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Workshop ventilation questions


Hi gang, Does anyone know if there’s an existingformula for
determining the proper CFM rates for airflow, when properly
ventilating a workshop? Now that the contractors have finished
framing-out my new rooms, I need to build and install something whose
ducts will fit into the ceiling joists, but don’t know what size
motor(s) to buy. FWIW, the lapidary workroom measures 9x11’, the
metalsmithing room is roughly 11x11’, both feature81" ceilings and
insulated floors, and each will soon have a window with one of those
4" round clothes dryer vents in it,for connecting toinlet and
exhaust hoses. I know that Charles Lewton-Brain had posted some great
articles on ventilation in the Ganoksin library in the past,but none
of them addresses this issue, directly. Might there be an HVAC Tech
lurking in our midst, somewhere? (Please say ‘yes’!) Many thanks, in
advance, for whatever help anyone can offer.

My best, as ever,
Douglas Turet


Doug, You want somewhere between 10 to 12 volume changes per hour. So
figure your rooms volume in cubic feet ( width x length x height)
then divide by the CFM rating of your fan this will give you the
number of minutes per air change and then divide 60 by that number
to get the number of changes per hour.

Example 10’ x 10’ room with an 8’ ceiling with a fan that exhausts
200 CFM : 800 cubic feet (10x10x8) / 200 = 4 minutes per change 60
(minutes in an hour) / 4 (minutes per change) = 15 (changes per

The trick is that the nameplate CFM listed on a fan is under ideal
conditions. Ducting length, size and number of turns all degrade the
ability of a fan to move air so the true CFM is much lower. Also a
big item that many people forget, you must have a fresh air supply
that is going to make up the air that you are pumping out with your
exhaust fan. You will need an inlet vent as well as the exhaust vent
and it should not be located near the exhaust vent or you will just
recycle all the stale air and fumes. – Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160

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