Duane Bozarth wrote:
Joseph Meehan wrote:
...
Not a good idea to use that vent. There is a proper vent that
has a
grill to prevent birds from entering but more important has a flap
type
valve to close the vent to prevent cold air from entering when it
is not
operating.
...

I've never seen one (the flapper part) that would work well for
vertical
application, however.

I've seen this used succesfully in vertical runs (as long as the flow
goes up):

Deflecto BD04 Draft Blocker

http://www.deflecto.com/OMS/tier2_te...?ProductID=912

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