Calculating Combustion Air

How to Calculate Combustion Air

Note: This calculation is not required for sealed combustion/direct vent

The amount of combustion air today is limited to extremely limited due to 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 in the basements 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 in the basement, 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 needed for safe combustion.

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 an odor free gas which gets inhaled and stops the body from absorbing oxygen. CO poisoning deaths happens every year in the USA. We must be more 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 effects of CO in the air.

As you can see it does not take much CO to start being a problem. When you think of this as parts per If just 200 molecules are CO and 800,000 are air it is a problem. Let's see how much air we actually need. The federal code requires 50 cu ft. of space per 1000 btu’s input from your heating appliance. Your state or local code may exceed that. If we know what the input of the appliance is and divide it by the total cu. Ft. of the mechanical room the answer would have to be t least 50 cu ft. per. If we do not have 50 cu. ft. per thousand btu's, we must bring combustion air in from the outside. If there are hallways and other rooms available due to no door being on the mechanical room we can also calculate these other areas.

We need to measure all the areas we can get combustion air from, until we come to a doorway with a door. The code states even if it is a louvered door, we still stop when we come to a door. If the utility room has a door mounted, even if it is open all the time you stop at the door. Since the formula uses btu's as MBH (thousand btu/h) we do not use the last three (3) zeros. For example, 130,000 would be just 130. Let's look at a mechanical room which would have a door.

If we do the math it would look like this;

32 feet long x 25 feet wide x 7 feet high
Boiler = 130,000 Btuh input and a 40,000 Btuh Water Heater
Volume = Length x Width x Height
Volume = 32' x 25' x 7' = 5600 Cubic Feet
Total Btuh Input = 130 MBH + 40 MBH = 170 MBH
5600 Cubic Feet / 170 MBH = 32.95 Cubic Feet per 1000 BTU
Combustion Air will be Required

Now, lets add other areas. If there would not be a door on the mechanical room we could pull air from all these other areas until we encounter a door. They would all be included in the calculation.


Now let's do the math on these areas.

You can see, even with all these areas we still need combustion air to satisfy the national code  for combustion air. Remember this calculation assumes no door on the mechanical room. If there is a louvered door, can it bring in enough air from these other areas? Since are assuming there are doors at each end of the hallway we still would be short. The better application would be to bring combustion air from outdoors. 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. 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.

When we look at the differences of free inches from metal grilles to wooden grilles and some of these decorative grilles may vary a lot. The grilles could lose 15% to 70% of free area.
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 inch per 1000 btu's
*Cross section of duct must equal opening free area free area
If you want to use round duct this
chart will give you square inches of round duct.

Let's look at an actual job site I was on and how it was calculated.

Boiler room 15ft x 15ft x 7ft
Boiler 130,000 btu/h
Water Heater 45,000 btu/h
15 x 15 x 7 = 1575 cu ft
Appliances 130 + 45 = 175 mbh
1575 cu.ft. / 175 mbh = 9 cu.ft. per 1000 btu's (should be a minimum of 50)
9 cu. Ft. / 50 cu. ft. required = 18% of required
175,000 /4000* = 43.75 free sq. in. needed
43.75 x 0.82 = 35.87 free sq. in.
* Direct communication with exterior of home

There are some states that have increased the 50 cubic foot per 100 btu's per 1000 btu's. This is a safer number as the homes are tighter today and the air required for combustion will come in slower. You can change the cu. ft. per thousand btu's in your formulas if you have tightened up the home or the home was built since 1970.

Here is the same formula above using 100 cu. in. per thousand btu's

Boiler room 15 ft x 15 ft x 7ft
Boiler 130,000 btu/h
Water Heater 45,000 btu/h
15 x 15 x 7 = 1575 cu ft
Appliances 130 + 45 = 175 mbh
1575 cu.ft. / 175 mbh = 9 cu.ft. per 1000 btu's (should be a minimum of 100)
9 cu. Ft. / 100 cu. ft. required = 9% of required
175,000 / 4000* = 43.75 free sq. in. needed
43.75 x 0.91 = 39.8 free sq. in.
* Direct communication with exterior of home

To determine the free square inches required when using a duct either horizontal or vertical we will not use the numbers from chart below depending on your application. The above formula examples used direct communication to outdoors or vertical duct numbers.



BTU's Per Square Inch 

Direct Communication to Outside (Grilles) 

4000 BTU's per 1 sq. in. 

Vertical Duct

4000 BTU's per 1 sq. in. 

Horizontal Duct

2000 BTU's per 1 sq. in. 

Grilles to interior rooms 

1000 btu's per 1 sq. in. 

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.