Vacu-Stack for Chimney Drafting Problems
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Vacu-Stack chimney caps are designed using the venturi effect creating an upward draft in windy
situations. It is important to select the proper size for your chimney. When your masonry chimney was
built the mason used a formula to determine the size flue according to the fireplace opening. To
determine the size vacu-stack cop needed multiply fireplace face opening width and height and divide by
ten, this will give you the square inches needed for proper draft.
Vacu-Stack Model
VSS-5” = 20 square inches in area
VSS-6” = 28 square inches in area
VSS-7” = 38 square inches in area
VSS-8” = 50 square inches in area
VSS-9” = 64 square inches in area
VSS-10” = 79 square inches in area
VSS-11” = 95 square inches in area
VSS-12” = 112 square inches in area
VSS-812 = 52 square inches in area (comes with masonry adapter)
VSS-1216 = 119 square inches in area (comes with masonry adapter)
For Fireplaces Multiply fireplace face opening width and height and divide by ten. Match that number to
the closest number listed in the cross sectional area chart listed above (NEVER DECREASE!). Example,
a fireplace face opening is 24” by 46”. 24” times 46” is 1104. 1104 divided by 10 is 110.4. This would
require a 12” Vacu-stack.
The Vacu-stack size listed is the BASE size. If the chimney is round (masonry or stainless steel), the
Vacu-stack is to fit on the INSIDE of the liner only. If the chimney is a square or rectangle masonry flue,
the Vacu-stack will fit on the inside of the flue as well, but you must use with a masonry adapter. Example,
you would use a AD6 or AD614 with a VSS6, an AD8 with a VSS8 and so on.
If the base of the required Vacu-stack will not fit in the flue, the flue is apparently too small (example, a
10” required, but the inside flue dimensions are 8”X8”). This most likely means that the chimney is too
small for the appliance, and that the problem is not caused by wind induced downdraft. Other measures
will need to be taken to correct the drafting situation, the Vacu-stack will hinder and not help in this
situation.
For woodstoves, wood inserts, and furnaces Vacu-stack size same as outlet size (example 6” outlet
on woodstove would require a 6” Vacu-stack). It is recommended for inserts to use a chimney liner which
is a flexible stainless steel coil pipe from the top of the insert to the top of the chimney. The liner should
be the same size as the insert opening. This bypasses the smoke chamber and increases draft. If a wood
stove is drafted into a masonry chimney a liner should also be used. When the smoke from a wood stove
is "dumped" into a masonry chimney (which many times has a larger flue size) the smoke becomes "lazy"
rolling around filling the larger volume flue tiles before rising to the top. Relining the chimney will keep the
same size pipe all the way to the top increasing draft and reducing creosote build-up.
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