How to Install an Air Duct Booster Fan

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How to install a booster fan in your ductwork to improve airflow in your central air conditioning system.

Most homes with central air conditioning and forced air heating systems have a single fan in the unit to distribute air throughout the home. In some instances, you may need to add a booster fan, also known as an inductor fan, in a branch duct to increase the airflow to a particular room. Long duct runs increase friction and reduce airflow as well as old leaky ductwork. A typical 6 inch booster fan uses less than 50 watts of power when running and can be connected to a switch or wired directly into the controls of your air handler so that it comes on when the unit is on. Increasing the airflow in a room will help mix the air and even out the temperature improving comfort and indoor air quality.

See my article on Duct Booster Fan Reviews.

Prior to installing a fan, you should inspect your system for any leaks in the ductwork and ensure that the fan and belt are functioning properly. See the end of the article to review proper belt tension and fan motor maintenance. You may also want to see my article about duct cleaning.

Purchasing the Proper Booster Fan

When considering a booster fan you must know a few things about your system. You need to know the duct size of the branch you are installing the unit in, the material your duct is made of, and if your ducts are connected by screws or by tape. Metal ductwork uses screws and fiberboard ductwork uses tape. Most branch ducts are round flexible ductwork, usually 6 inches in diameter, but can be 5” to 10”. Have all of this information on hand when you go to purchase your fan.

6 inch In-line Booster Fan

Locating the Fan

When picking a location for the booster fan you should consider how close it is to available electrical connections, whether the unit is hard-wired or plug-in, ease of access for replacement or cleaning, and keep the fan at least 5 equivalent duct diameters away from any connections or the room register. An equivalent duct diameter is the size of the duct the fan is located in multiplied by 5. So a booster fan in a 6 inch diameter duct should be 30 inches away from the connection from the main trunk of the duct, usually a rectangular duct, and 30 inches away from the register in the room. This will help keep the noise down in the room and deliver the stated amount of air. Turbulence occurs after elbows and takeoffs in ductwork and the farther the fan is from them, the better it will work.

Installing the Fan in the Duct

Follow the installation instructions included with the fan. For round flexible duct you will probably only need two quick-ties included in some kits and duct tape to seal the insulation around the connections. You can cut flexible duct with a utility knife or scissors and a pair of pliers to cut the spiral metal wire that gives the duct its shape. For installation in metal round duct, you will need to cut the ductwork with a reciprocating saw or sheet-metal shears. You should remove the length of duct and make your cuts on a workbench or similar area. The duct can then be slid over the connectors on the fan and screwed together with self-tapping sheet metal screws and then sealed with foil duct tape. If you have rectangular ductwork going to the register, you will need to install a transition on either end of the fan. This changes the fitting from a rectangle to a circle. When purchasing the fan and transitions, the diameter of the fan should be the same as the smallest dimension of the rectangular duct. Example: a 6 by 9 inch duct would have a 6 inch booster fan.

Duct Transition

The fan will have an arrow on the side showing the direction of the airflow. Make sure you install the fan with the arrow pointing towards the room register.

Spin the fan blades of to make sure that there are no obstructions and they move freely. You may want to test the fan prior to completing the installation, if it is a plug-in type, simply plug it into a receptacle keeping your hands clear of the spinning blades. If it is a hard-wired type, you may want to temporarily wire a plug to the wires and plug it in. Follow the directions on the plug you are using; the black wire connects to the brass screw and the white wire is connected to the lighter aluminum screw.


Take the wires of your air duct fan from the side opening of the unit and then attach them to the duct with the use of tape or loose quick-ties. Your booster fan will have specific instructions when it comes to connecting your unit to a power source. But if you want to use a switch to turn on the fan, splice the fan wire and attach it to a plug which you can then plug into a circuit breaker that has a switch or run the wires to a junction box and then to a light switch.

Belt and Fan Maintenance

Most homeowners forget about them until they break, but the belt that connects the fan pulley to the motor stretches and cracks before eventually breaking. Most of the time they will break as soon as the fan starts since the tension will be greatest to overcome the inertia of the blower and air in the ductwork. Before breaking the belt becomes looser and more brittle, it slips on the pulley reducing the amount of airflow through the system. This causes the unit to remain on longer and could cause the evaporator coil to partially freeze, thus reducing the airflow even more.

A quick visual inspection can determine if the belt needs to be replaced. Turn the power off to the unit with the electrical disconnect or breaker, remove the access panel and press down on the belt at the midpoint between the 2 pulleys. It should not depress more than an inch, but if the belt is not frayed or cracked, you can loosen the motor mount bolts and slide the motor back slightly to take up the slack. New belts can be ordered on line, or purchased at most auto parts stores. Smaller belts wear out sooner than larger ones since they heat up faster.

Your fan might also have grease fittings or oil ports for lubricating the bearings on the fan shaft and motor. Inexpensive grease guns and oil cans can be purchased at home improvement centers. Don’t use spray lubricants as they can get on the surface of the pulley and belt and cause it to slip and deteriorate more quickly. If oil or dirt enter the windings of the motor they can create hot spots that could cause the motor to burn out prematurely. Newer units will most likely have sealed bearings that don’t require lubrication.

Depressing the belt more than 1" could mean that it's time to be replaced.