement of exterior intake grilles to prevent intake of polluted air.

Condensation occurring within ductwork can deteriorate duct, fan, and building components. Be sure to sufficiently insulate ductwork that runs outside of the insulated building envelope, always sloping it to minimize drainage toward the fan assembly.

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Code/Regulatory

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Bathrooms are required by code to have either an operable window or a minimum 50 cubic feet per minute (cfm) capacity exhaust fan. Kitchen ranges are also required to have an exhaust fan.

Domestic clothes dryers are required by some building codes to be exhausted to the outside. The exhaust ducts cannot terminate beneath the building or in an attic, and must also have a smooth, noncombustible, nonabsorbent surface (e.g., vinyl flex duct does not meet code).

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Availability

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In-line fans are available in a range of airflow capacities, from about 100 cubic feet per minute (cfm), suitable for most housing needs, to 2000 cfm. Manufacturers offer a variety of unit sizes, flow capacities, ducting configurations, controls, and accessories (e.g., grilles).

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Contact(s)

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Do you have a specific question about this technology and/or its 'real life' applications? Try the contacts listed below:

Home Ventilating Institute
c/o Air Movement and Control Association
30 West University Drive
Arlington Heights, IL 60004-1893
Phone: (847) 394-0150
URL: http://www.amca.org/

Manufacturers:

• American ALDES Ventilation Corporation: (800) 255-7749; http://www.americanaldes.com
• Broan: (800) 548-0790; http://www.broan.com
• DEC International/Therma-Stor Products Group: (800) 533-7533
• Fan America: (800) 838-4074, http://www.fanam.com/
• Fantech: (800) 747-1762; http://www.fantech-us.com
• NuTone: (800) 543-8687: http://www.nutone.com