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Understanding Combustion Air and Tight Homes

The amount of combustion air today is very limited due our attempt to reduce our fuel consumption. We are replacing our windows, doors, weather stripping, caulking and adding insulation. When I started in this trade there were no double pane windows, much less steel insulated doors in the basement leading to the outside. There were old single pane glass windows and old wooden doors which, a lot of times you could see daylight around the door frame.

Today we have added thermopane windows, new steel doors complete with magnetic seals. We even insulate the exterior perimeter between the floor joists. All these ideas are good, but they steal much needed combustion air required for a safe combustion process. If your heating appliance is using indoor air for combustion you must be aware of the amount of combustion air that is required.

The Combustion Process

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.

PPMSymptoms / Adverse Affects of CO
200Slight headache, tiredness, dizziness and headache and nausea after 2-3 hours
400Frontal headaches within 1-2 hours, life threatening after 3 hours
800Dizziness, nausea and convulsions within 45 minutes. Unconsciousness within 2 hours. Death 2-3 hours
1600Headache, dizziness, and nausea within 20 minutes. Death within 1 hour
3200Headache, dizziness, and nausea within 1-2 minutes. Death within 10-15 minutes
6400Headache, 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 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. If you have questions, please call Tyler at Steel Rhino in Merritt Island, Florida.

Jim Michael
Steel Rhino Property Inspections
President Inter-NACHI Colorado