Air cleaner fan flow reduction

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Forum topic by jayscott posted 03-16-2012 09:30 AM 1908 views 0 times favorited 15 replies Add to Favorites Watch
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19 posts in 2472 days

03-16-2012 09:30 AM

Topic tags/keywords: question tip

I am using an old squirrel cage furnace fan with a direct drive, 4 speed motor, for cleaning the air in my shop. I put the fan in a box with openings on each side that are covered with replaceable filters. My problem is that, even on the lowest motor speed, the fan moves way too much air. It’s noisy and windy.

Has anyone any ideas on how to reduce the air flow? I have been thinking of cutting every other fan blade, or filing the space between every other blade with something, or ??? (I have no idea what could be used to fill the spaces between the blades.) Can’t afford a variable speed drive and I don’t know if a VFD would work on that kind of motor.

I’d appreciate any suggestions, especially those that aren’t too expensive. :-))


15 replies so far

View Fred Hargis's profile

Fred Hargis

5072 posts in 2614 days

#1 posted 03-16-2012 11:09 AM

I had that same problem with mine. This one was a 240V motor and I cobbled together a switch that allowed me to run the 2 lowest speeds…the low one was so fast I don’t remember ever using the second one. I worried that any attempts to change the fan would only screw it up, though I did think about making the box bigger, thinking the increased area would lead to slower velocity. Never got to try it since we moved and I left it hanging in the old shop.

-- Our village hasn't lost it's idiot, he was elected to congress.

View DIYaholic's profile


19647 posts in 2795 days

#2 posted 03-16-2012 12:05 PM

I’m no mechanical engineer, but you should/may be able to put a smaller pulley on the motor & a larger pulley on the fan. This would decrease the fan RPMs, hence reduce air flow. Greater minds may be able to help, when they chime in.

I’m using a repurposed squirrel cage furnace blower for my air cleaner also. However my fan is a direct drive, variable speed (variac??, think router speed control), unit so no pulleys.

Good luck.

-- Randy-- I may not be good...but I am slow! If good things come to those who wait.... Why is procrastination a bad thing?

View jayscott's profile


19 posts in 2472 days

#3 posted 03-16-2012 12:47 PM

Fred, I found a place on the ceiling of my shop to place the fan so it isn’t too much in the way. This meant the box is as small as possible so the fan outlet extends 1/4” through the side of the box. But, your comment leads me to wonder if I could put some kind of diffuser on the outlet to spread out the flow so it wouldn’t be as windy. That probably wouldn’t cut the noise but it would be a start. Thanks.

DIYaholic, I am using a direct drive unit as it is smaller than a belt drive fan so there are no pulleys to change. As I mentioned to Fred, I wanted the smallest box possible.

Thanks for the comments.

View ChrisK's profile


1984 posts in 3202 days

#4 posted 03-16-2012 01:03 PM

A VFD might work with motor set to the highest speed. Can see how the speed selector works? Is it switching the motor windings or is it a simple VFD. A diffuser on the outlet will help but it has to be long for air to follow the shape and start to slow down. The length should be at least 10 times the width of the opening.

Messing with the blower fan may cause the unit to become unbalanced, not good for the bearings.

-- Chris K

View bondogaposis's profile


4931 posts in 2472 days

#5 posted 03-16-2012 01:07 PM

Be careful modifying the fan blades. If you get it out of balance it will really make a racket.

-- Bondo Gaposis

View Woodknack's profile


12340 posts in 2500 days

#6 posted 03-16-2012 01:15 PM

I have a squirrel cage with that problem. My neighbor has an HVAC business and told me those fans are designed to run with more air resistance (ductwork from a whole house). The fan pulls air from both sides, I screwed pegboard over one side to restrict the airflow. It slowed the fan and now the motor runs much cooler.

-- Rick M,

View jayscott's profile


19 posts in 2472 days

#7 posted 03-17-2012 01:51 AM

ChrisK, the motor speed is changed by connecting different wires, i.e., changing windings. It looks like I’ll have to experiment with wormil’s suggestion and try reducing the inlet area – I like the idea of using perf board.

Removing blades also scares me. I have placed a piece of tape across every second blade, on the inside and outside of the blower cage. It does reduce the flow somewhat but not as much as I would like and I’m afraid it will come off over time. If I could find something that would completely fill the space between two blades, and not peel off, I’d try it, especially if I could fill 3 spaces out of 4.

Any suggestions for something that wouldn’t affect the balance of the wheel?


View BobM001's profile


388 posts in 2451 days

#8 posted 03-17-2012 04:13 AM

DO NOT mess with the fan wheel in any way. It is factory balanced. You might better restrict the inlet to reduce the amount of air it moves. As for a VSD these fractional HP fan motors ARE NOT “inverter rated”. You most likely would SMOKE the motor if you connected one to it.

-- OK, who's the wise guy that shrunk the plywood?

View Loren's profile


10477 posts in 3768 days

#9 posted 03-17-2012 04:48 AM

My JDS air cleaner has a diffuser filter over the outlet end. It
also produces a feelable amount of “backwash” at the inlet side.
The air is pulled through an electrostatic inlet filter, a “baghouse”
cloth filter, then the blower. It is pushed through the diffusing

View Jim Finn's profile

Jim Finn

2670 posts in 3042 days

#10 posted 03-17-2012 09:17 PM

wormil has it right….just block off part of the inlet side of the box and it will slow the air coming out and will use less power to run the motor.

-- No PHD just a DD214 Website>

View TopamaxSurvivor's profile


18358 posts in 3796 days

#11 posted 03-18-2012 12:25 AM

If you can turn ther fan around on the shaft so it runs backwards, it will move less air.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View jayscott's profile


19 posts in 2472 days

#12 posted 03-20-2012 12:34 AM

I looked at turning the fan wheel to rotate in reverse but wasn’t able to get my hand past the motor to tighten the nut at the hub. I could bend a blade to let me inside the wheel but it will be a last resort trial. A diffuser on the outlet also has possibilities that I’ll have to investigate.

The most promising is closing off the inlet but during rough trials I did when I first got the fan, closing it off too much made the fan vibrate too much. I’ll have the find the sweet spot.

Thanks to all those who commented. The suggestions were very much appreciated. If I make a break-though, I’ll post the results.

View jayscott's profile


19 posts in 2472 days

#13 posted 04-01-2012 03:11 AM

Well, I tried a number of things with not the best results so I thought I’d report in.

1) I did manage to turn the direct drive fan cage around and get the set screw tightened but the fan vibrated at high, low, and medium speeds. Turning the cage back to the correct orientation, the vibration stopped and the fan run smoothly. I guess they are not designed to run backwards.

2) When I did close off the inlet enough to get the flow I wanted, the direct drive fan vibrated so much I couldn’t use it.

3) I turned the cage on a belt drive fan backward and it runs smooth as silk. The flow was still higher than I wanted but I could live with it if it was all I had. I can also change pulley sizes to adjust the flow to reasonable levels.

4) During all the testing, I found a 10” 1500 rpm propeller blower. I installed it in the box from the first trials and it seems to be working quite well. I’ll stick with it for a while.

Thanks for all your help.

View longgone's profile


5688 posts in 3429 days

#14 posted 04-01-2012 03:55 AM

I use dual filters on the intake side and it creates resistance that slows the fan and quiets the sound.

View BobM001's profile


388 posts in 2451 days

#15 posted 04-01-2012 04:18 AM

The vanes on a backward curved fan wheel are generally further apart and are longer. Likened to those on a pump impeller.

-- OK, who's the wise guy that shrunk the plywood?

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