Do I Need More Combustion Ventilation Air (CVA)?

Do I Need More Combustion Ventilation Air (CVA)?

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Determining if More CVA is Required

The first step is to determine the input BTUs of the combustion appliances in a zone. You need to have enough ventilation air for all the appliances in a zone, not just one. Often times you will see this situation with both a furnace and water heater located in a garage.

The next step is determine of the combustion zone (closet, garage, etc.) is considered a "confined space." The existing area in cubic feet is measured by multiplying the length, width, and height of the enclosure or room. The required cubic feet is determined by totaling the input BTUs, dividing by 1000, and then multiplying by 50. There is an easier formula that achieves the same results. Divide the total BTU input by 2, and then drop the last zero. Example: 44,000 BTU input divided by 2 = 22,000. Drop the last zero = 2,200 cu. ft. If the "existing area" is less than the "required cubic feet," then CVA is required.

Rules for Combustion Ventilation Air

Once it has been determined that the combustion appliance zone requires CVA, then the Net Free Vent (NFV) area, per opening(s) must be determined. This is achieved by taking the total kBTU input and divide by the CVA rule that is being used. Example: The total BTU input is 80,000 BTUs. You have chosen rule 4. 80,000 divided by 1000 = 80. 80 divided by rule 4 = 20 sq. in. required NFV area. Below are the 4 rules that can be used for CVA.

Rule 1: Requires two opening. CVA from conditioned space (living space, not garage) requires that each opening shall have a NFV area of at least 1 sq. in. for every 1000 BTUs input. 1 upper vent within 12" of ceiling and 1 lower vent within 12" of floor venting to unconfined space (attic / crawlspace). Each opening, minimum 100 sq. in.

Rule 2: Requires two openings. CVA supplied by horizontal ducts to the outside (unconditioned space). 1 upper duct and 1 lower duct. Each opening requires a NFV area of at least 1 sq. in. for every 2000 BTUs input.

Rule 3: Requires one opening. CVA to outside (unconditioned space). 1 upper opening may be used to provide the combustion air. The vent must provide 1 sq. in. NFV area per 3,000 BTUs input. With Rule 3, appliances must have a clearace of 1 inch on the sides and back and 6 inches in front from appliance to wall / door.

Rule 4: Requires two openings. CVA to the outside (unconditioned space). 1 upper and 1 lower vent or vertical duct opening is required. Each opening shall have a NFV area of at least 1 sq. in. for every 4000 BTUs input. If the combustion appliance zone is an unconditioned garage, then only one opening is required.

Below are the vent opening multipliers to calculate NFV area.

  • Mesh 1/4 in. or larger = 90% of the actual vent opening
  • Mesh, less than 1/4 = 50% of the actual vent opening
  • Metal louvers = 75% of the actual vent opening
  • Wooden louvers = 25% of the actual vent opening

In the Sacramento, Folsom, El Dorado Hills area, we see some of these rules and vents more than others. Vent openings usual use 1/4 in. or larger mesh. Rule 1 is typically used for utility closets, and Rule 4 is used for unconditioned garages.