Installing a Duct Fan Instead of Adding a Second Air Conditioning Unit

Why did we add a duct fan to our house? The has a second floor that includes an office, a bathroom and what we call the craft room. During the summer months, it was always too hot and during the winter months it was always too cold.

We had a new HVAC system installed in our house last year and had the unit sized to provide enough air flow to heat and cool the entire house. The problem turned out that when the house was built, the initial contractor made some glaring mistakes.

We had ducts that did not go anywhere and/or were not sized correctly for the air flow required. When the new air conditioner was put in, that was all fixed except for the duct going to the second floor.

The twelve inch supply duct was not large enough. The twelve inch return duct was okay.

We figured out that there simply was not sufficient air flow in the twelve inch duct to keep the rooms upstairs as cool as the rooms downstairs. We had to take into consideration we were only trying to heat or cool 450 additional square feet of floor space or 3600 cubic feet.

To the right is a picture of a duct fan.

There were three solutions:

  • First, install a second system to supply the second floor only. This solution cost several thousand dollars and we would have a higher electric bill.
  • Second, install a new circuit controller board in the unit and add zone controls. Add a zone damper switch to switch the air supply from the first floor to the second floor. Think of this as a big valve to switch the air flow between the two floors. This would cost several thousand dollars.
  • Third, put a duct fan in the riser duct to pull more air upstairs. The cost of a duct fan at most would be several hundred dollars.

We decided on the third choice of using a duct fan.

The Choice Has Been Made

After careful shopping around and getting all kinds of advice, some of it good and some of it bad, I found the configuration I wanted.

One of the suggestions was to install some adapters in the 12 inch duct to expand the duct size to 14 inches. “This will give you more air flow”. It would have given 1600 cubic feet of air per minute (“cfm”). I did want some air to stay downstairs so I did not take that advice.

I finally found a 12 inch duct fan on the internet that would pull 975 cfm with the electric current draw of 65 watts or .80 amps which is the range of a light bulb. Therefore, it is energy efficient. Other fans I looked at moved less air and had higher current usage.

The final deciding factor was the amount of heat the unit could withstand before breaking down. After all, I will be installing this in the attic.

Some units could only be used at temperatures below 130 degrees. The unit I chose was the only one that stated “this unit is certified to work at temperatures up to 257 degrees F and is rated for continuous duty”.

That was the clincher. Most of the time when the air is on, the house stays around 78 degrees. However, there are times when the unit is off and no one is home. I know the attic gets hot and I did not want any problems.

How I Installed the Duct Fan

Here is all that was required to install the duct fan.

  • Slice open the supply duct before it went into a supply distribution box. The distribution box supplied the four registers feeding the upstairs area. This was a 12 inch duct, so inserting a 12 inch fan was not a big deal.
  • I cut into the flex duct and pulled the Mylar and fiberglass material back.
  • I checked for the air flow direction arrow on the side of the fan to make sure I did not have the fan facing the wrong direction. Then in it went.
  • The duct fan itself is just a hard galvanized metal tube with the fan mounted in it and two wires hanging out through a plastic grommet. I mounted a standard metal electrical box to the side of the fan housing that will allow me to connect all wiring in a protected enclosure.

NOTE: I really hate to see wires loose and just connected together. Also, the National Electrical Code requires it.

    • The next step is to reattach the supply duct to the fan. Once again slide the Mylar liner over the outside of the fan housing and tape it using Foil Sealant tape, mastic tape or butyl tape. All these tapes are a metal faced tape with a soft foam lining to seal the tape to the duct. It is really sticky stuff and it works great!

Make sure you seal everything as you do not want any air leaks.

    • Then cover the entire assembly with more duct insulation to keep all air passing through the ducts and not leaking out. If any air leaks out, you can have condensation on the outside of the fan housing. This water can drip down and ruin your ceiling.NOTE: I use foil tape with a fiberglass reinforced lining. I also use a foil tape for sealing joints and seams against moisture and vapor on foil jacket insulation.NEVER use the gray cloth tape as it is not rated for this type of usage.
    • After the electrical work is completed, the last remaining piece of insulation will be sealed around the fan.

Notice the two blue plastic conduit pipes. This blue plastic conduit is called Smurf tubing by the electricians. Wonder where they got that name?

As a temporary measure to see if everything worked, I wired the fan motor to an extension cord and plugged it in. Immediately, there was cool air in the office area. Within the hour, the temperature was the same on both floors.


Now, let us make this permanent. I installed a 4×4 electrical box on the craft room wall. It was equipped with an auto/manual override switch and a variable speed motor control.

An air pressure switch was installed in the riser duct to automatically turn the fan on or off when the air conditioning unit turns on and off downstairs. The objective is to have fully automatic action with minimal electrical usage.

Most of this Project

The total cost to install the duct fan and the controls was less than $200.

The window air conditioner unit on the second floor will be removed this weekend. We will have to wait and see how much our electric bill will go down?

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