I don't know where else to ask this, and it seems like this group would
probably have some expertise in this area.
I want to use an old shop-vac as a heavy-duty exhaust fan. I have an
application where I need to pull hot air out of an equipment area. I
made a serious mistake and built 3" flexiduct into the walls to the
exhaust fan area. The 3" flexi is a very thin straw, and normal fans
can't suck enough air through them. (I've tried 5 different fans of
varying types and they can't hack it.) I can't change the flexi without
ripping out several walls -- not gonna happen. Hence the shop-vac.
The shop-vac can pull LOTS of air through the 3" duct -- TOO much. I
need to reduce the amount of air it pulls. I could just introduce a
"leak" in the duct but that seems like a very bad solution.
I'd much rather slow the fan down to just the right level I need, which
will also reduce the noise level.
Is a router speed control like this one OK to use on the kinds of motors
used in shop-vacs?
It's rated for 15A, and the shop-vac only pulls 5.3A.
BTW the vac will run for 2-4 hours at a time. That's probably more than
the recommended duty cycle for a shop-vac but hopefully it will be OK.
Especially if I slow it down.
Hm, I should have dug deeper before I posted. I found a thread where this
has been discussed before:
The consensus there seems to be that shop vacs (and any other "noisy" tool)
always use "universal" motors, which should have no trouble being
controlled by the Harbor Freight speed control.
So unless someone has a good reason not to, I'll go make a run to Harbor
Freight. Or if someone has a suggestion for a fan that really can pull a
lot of air through a 3" duct -- for cheap -- I'd like to hear that too.
(Anybody want to part out their old dust-collector fan/motor?? :-)
Maybe I should try an inexpensive blower/vac, e.g.
bwr.html ? I'd still need to slow that one WAY down. They claim 100cfm,
and I need to cut it down to 50-60 or so. So I'd still need the speed
BTW I'm quite concerned about the safety issues of this idea, since the vac
will be stashed off in a closet and not immediately visible. I definitely
don't want it to overheat and start a fire.
Wed, Feb 1, 2006, 6:07pm (EST+5) firstname.lastname@example.org (Gary Fritz)
doth admit amazement:
Hm, I should have dug deeper before I posted. I found a thread where
this has been discussed before: <snip>
The real trick would be finding a subject that "hasn't" been
Shhh... that's the sound of nobody caring what you think.
I seriously doubt the motor would last using it that many hours at a time
and I seriously doubt that you will reduce the motor noise very much either.
I would either adapt a low end dust collector to it or visit a local Air
Condition/Furnace dealer and see if they have any old furnace blower
motor/fans. I have seen these at garage sales for $5-$10. I think an
induction motor is going to serve you better.
The noise I can live with. Motor not lasting forever wouldn't kill me
either, as this is a freebie scrounged vac anyway. Just so it doesn't die
in a burst of flames.
Suggestions for low-end dust collector? I can't find much of anything for
less than $150 or so. I'm trying to make this as close to zero-cost as
Furnace blowers: I haven't seen anything that can move enough air through
a 3" duct with high static pressure. They are all designed to move air
through large, smooth ducts, and their CFM drops to near zero with a high-
resistance load. If you know of a fan or class of fans that would work,
I'm all ears.
Might work for the airflow, but I don't want to blow air into the hot area.
I need to remove heat FROM the hot area. I really need to suck the air
I tried a Nutone 696N fan, nearly identical to the 695 fan you linked to.
It's rated at 50-75 cfm. It barely pulled 25cfm through my duct. The
simple cheap Nutone fans simply do not have the oomph it takes to pull
sufficient air through this duct.
Well, I went to Harbor Freight and got the speed control. It appears to be
working like a charm. I can tune the airflow to exactly what I need. The
shopvac is **considerably** quieter at the slower speed.
Thanks for the help!
I didn't mean to reverse the direction of the air flow.
Put the blower in the hot area pushing the air out.
Most blowers will move a lot more air into a restriction
than when trying to suck thru the same restriction.
Not gonna work. Here's the deal: the "hot area" is an enclosed sound-
absorbing box for a noisy and hot Home Theater projector. The projector
puts out about 800-900 watts of heat. It will heat the room on its own
quite handily, and since it's mounted on the ceiling, it ends up baking
in its own hot air. Plus the people watching the movie each put about
100W of body heat into the room. (The first night in the theater, before
I built the sound box and the exhaust system, the projector and the 8
people watching generated so much heat that the projector overheated and
shut down! I was afraid I'd fried it, but fortunately there was no
I don't WANT to push all that hot air into the room, and try to
eventually vent it out. I want the hot air OUT of there.
Ah. Yes, and the flexiduct would inflate and expand, instead of
shrinking and further restricting the airflow. That would work better,
IF it was an option to put the fan in the hot area. But it's not --
there's no room for a fan in the projector box, and I can't have the
noisy fan/vac in the theater room. It needs to be several walls away, on
the other end of the duct.
A pool contractor I know uses the shop vac blower motors in
his buisness. The vacs runs continuously sucking air from behind the
vinyl liners as the pool fills up. This can take the best part of a
day on on the average pool. I know the vacs don't last forever like
this( I have the extra hoses to prove it) but they do last longer
than one might expect, and four hour run time is no big deal. Good
Why don't you just use a cheap $15 bath exhaust fan? They are meant to
push 50-60 cfm through a 3 or 4" duct. Here's an example.
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