– 13 –
Ductwork Calculations
Note: Do not use flexible plastic ducting.
If flexible metallic ducting is used, all the equiva-
lent feet values in the table should be doubled.
The flexible metallic duct should be straight,
smooth, and extended as much as possible.
Vent installation should not exceed the equivalent
ductwork length of 100 ft.
Blower is rated at 400 CFM at 0.1 inch of water
back pressure.
*
Equivalent lengths of duct pieces are based on actual tests
and reflect requirements for good venting performance with
any downdraft cooktop.
†
Measure and list feet of straight duct used. Count and list
the quantity of all other duct pieces for the “Number Used”
of each type.
Note: For maximum efficiency, use the shortest
and straightest duct run possible, with as few
fittings as possible.
For satisfactory performance,
the duct run should not exceed 100 ft equivalent
length.
Venting performance is improved by using larger
diameter duct.
Equivalent
Number
Equivalent
Duct Pieces
Length* x
Used = Length
6" round
to 3
1
4
" x 10"
transition
90 elbow
20 ft.
x
(
) =
ft.
3
1
4
" x 10"
to 6" round
transition
5 ft.
x
(
) =
ft.
3
1
4
" x 10"
to 6"round
transition
90 elbow
12 ft. x
(
)=
ft.
6" round
wall cap
with damper 21 ft.
x
(
) =
ft.
3
1
4
" x 10"
wall cap
with damper 27 ft.
x
(
) =
ft.
6" round
roof cap
20 ft. x
(
) =
ft.
6" round
roof vent
24 ft
x
(
) =
ft.
Subtotal Column 2 =
ft.
Subtotal Column 1 =
Should not exceed 100 feet.
ft.
TOTAL DUCTWORK =
ft.
GEA00203
TABLE 1
Calculate Total Equivalent Ductwork Length
Equivalent
Number
Equivalent
Duct Pieces
Length*x
Used = Length
6" round
straight
1 ft.
x
( )=
ft.
3
1
4
" x 10"
straight
1 ft.
x
(
) =
ft.
6", 90
elbow
15 ft. x
(
) =
ft.
6", 45
elbow
9 ft.
x
(
)=
ft.
Flexible
Metal Offset See Calculations
Adapter
in Steps 5A and 5B
ft.
3
1
4
" x 10"
90 elbow
16 ft. x
(
) =
ft.
3
1
4
" x 10"
45 elbow
5 ft.
x
(
)=
ft.
3
1
4
" x 10"
90 flat elbow 18 ft. x
(
)=
ft.
6" round
to 3
1
4
" x 10"
transition
7 ft.
x
(
) =
ft.
Subtotal Column 1 =
ft.
GEA00202
