I’ve found that the metal 4″ duct fans (or duct “booster” fans) commonly found in the $20-30 price range don’t have a lot of oomph. Rated at somewhere between 65-100cfm, I’d be surprised if they move that much air. Incidentally, these tend to be the common budget-end Vivosun, Vivohome and Ventech fans.

Ask these fan ducts to move air through a restriction, and all bets are off. These little axial fans just don’t create enough pressure. The higher-cost centrifugal ones fare a little better, but the budget centrifugals in the $30-40 range tend to be… let’s say… very poorly rated when it comes to longevity.

I was recently in the situation where I needed to push some air past one of the cheap plastic 3-fin dryer vents. The hope was that the 4″ duct fan would do it.

It didn’t. Only 2 of the 3 fins opened, and only slightly – there just wasn’t much pressure. Looking inside the unit, I noticed there was a lot of space between the fan blades and the shroud/housing. Obviously, air pressure is going to take the path of least resistance, and that big gap has far less resistance than the plastic dryer vent fins!

I decided to try reducing that gap. After thinking about a few options I decided I may as well do a quick test with some caulk. Here’s what it looked like in the end:

Steps were pretty simple:

1. Grab a tube of caulk (I wasn’t picky, and just used typical indoor/outdoor caulk we had laying around).
2. Fill the gap between the fan blades and the housing. I spent about 30 seconds doing this as I didn’t care if it was a sloppy job. The fan blade tips will get some caulk on them: that’s fine for now, it’ll be washed off later.
3. By hand, rotate the fan blades in reverse: this smoothes/spreads the caulk nicely (if you rotate the blades in the forward motion the blades will act like a scoop). You may want to slightly squeeze the housing/shroud in each direction while doing this to create a slight gap now, which may avoid the need for step #5.
4. Remove/clean/reassemble fan: Once the caulk is a little dry, the tiny spring that holds the fan blades to the shaft can be worked off with fingernails. Then the fan blade assembly should pull off the shaft (check to see where the fan blade assembly lines up with the shaft beforehand). Clean the blades with water and mild scraping. Dry the blades and reassemble.
5. With the caulk being slightly dry/skinned, try plugging the fan in briefly. If it makes a loud racket (like the fan is hitting something), you probably have some caulk rubbing up against a fan blade. You can either squeeze the housing while it’s running and hope the fan blade rips off whatever chunk of caulk it’s catching on, or unplug the fan, rotate the blades by hand and see where it’s catching: use your fingers to press down the caulk in these areas (the caulk should hopefully still be soft).
6. Once dry, assuming the caulk isn’t hitting the fan blades, you should be good to go.

Note that the fan did make a little more noise with this mod. However, it did force more air through the dryer vent I was using, which was a fair tradeoff for me to make.

Enough pressure to open all 3 dryer vent flaps now. Hurray!

To be clear, this mod doesn’t suddenly turn the duct fan into a powerhouse or anything. It’s a measurable difference, but not a night-and-day difference. I’d also be surprised if it increases the flow rate much (or at all) if you don’t have a restriction. Whether the caulk will last/hold in the long term is something only time will tell (if it doesn’t, it’ll likely jam up the fan).

…that said, if you’ve got one of these 4″ duct fans kicking around and really need a slight pressure bump for a project, this mod could be worth a shot.