What is a Duct Booster Fan and Do I Need One?

“Dad, my bedroom is so hot, I can’t sleep at night!”   “Mom, I’m freezing in here!” 

Have you ever heard something like this from a member of your family? If so, you might need to consider a duct booster fan to push more “treated” air to that room.

Knowledge Tip: "Treated air" is the term used for air that has been through the air handler or furnace and is air conditioned (cooled and dehumidified) or heated. 

What a Duct Booster Fan Is

A duct booster fan is a small fan that is installed inside an air duct of a forced air system – forced air meaning that a blower fan is employed to pull air into the air handler or furnace and push it through the ductwork into each room.

These fans can be used in main ductwork trunks, but more often, they are installed in smaller ducts serving a single room.  Cheap duct booster fans run all the time. Some are even plugged into a standard outlet. Better fans have programmable controls that allow you to determine when they come on to increase the airflow coming through the duct when your furnace or A/C is running.

The obvious and intended result is that more treated air will be pushed into the room to make it more comfortable throughout the year. 

2 Types of Duct Booster Fan

Which type you need is determined by factors such as current airflow, ease of installation

In-the-Register Booster Fan

The easiest to install type of duct booster fan is a rectangular unit that simply replaces the register through which either heated or air conditioned air comes into the room. They are available in several different sizes to correspond to the typical size of a floor or wall register. 4” X 10”, 4” X 12”, 6” X 10” and 6” X 12” are four of the more common sizes, though others can also be found. 

These units can be installed in a floor register by simply removing the original register and setting the booster fan in the now-vacant hole. Some units can be secured in a wall or ceiling register with screws, but others require a special wall or ceiling kit which must be purchased separately. The unit must then be plugged into a standard 110V outlet by means of an included power cord. 

In-the-Register booster fans are available with motors of various sizes that can move anywhere from 45 CFM (cubic feet per minute) to almost 300 CFM of air. Some of them can be pre-set at different fan speeds, while others will automatically change fan speeds depending on conditions and settings. 

In-the-Duct Booster Fan

Duct booster fans of this type must be installed directly inside the HVAC ductwork itself. Also called an inline booster fan, most produce enough airflow for one duct that feeds air to a particular room. A few are large enough for the larger “trunk” duct that supplies the whole house. However, if you need a larger booster fan to push air to several rooms, it is an indication that your blower motor is undersized for its job.

Most in-the-duct booster fans are for round ducts, but a few manufacturers produce models that can be installed in either round or rectangular ductwork. DIY requires decent handyman skills – or simply call an HVAC contractor to do the work. Make it simple by calling the 888 toll-free number on this page.

In-duct booster fans also need a 110V power outlet and might have simple controls that cause them to turn on or off when the HVAC unit turns on or off. They might also be programmable, either by a separately purchased control unit or by an internal controller. Some of these are programmed manually and some of the more advanced units are programmed and controlled wirelessly. This type of booster fan typically moves between 275 CFM and 400 CFM of air.

How much is 400 CFM? Well, the typical 80,000 BTU gas furnace has a blower that runs from about 1200 to 1600 CFM. 400 CFM represents an increase in airflow of 25% to 33%. That’s a pretty good boost! Again, if any room in your home needs more of a boost than that to prevent it from being a cold spot in winter or a hot spot in summer, your air handler’s blower motor isn’t big enough or it is wearing out – or there is some other reason for low airflow. 

Why You Might (or Might Not) Need One

If you have a room or rooms that are either too warm in summer or too cold in winter, a duct booster fan might be the right way to correct the problem. But there are several possible reasons why that room is too warm or too cold, and you should always know what that reason is. It’s possible that the cause of your problem will not be corrected by a booster fan. Let’s look at some of these possibilities: 

Leaking HVAC Ductwork. Proper heating and cooling of the rooms in a house depend on an HVAC system that can operate the way it was designed. This includes having no significant air leaks. Some of the more common ways that air can leak from your ductwork are poorly installed or loose joints, seams or fittings anywhere along the line. With flexible ductwork, tears can occur from people working too close to it or even by rodents and other unwanted critters. If the duct supplying air to a room that is consistently too hot or cold has significant air leaks, this could very well be the problem. The leaking ductwork should be fixed to see if that solves it.

