1. Determine duct size 

• Follow work order and IRC G2407.6 or IFGC 304.6 to determine appropriate combustion air duct sizing 

2. Determine duct route 

• Determine most direct route to run combustion air opening. 

• Combustion air openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors. 

3. Cut hole for termination 

• Follow steps 3a-3k to cut exterior hole for termination 

3a. Unobstructed termination 

• Ducts shall terminate in an unobstructed space allowing for free movement of combustion air to the appliance. 

3b. Mark hole interior 

• Mark hole from interior of home. 

• Take care to avoid structural members and electrical and plumbing lines. 

3c. Drill pilot holes 

• Where exterior cladding permits, drill pilot holes from interior of home to indicate where the edges of the hole will be 

3d. Mark hole exterior 

• Use the pilot holes as a reference and mark the location of the hole on the exterior of the home 

3e. Cut hole 

• Determine duct size and cut hole in exterior wall slightly larger, but not more than 1/4" larger. 

• Take care not to cut any plumbing or electrical lines. 

3f. Drill hole 

• If installing termination through masonry or stone, use a core drill to cut hole 

3g. Install flashing 

• Whenever possible, install flashing under cladding 

3h. Install termination 

• Position Termination and orient with louvers in a downward position to prevent precipitation from entering the home. 

• Secure to house using 4 or more exterior grade fasteners. 

3i. Seal termination 

• Seal gap between termination and exterior of home with exterior grade caulk to prevent water intrusion. 

3j. Louvers and Grilles 

• Louvers and grilles shall be fixed in an open position. 

• Grille and screen openings cannot be smaller than 1/4". 

• Ducts shall not be screened where terminating in an attic space. 

3k. Pest screen 

• Vent termination shall have screen material with no less than 1/4" and no greater than 1/2" hole size in any direction to prevent pest entry. 

4. Install duct  

• Follow steps 4a-4e to install duct 

4a. Ducts must be galvanized steel 

• Ducts shall be constructed of galvanized steel complying with chapter 6 of the IMC code or material having equivalent corrosion resistance, strength and rigidity. 

• Flexible duct shall not be used for combustion air. 

4b. Assemble duct  

• Cut and assemble duct 

4c. Install fasteners  

• Install fasteners per code to hold duct together and to connect to termination collar 

4d. Seal with mastic 

• Seal all duct seams and fittings with mastic 

• Follow instructions under Air Seal Duct to seal the duct.

4e. Insulate the duct  

• If any portion of the combustion air duct runs through the unconditioned space, the duct shall be insulated to an R-8 or greater. 

• Follow instructions under Duct Insulation to insulate duct. 

5. Worst Case Draft Test 

• Conduct a Worst Case draft test to verify Category I, II and III appliances are venting safely before leaving home for the day. 

• Testing shall be recorded on the WAP Worst Case Draft Test form. 

6. Client Ed 

• Educate the client on purpose of combustion air opening. 

• Warn client not to block or plug the opening