I have installed two inline duct booster fans to increase the airflow for my furnace/ac in a new addition that I got built. The new run that I installed is an 8" flex duct that runs about 25 ft. then splits into two 6" flexible ducts running to the two registers that I installed for that addition.

I checked the new run from beginning to end and there are no obstructions or slack in it. Right now I have one booster fan in each of the 6" split runs and they are controlled by a Suncourt ducstat.

I have two issues that I am puzzled with:

  • First, The fans are really not blowing a lot of air through the registers, although they are positioned about 3-4 feet away from the registers.

  • Second, The fans turn on for a few seconds only and then turn off, then a few minutes later they come on briefly and then shut off (they don't stay on continuously for the entire furnace blower cycle) I have followed the Suncourt Ducstat instructions carefully and set it up correctly.

Any advice as to what the issues could be will be appreciated.

Before anybody asks the obvious, the fans are installed correctly with the correct airflow direction!!

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    – Niall C.
    Commented Mar 26, 2021 at 0:01
  • I've never heard of a "booster fan" used this way and the idea of it seems unrealistic. The main fan will produce a certain amount of air flow that will be divided among the ducts according to their sizes and lengths. Adding a fan to one duct might produce a slight increase in flow through that duct, at the expense of all the other ones, but the effect will be slight if it works at all. Is there a good return air path from the extension to the main blower? Is the blower big enough for the home with the addition? Some blowers need a certain amount of back pressure. Could that be a factor?
    – jay613
    Commented May 11, 2021 at 14:11

3 Answers 3


Has your furnace/AC unit been sized for the addition?

Try unplugging the fans from the ducstat and plug them into a regular outlet and see if they continue to cut out.

Keep in mind that these fans will only help move the air that's available from the main blower, they won't increase the air available. Your run of 8" duct and then split into 2-6"ducts is going to have a lot of resistance to air flow. Those two fans will help accelerate the existing air to it's destination. The plenum and main blower will supply a certain amount of air for the entire house, the fans won't increase that amount. The strain on the fans due to the lack of air could be overloading them and causing them to kick out.

I've seen this a lot on this site and one of the good solutions is to add a mini split system for the addition because you're probably taxing your existing system.


Where on the existing duct did you "tap in" or install the 8" round supply? If it is in the end of the existing duct, that will not work. Also, flex duct has a lot more resistance to air flow than does hard metal smooth pipe. I have found those in-line duct fans to be almost useless. They usually make more noise than necessary for the little amount of "push" that they provide.

  • D.George: The 8” flexible duct is coming straight out of the furnace plenum. Thanks!!
    – user132109
    Commented Mar 25, 2021 at 23:40
  • can you post a picture of the new duct or a detailed drawing of the whole duct system?
    – d.george
    Commented Mar 26, 2021 at 9:59

It will work on low demand days but when extremely hot or extremely cold it will make no difference.what you are doing is placing a fan in line which will simply circulate air within the duct. This does not guarantee an increase in air pressure especially if the unit is not sized right. Try closing some vents in other rooms and see if you get an increase in air flow. If you do an in line fan...

  • It seems that you forgot to finish your thought there at the end. Please edit in the rest of what you were going to say.
    – FreeMan
    Commented May 11, 2021 at 13:09

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