Find and Fix the Cause!!! Here are two guides that should be read before you have a ductwork booster fan installed. If your blower motor/fan is large enough but you’re still getting low flow causing cold spot / warm spot issues, the information in them will help:

Blocked or Restricted Ductwork. Air must be able to flow freely through the ductwork all the way from the air handler to the room register to keep the room at the right temperature. The airflow can become restricted by the duct being crushed – perhaps by someone having stepped on it in an attic. Flexible ductwork is often restricted by improper installation. Kinks and bends that are too tight will collapse the inner lining where the air flows. This can also easily occur if someone is working on something else close by the flexible ductwork and kinks or crushes it. This might be your problem. 

Animal Intrusion. It’s not something we normally think about, but especially in cold weather, many animals look for a warm place to spend the winter. Mice, squirrels, racoons and others can and will sometimes get into houses and find a way to make a nest in that nice, warm ductwork. This is probably a rare occurrence, but it can happen, and might be the cause of one of your rooms not heating or cooling properly. 

Damper Misaligned. Many HVAC installations call for dampers to be placed at some or even all branches where they come off the trunk of the system. This is so that the airflow can be manually regulated to different parts of the house so that all rooms are more or less the same temperature. One problem with this is that the dampers might all be wide open, or one or more might be almost closed. The result could be exactly what you are experiencing with a hot spot or cold spot in your house.

The above issues are examples of things that should be corrected before deciding that a duct booster fan is needed in your HVAC system. After they are corrected, you may find that you do not need a duct booster fan. That would be a great outcome. See the Pick HVAC guide Low Airflow Causes and How to Increase It for full details on this common ductwork problem.

There are also some issues that will make the purchase and installation of a duct booster fan a wise choice. Here are some of them: 

HVAC System Too Small for the House. This is a bad situation because it will usually take a while for the homeowner to finally figure out that this is the problem. By that time, it may be too late to require the company that originally installed the system to do something about it. And the only real solution is to replace the heart of the system, which is very expensive. If an undersized HVAC system is causing one or two rooms in your house to be too hot or too cold – but the rest of the house is relatively comfortable – a duct booster fan will probably help solve your problem in that room. 

Afternoon Hot Spots. If a room that is too warm faces the afternoon sun - and especially if it is a corner room and gets heat on two sides - it may very well be that the best solution is to install a duct booster fan to bring more cool air in. Of course, good insulation should also help in this situation, but in hot climates and on very hot days, those South and West-facing rooms will tend to be warmer than others. Another good option for a hot spot in summer is to use a window air conditioner for that room. They’re cheaper than a new HVAC system, of course, and also cost less than the installation of most duct booster fans.

Likewise, a room that faces the North might tend to be colder in the winter – especially in cold climates and when the North wind is howling. For these rooms, if you don’t want the expense of a duct booster fan installed in your ductwork, perhaps a large space heater will do the job.

Rooms Farthest from the HVAC Unit. Some houses are constructed so that they have a long “footprint”, and the HVAC unit might be at one end. This means that it’s a long way to the room farthest away. One result of this could be a room that is too hot or too cold, and a duct booster fan may very well solve the problem. 

Cost of a Duct Booster Fan 

If you are struggling with a room or rooms that are uncomfortably warmer or cooler than the rest of your house, it’s good to know that duct booster fans are not especially expensive. The least expensive inline booster fan is around $20.00 and those that are larger with more sophisticated controls are usually still under $200.00. In-the-register booster fans can be found for around $50 to $100.00. If you have a pro install the unit, the cost will be $100-$200 for register installation and up to $600 for it installed inside ductwork. 

Have a Plan

You have options and costs to consider based on the information. Here’s a suggested plan of action.

  • First, check your ducts for leaks. Leaky ductwork dramatically reduces airflow and wastes a lot of money.
  • Secondly, seal leaks you find.
  • Thirdly, consider insulating all exposed ducts to save on heating/cooling costs.
  • Finally, when those issues are resolved, if you still have a hot spot or cold spot, consider a ductwork booster fan. 

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