I don’t speak much about dryer vent booster fans because for the most part, I never have had to deal with them. In general, adding a fan to a dryer vent isn’t something that is done and is usually prohibited by code unless specifically requested by an inspector. They are usually found in homes or buildings where the dryer vent run is exceptionally long and the dryer itself is simply not going to be able to get the job done.
Most of my experience with these booster fans is I remove them and install proper ducting to code and eliminate the potential problem. I say problem because most every booster fan I have found is the same. The ducting before the fan is blocked with lint while the ducting after the fan is spotless. The reason is simple. It gets stuck in the fan blades.
The logic behind these booster fans makes simple sense. The dryer fan is capable of moving air efficiently X number of feet. If the dryer ducting is going to be more than that, add a booster fan and the fan can help move the air that much further. Unfortunately, it seems the companies that are installing these booster fans are not reading the directions as to the proper placement of these devices and simply finding a convenient place to put them. Thus the reason I see so many filthy ducts up to the booster itself.
And then what I would see as the biggest problem with using one of these fans is the periodic cleaning required which in itself isn’t bad. The problem occurs when the homeowner simply doesn’t even know the fan exists to even have it clean. Which is exactly what happened on this recent service call to a home that was experiencing a rather unsettling noise and didn’t know where it was coming from.
After tracking down the noise deep inside a rather cavernous crawlspace (no crawling required, actually my 16 foot ladder was needed to get to the blower), I did some investigating and found a rather creative booster fan installation. My speculation (I do that a lot since many things I see I simply shake my head) is the inspector stopped by during construction, saw the size of the house and said a booster fan needed to be installed. Why bother with simple math because when I measured the duct the equivalency, it turns out this dryer ducting was under 25 feet in length, well below the code requirement and the capabilities of the clothes dryer.
So I am sure the builder complied but in doing so, created an nearly impossible airflow condition for the booster fan because the exit went into a 180 degree turn followed by an immediate 90 degree bend. The results were lint buildup in the booster fan due to significant duct back pressure and the result was noise. And since the booster was mounted to the house frameworks, its vibrations were transmitted up the wall and could be heard all over the home.
The solution was simple, remove the booster fan and put the ducting back the way it should have been from the start. The moral of this story is do not assume. Do the proper duct calculations, put it together per code requirements and all will be well. And if you do have one of these in your home, have it cleaned. The manufacturers recommend cleaning every 6 months. But you likely weren’t even told you have one so just call me when it starts making noise.