Vent systems shall be sized and constructed so as to develop a flow adequate to convey all combustion products to the outside atmosphere. The area of the vent shall not be less than the area of the largest vent connector inlet. Refer to Appendix C: Venting Tables.
When two or more vent connectors are joined together, the area of the vent or chimney shall not be less than the area of the largest vent connector inlet plus 50% of the areas of all additional inlets.
Appliances that are connected to a common venting system shall be located within the same story of the building, except venting systems designed in accordance with accepted engineering methods when the building official has approved the design method. Where the same vertical vent, or chimney is used for gas appliances on more than one floor, all appliances except those on the lowest floor shall be provided with as much vertical vent connector as possible before connecting into the common vent.
When two or more vent connectors enter a common vertical vent or chimney, the smaller vent connector shall enter at a higher level and in no case shall any two vent connectors enter in the same horizontal plane.
The following shows the size of vents and cross-sectional areas.
No vertical metal vent or masonry chimney shall have any area less than 7 square inches (3 inches diameter).
The cross-sectional area can be determined by using the following formula:
Nominal area square inches = D2 x 0.7854
A 6” vent has 28.27 nominal area square inches.
6” x 6” x 0.7854 = 28.27 sq inches
The common vent size can be determined by using the nominal area square inches formula and using the nominal area square inches of the largest vent connector inlet plus 50% of the nominal area square inches of all additional inlets.
Another means of determining a common vent size can be achieved by using the square root method.
A 4” and 5” vent connector:
5” x 5” x 1(100%) = 25”
4” x 4” x .5(50%) = 8”
25” + 8” = 33” requiredCommon vent size required is 6”
6” x 6” = 36”
36” is greater than the 33” needed. If the square root works out to be a lesser number, use a larger number that will be just larger than the number needed.
NOTE: Do not use these methods for sizing Category I fan-assisted appliances – use the AGA/GAMA tables. For additional assistance with vent sizing, please consult The Gas Company. Venting tables are provided in Appendix C: Venting Tables.