Model CFC
ClearFire Commercial Boilers
61
Rev. 05-2012
Table B5-26.
STACK SIZING USING OUTSIDE AIR FOR COMBUSTION (DIRECT VENT COMBUSTION)
DIRECT VENT COMBUSTION AIR -- MAXIMUM ALLOWABLE VENTING LENGTH
Boiler
Boiler Stack/Flue
Connection
Combustion Air
Intake Duct &
Connection
Boiler
Flue/Stack
Size
Maximum length of
Flue Gas Vent in
Feet*
Maximum length of
Air Intake Duct in
Feet**
Direct Vent
Combustion Air Kit
Flue Connection
Part No.
CFC 500
6" Standard
4"
6"
75
75
880-01312
039-01704
CFC 500
6" Standard
6"
6"
80
80
880-03736
039-01704
CFC750
6" Standard
4"
6"
40
40
880-01312
039-01704
CFC 750
6" Standard
6"
6"
50
50
880-03736
039-01704
CFC1000
6" Option
4"
6"
30
30
880-01312
039-01646
CFC1000
6" Option
6"
6"
40
40
880-03736
039-01646
CFC1000
8" Standard
4"
8"
50
50
880-01312
039-01647
CFC1000
8" Standard
6"
8"
60
60
880-03736
039-01647
CFC1000
10" Option
4"
10"
70
70
880-01312
039-01705
CFC1000
10" Option
6"
10"
80
80
880-03736
039-01705
CFC1500
8" Option
6"
8"
30
30
880-01313
039-01645
CFC1500
8" Option
8"
8"
40
40
880-02451
039-01645
CFC1500
10" Standard
6"
10"
40
40
880-01313
039-01644
CFC1500
10" Standard
8"
10"
60
60
880-02451
039-01644
CFC1500
12" Option
6"
12"
60
60
880-01313
039-01688
CFC1500
12" Option
8"
12"
80
80
880-02451
039-01688
CFC1800
10" Option
6"
10"
40
40
880-01313
039-01644
CFC1800
10" Option
8"
10"
55
55
880-02451
039-01644
CFC1800
12" Standard
6"
12"
50
50
880-01313
039-01688
CFC1800
12" Standard
8"
12"
80
80
880-02451
039-01688
CFC 2500
12" Standard
8"
12"
100
100
880-02451
039-01688
CFC3300
12" Standard
8"
12"
55
55
880-02501
039-01761
CFC3300
14" Option
8"
14"
100
100
880-02501
039-01762
* Each additional 90 elbow equals 5 equivalent feet of ductwork. Flue terminations may add 5-10 feet
to the equivalent length and should also be included in the equivalent length calculation.
** Increasing the diameter of the air intake will reduce the overall pressure drop and thereby allow
longer total vent lengths.
Draft tolerance at boiler flue connection during operation is +/- 0.25” WC.