I've a fan/motor assembly out of a house central air/heat air handler, it
probably moves around 300 cfm. I'm planning on putting it in a box with a
std filter (it used a 20 * 20 inch) on the inlet. Will I notice an
improvement in the airborne dust in my shop (30 * 20 ft)? Any tips?
I've a motor/fan assembly from an old air handler ( off a 3 1/2 ton AC,
ballpark 300 cfm), it was working when it came out. The plan is to make an
air cleaner to reduce dust in the workshop by putting hte fan in a box with
a filter on the intake. When running as an air handler it took a 20 * 20 in
filter; I'll stick with the same. Am I wasting my time, any tips before I
That fan is more likely a 1200-1400 cfm fan if it came off a 3 1/2 ton
system. It should make a pretty good air filter for a workshop, but locate
it where the output won't blow in your direction or working may feel a bit
uncomfortable. Your design needs to make provision for several filters, a
mesh pre-filter followed by one or two of the newer paper sub micron type
"Peter Wells" < firstname.lastname@example.org> wrote in message
3-1/2 tons comes out to 1400 CFM, but yes it will work fine for a air
cleaner. I did the same using an old furnace blower. I use two 20"x20"x2"
standard pleated paper filters back to back. When the outer one gets packed
I toss it, move the inner one out and stuff a new one in the inner position.
My nose used to get full of crap after wood working or metal working in the
garage, but not any more.
Pretty soon someone will come along and tell you it is a waste of time, but
don't believe him!
I'm another person with a shop-made air cleaner, using a blower
salvaged from an old furnace. Don't know the cfm of the blower, but it
does a pretty good job hanging from the ceiling of my 20'x25' shop.
I installed the blower at one end of a box, put a large pleated fine
furnace filter in the middle, mounted on an angle to reduce vertical
height of the box, and then used a couple of coarse washable fibreglass
filters at the far (intake) end. The blower sucks air through the filters.
Part of one side of the box is removable to get access to the filters. I
wash the coarse ones periodically, but the fine one gets replaced when it
The blower is a two-speed unit, so the box has two switches: a normal
on-off switch, and a three-way switch that changes the blower speed. I
normally leave it running on low most of the time I'm working, and then
turn it to high if I'm doing major dust production with tools that don't
hook up to the dust collection system (e.g. sanding). On high, it moves a
lot of air and clears things up in a hurry.
Air cleaner, downdraft sanding table, outfeed table/cart
diesel truck or pool/spa filters to catch the really fine stuff
- the little ones that get WAY down in your lungs.
I made something similar - basically a sealed box with a 320cfm
squirrel-cage blower. I was going to give the opposite advice - find
some NICE filters (even the super-micro-ultra-allergen are maybe
$20?). Protect your lungs! This is still a LOT cheaper than any of
the commercial air cleaners, and I've found the nice filters last a
long time when you get some cheap electrostatic filter as a pre-
filter, and that can even be vacuumed off a couple times to extend its
To answer your original question, from my experience, with the double-
filter setup as described above, a home-made air cleaner will
definitely reduce the amount of airborne dust in your shop. (Notes:
my shop is very small - maybe 800 cu ft., I still wear a respirator
when sanding or doing anything dusty, and I SUSPECT that placement of
the unit and direction of exhaust and shape of the room have a lot to
do with overall efficiency.)
I'd agree. My air cleaner has a standard pleated furnace filter for
the fine collection, and a [cut with a utility knife] standard washable
fibreglass furnace filter for the coarse filtration. About $5 for the
fancy one; $2-3 ea. for the coarse ones.
The design process was:
- decide how big a box I needed to hold the furnace blower, and how to
mount the blower so that its output was all directed outside the box and
it could suck air from the other end of the box. (It's bolted to one end,
with a standard HVAC grill covering the hole.) I made the box using pieces
of broken wooden hockey sticks for framing and MDF (probably 3/8") to form
the sides, top, bottom, etc. You could probably use cheap plywood and skip
a frame. I used scraps I had on hand.
- decide how to mount the largest common fine filter size I could fit
in that box, so that the fan had to suck air through it.
- leave the other end open, with enough bits of wood glued on to form a
slot I can stick the coarse filter in.
- find a spot to mount an electrical box for the switches. I used one
of those plastic boxes with the foam covers that are supposed to seal
against vapour loss on exterior wall, so that dust wouldn't get into the
box from inside the air cleaner. (The electrical box is mounted inside the
air cleaner. You could moutn it outside to avoid all that.) I used an
extension cord with the end cut off to give me my wiring to plug into the
That's about it. Crude, quick, easy, and after about 10 years I've had
no urge to change it.
Peter, if you want the best filtration with the maximum flow, you may
want to check our an HVAC supply company and get a 'bag' filter. These
are the ones that are 6" to 12" (or more) deep. The pockets vastly
increase the surface area, enabling you to move more air. To do it
right, you would also add one of the standard fibre filters (not the
pleated ones) in front of the bag filter to trap larger particles.
May be more money, but it will last longer and trap more dust.
I made my shop air cleaner out of a 'donated' furnace blower & motor. To
this I added a scrap plywood box sized to serve as a tall workbench
(grinder, belt sanders and bench-top arbor press) with 2 - 20"x20"
filters taped edge to edge to make a single layer of filter media. These
were then taped to two more layers (making a 6 filter stack, 3 high, 2
wide) using a standard coarse furnace filter on the outer panel, a
medium (~5 microns) middle layer and a fine (~0.3 microns) inner panel.
The bench gets its air intake from both sides, each of which has a stack
like this, making some 800 sq. in. of filter face. The inner filters are
pleated so there is quite a bit more filter surface than is at first
evident ... I'd guess about 3x. The coarse filters are that woven
fiberglass stuff meant to keep small birds and large bugs out of the
furnace. I spray them lightly with adhesive when I first install them so
they'll build a bit of a cake faster.
I do not secure the filters in place, they are held against a cleat by
suction. This gives me an audible clue (by how loud they slap the cleat)
to their level of dirtiness. The dirtier they get, the louder they are.
With fresh filters, there is an obvious, but not (to me) uncomfortable
flow of air in the shop.
My shop is about 2,700 cu ft, so I estimate that my air is getting
filtered roughly 30 times an hour.
I don't count on my (so-called) dust collector for much ... it emits ~30
micron dust and sits downstream in the air flow from where I am working
when it is operating.
The air coming out of the whole-shop filter is good enough to place a
freshly varnished piece in its output flow and go on back to work.
Sure beats a box fan with a furnace filter! (Which sure beat nothing at
PS, keep wearing that respirator ... you are closer to the source of
dust than the filter is, so you get first dibs on it. When possible,
work with your back to the filter ... let it blow the dust away from you.
I am disillusioned enough to know that no man's opinion on any subject
is worth a **** unless backed up with enough genuine information to make
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