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The combustion process requires the air being burned to contain 20.9% oxygen. If it does not, the flame will start producing CO (carbon monoxide). This is a colorless and odorless gas which when inhaled, stops the body from absorbing oxygen. CO poisoning happens every year. We must be aware and concerned about this when we tighten up our homes and have appliances which use indoor air for combustion.

Below is a chart which shows concentration values and the adverse affects of CO in the air.


Symptoms / Adverse Affects of CO


Slight headache, tiredness, dizziness and headache and nausea after 2-3 hours


Frontal headaches within 1-2 hours, life threatening after 3 hours


Dizziness, nausea and convulsions within 45 minutes. Unconsciousness within 2 hours. Death 2-3 hours


Headache, dizziness, and nausea within 20 minutes. Death within 1 hour


Headache, dizziness, and nausea within 1-2 minutes. Death within 10-15 minutes


Headache, dizziness, and nausea within 5-10 minutes. Death within 30 minutes

When bringing air from the outside you only need to bring in the balance of what you need. When in direct communication with the outdoors you need 1 sq in of free space for every 4000 btu's. This would mean a hole cut through the wall and a grille installed. See below for other calculations.

When you determine the total appliance input divide this by 4000 to get square inches of free area required, (4000 if the fresh air is direct or a vertical pipe, 2000 if the pipe is horizontal). In our above example we have a total input of 170,000 btu's divided by 4000 equals 42.5 sq inches of free inches. For ease of explanation we will round this up to 50 sq. in. this sounds like easy to determine a grille size. 10" x 5" - 50 sq in. It is not as easy as that. We are talking about free area of the grille face. The manufacturers of the grilles will have charts for free area.

To calculate area for a round hole, take the radius times itself, times Pie; for example a 5” round pipe has the radius of 2.5”: 2.5 x 2.5 x 3.14 = 19.6”

The way we bring in combustion air makes a difference.

Direct communication with outdoors (grille through wall) - 1 sq in per 4000 btu's input

*Vertical duct - 1 sq in free area per 4000 btu's input

*Horizontal duct - 1 sq in per 2000 btu input

Indoor air grille through interior wall - 1 sq in per 1000 btu's

When you purchase a home, finish a basement or replace windows, you should ensure that you have sufficient combustion air supplied to your gas fired appliances. For specific verification the air source should be evaluated by a National Association of Technical Excellence (NATE) Heating and Cooling Specialist.

Jim Michael

Steel Rhino Property Inspections

President Inter-NACHI Colorado