How to Install a Duct Booster Fan

If a particular room in your house is colder or warmer than it should be, you might consider rectifying the situation by installing a duct booster fan​.​ The purpose of this fan is to increase air flow from the central air blower of your HVAC system to the affected room. Booster fans don't cost very much, and installation is fairly simple, depending on what type you purchase. The only problem is that the booster fan might not make a notable difference. It all depends on the reason for the poor air flow.

The simplest booster fans are those that fit inside an existing duct boot, also known as a register. In some cases, you can simply remove the register cover, drop in the fan and plug it in. Your register opening may be too small for the fan you purchase, in which case, you'll have to widen the opening, which may involve cutting through the subfloor. If you'd rather not do that, you can buy a booster fan that installs directly into the duct pipes and avoid messing with the register altogether.

If the duct leading to a room far from the central air conditioning system is too long, the air flow may be less than it should be, and the room won't receive as much hot or cold air as the rest of the rooms in the house. A booster fan can correct this problem, because it sucks air from the central air system and blows it toward the vent opening. This strategy has some limitations, though. For it to work:

• The furnace, heat pump or air conditioner must be in good working order.
• The ducts must all be properly constructed with smooth-wall galvanized steel, rather than corrugated flex pipe. The corrugations create turbulence that impedes air flow, and a booster fan will just add to the turbulence rather than facilitating the flow of air.
• The ducting system must be balanced, meaning that air is evenly distributed throughout the house. If a disproportionally large percentage of rooms are on one side of the house, those on the other side probably won't get enough air, even if you add a fan.
• The duct system must be properly constructed with all the right fittings, and all the joints must be properly sealed.

Even though installing a booster fan is easy enough for most homeowners to accomplish, it's important to first have the entire air system evaluated during an HVAC tune-up by an HVAC professional. Otherwise, you could just be wasting time and money.

It's also important to consider the impact of a booster fan on the sound levels inside your house as well as its impact on your energy bill. Less expensive booster fans tend to be noisy, and you have to run them day and night to get proper benefit. That could be a problem if your sleep is easily disturbed. Furthermore, running a fan 24/7—even if it draws a modest 400 watts—could end up adding almost $40 to your monthly electricity bill, assuming you pay the 2019 national average of $0.13 per kilowatt-hour.

By far the easiest type of booster fan to install is one that fits inside a standard duct register in the floor or wall. You simply remove the louvered cover and replace it with the fan, which usually comes with its own register grille. Screw the booster fan to the sub floor, plug it in, attach the cover and you're done.

The procedure is a little more complicated if your duct boot isn't a standard size and it's too small to accommodate the fan. This is most common with floor ducts. In this case, you have to replace the duct boot with a larger one, and you'll only want to do this if the floor covering is carpet or some other material that's easy to remove and replace (not, for example, hardwood or tile). Here's how to do it:

1. Pull back the floor covering to expose a 2 x 2-foot section of subfloor.
2. Cut out a section of the subfloor that exposes the connection between the duct and the boot, using a circular saw. It's best to cut out a full section the extends from one joist to another so you can easily reattach it. Save this cutout piece.
3. Unscrew the sheet-metal screws holding the boot to the duct, using a screwdriver. Lift out the old boot.
4. Widen the hole for the register outlet, using a jigsaw.
5. Connect the new boot to the duct, attach it with screws and wrap foil tape (not traditional duct tape) around the joint to seal the connection.
6. Replace the cutout piece of subfloor and screw it back to the joists.
7. Replace the floor covering.

Once the new boot is in place, you can drop in the booster fan, screw it to the subfloor and plug it in.

If you want to install a booster fan to service a room with hardwood or tile floor, or there is some other reason a drop-in model won't work, you have the option of installing an in-line duct fan directly into the duct pipe. If possible, choose a location close to an existing electrical circuit to avoid having to run new wires, which is doable but sometimes troublesome. The optimal and most efficient way to wire this is integrating into the existing fan circuit of the HVAC equipment. This may require some professional help if your not comfortable with wiring.

The procedure may vary somewhat depending on the fan model. The most common models consist of a fan mounted inside a standard coupler. You install the fan by cutting out a section of a straight duct pipe and substituting the coupler for the piece you remove. The point of installation should be close enough to an outlet so that there is an easy place to plug in the fan.

Some models are designed to be installed inside the duct through a cutout in the duct wall. Although you should always refer to the instructions for your specific model, here is a general procedure:

1. Mark a rectangular outline on the duct with a felt tip marker. Check the product specifications for the proper dimensions.
2. Cut along the outline with a jigsaw fitted with a metal-cutting blade or an angle grinder. If you use a jigsaw, you'll need a pilot hole for the blade. Make this with a power drill and a 1/2-inch drill bit. Remove the cutout.
3. Insert the fan into the cutout. Make sure that the fan blades clear the duct walls so they can spin freely and that they face the right direction—so that air blows toward the duct opening, not towards the furnace.
4. Support the fan with one hand while you attach it to the duct wall with machine screws. The screws usually come with the fan.
5. Seal the edges of the fan with foil ductwork tape (not standard duct tape).
6. Connect the fan to power either by plugging it in for the least efficiency. Hardwiring it to the existing HVAC fan circuit for most efficiency. Either way make the connection inside an approved electrical box.