My crawlspace has high humidity and a musty odor.
Since it is now warm and sunny outside, I want to vent it out.
The idea is to put a 8-inch centrifugal duct fan in the middle of the
crawlspace and connect a flex duct from the fan's output to one of the
So if there are 10 crawlspace vents, 9 will be sucking air and one
blowing air. If this creates a slight negative pressure in the
crawlspace that would be even better; it would counter the tendency for
crawlspace odor to migrate into the living are.
Question is how best to connect a 8-inch round flex duct to a
rectangular crawlspace vent (about 16-inch by 7 inch inside dimension).
I don't think there is a "round duct to crawlspace vent adapter".
Also, the crawlspace wall is concrete so it's not easy to attach
anything to the wall.
Well, 8" dia is has a 8x3.14 = 25 " circumference.
The rect vent has a 2x16x7 = 224" circumference, so you will have to
fit the round vent *into* the rect vent as you probably already
How about squishing it in, packing the empty spots with crunched
plastic bags and covering with duct tape. Might that work?
Web based forums are like subscribing to 10 different newspapers
and having to visit 10 different news stands to pickup each one.
Buy or have custom made a "boot" like is used to put a rectangular
floor register on the end of a round heat pipe. Don't know if there is
a "stock" one that will do the job but any good HVAC sheet metal shop
should be able to cobble one up for you.
I suggest the OP get a piece of sheet metal cut that fits the hole with an
8" round hole cut in it . Got to the local HVAC supply and purchase a boot
like they use to connect those ducts to the plenum on the furnace . They
come with a double-sticky gasket that will hold it in place while you put a
few # 6 x 3/4" self tapping screws in . The type of canned foam used around
doors and windows doesn't expand much and remains flexible , use a bead of
that around the perimeter of the sheet meal where it meets the masonry (from
the inside) .
Ignore the above , I missed the part where the hole is only 7" tall .
Instead of the plenum boot you need a transition for a 6X12 floor or wall
vent attached to that piece of sheet metal . I'm not sure if that size has
an 8" inlet , might also need a reducer to 6" .
I'd call some HVAC places near you, and see if any of
them can get you a transition from their stock, or
from a parts house.
Since 7 inch isn't that far from your 8 round, maybe
you can flatten the round flex, and let it hang out
the opening a foot or so.
Christopher A. Young
learn more about Jesus
On Tuesday, April 21, 2015 at 7:05:33 AM UTC-4, Stormin Mormon wrote:
Either that or just look for a fan that is made
to go inside round ducts to begin with. Either the
fan or the transition piece sizes available should be
Does he even need a duct though? I would think if
you just mounted one of the fans made for attic
gables in one vent, consistent with the prevailing
winds, it would probably be effective enough at
pulling air through it.
If the ducts are made of metal, the connector should also
be of metal, thus made by either:
-- a skilled and competent metalworker, or
-- an amateur willing to learn beforehand for this one job
the necessary skills, that he will never use again.
Have you checked with an HVAC supplier? There are an amazing number of
transition units out there. You might even find what you need at the local
big-box store -- they don't carry the full range but they certainly have
adapters to go from round duct to standard registers. Even if you do find
something it may not last long if the environment is really humid since
most of them are made from the thinnest sheet metal. As a last resort, any
sheet metal shop can make what you describe from heavy galvanized which
will last longer than the flimsy stuff.
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