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?
http://www.harborfreight.com/cpi/cta...emnumber=43060
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.

Thanks,
Gary