15
34-55 08/14
7.3.1 Maximum Category III or Category IV Vent Length
(Equivalent Length)
If a vertical or horizontal Category III or Category IV vent system is used with this appliance, the maximum length of
field supplied venting is shown in the chart below titled Maximum Category III – Category IV Vent Equivalent Length.
Maximum Category III – Category IV Vent Equivalent Length
Vent Size
6” Vent
7” Vent
8” Vent
9” Vent
Max Equivalent Length
100 feet
130 feet
250 feet
450 feet
Pipe fittings reduce the maximum allowable vent length. Use the Category III or Category IV vent manufacturer’s
equivalent length deduction for all elbows, terminations, etc. If the information is not readily available from the vent
manufacturer, use the Vent Fitting Equivalent Length chart below to find the total equivalent length for all vent fittings
in your combustion air system. Then subtract this number of feet from the total equivalent length allowed in Maximum
Category III – Category IV Vent Equivalent Length chart above. The sum of this calculation is the maximum length of
straight vent allowed. If a longer length is required, repeat the calculation using a larger vent size. When using this
chart, no additional deduction is required for the addition of the vent system terminal.
The following vent information is provided for use in design calculations, if needed.
Venting Specifications
Boiler
Input MBtu/h
Combustion Air
Volume (cfm)
Max Vent
Press. “W.C.
399 136 0.5
500 170 0.6
650 221 0.8
750 255 1.0
850 289 1.2
900 306 1.2
1000 340
1.3
7.3.2 Vertical or Horizontal Vent Termination
1. The vent terminal must have a minimum clearance of 4 feet (1.22 m) horizontally from, and in no case be located
above or below, electric meters, gas meters, regulators and relief equipment.
2. The vent cap must terminate at least 3 feet (0.91 m) above any forced air inlet within 10 feet (3.05 m).
3. The vent shall terminate at least 4 feet (1.22 m) below, 4 feet (1.22 m) horizontally from or 1 foot (0.3 m) above
any door, window or building air inlet to the building.
4. The vent system shall terminate at least 1 foot (0.3 m) above grade, at least 1 foot (0.3m) above highest possible
snow accumulation levels and shall terminate at least 7 feet (2.13 m) above grade when located adjacent to
public walkways or gathering areas.
5. To avoid a blocked flue condition, keep the vent cap clear of snow, ice, leaves, debris, etc.
6. The vent must not exit over a public walkway, near soffit vents or crawl space vents or other areas where
condensate or vapor could create a nuisance or hazard or cause property or could be detrimental to the
operation of regulators, relief valves or other equipment.
7. A horizontal vent must extend one foot beyond the wall.
8. A horizontal or vertical vent terminal must not be installed closer than 3 feet (0.91m) from an inside corner of an
L-shaped structure.
Vent Fitting Equivalent Length
Vent Pipe:
6” Vent
7” Vent
8” Vent
9” Vent
90º Elbow
8 feet
9 feet
10 feet
11 feet
90º Long Radius Elbow
5 feet
5 feet
5 feet
6 feet
45º Elbow
5 feet
5 feet
5 feet
5 feet
