Calculating Combustion Air Drawings on this page are from my online or live seminar How to Calculate Combustion Air

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

Nowadays, the amount of air for combustion is pretty limited because we're trying to cut down on fuel use. We're swapping out our windows, doors, weather-stripping, caulking, and adding insulation. Back when I first started in this field, there weren't any double-pane windows in basements, let alone steel insulated doors leading outside. Instead, there were just old single-pane glass windows and wooden doors that often had gaps where you could see light coming through the frame.
These days, we've got thermopane windows in the basements and new steel doors with magnetic seals. We even insulate the area between the floor joists on the outside. While all these upgrades are great, they end up taking away the combustion air that's crucial for a safe burning process. If your heating system is pulling air from inside your home for combustion, you need to keep an eye on how much air is actually needed for it to be safe.

Examine this image of a residence. I assert that all heating devices should have combustion air supplied directly to them. You might be curious about the reason for this. When the air within the home is utilized for combustion, it decreases the pressure inside the house. This reduction in pressure causes cold air to be drawn in through any gaps and openings, which can lower the home's temperature and necessitate heating recovery. Supplying outdoor air to the appliance can prevent this issue. Insufficient combustion air may lead the appliance to draw air down the chimney, which can result in the intake of combustion byproducts, including carbon monoxide (CO).

A residence can lose combustion air through kitchen and bathroom exhaust fans, clothes dryer vents, and woodstoves or fireplaces that lack an outdoor air connection, which can diminish the oxygen levels within the home. Additionally, outdoor air temperature influences the availability of combustion air; as outside temperatures drop, the structure experiences greater heat loss. It is important to note that heat loss refers to air escaping from the home. Furthermore, the height of the building can enhance thermal circulation, leading to increased heat loss.

The combustion process necessitates that the air used contains 20.9% oxygen. If the oxygen level is lower, the flame will begin to generate excess carbon monoxide (CO), an odorless gas that can be inhaled, hindering the body's ability to absorb oxygen. Each year, CO poisoning results in fatalities in the USA. It is essential to be increasingly vigilant about this issue as we enhance the insulation of our homes and utilize appliances that rely on indoor air for combustion.

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

It is evident that even a small amount of CO can become problematic. When considering this in terms of parts per million, it is quite striking. Let us examine the required air volume. The federal regulations stipulate that there should be 50 cubic feet of space for every 1,000 BTUs of input from your heating appliance. Local or state codes may have stricter requirements. By determining the input of the appliance and dividing it by the total cubic feet of the mechanical room, the result must be at least 50 cubic feet per 1,000 BTUs. If the space does not meet this requirement, it is necessary to introduce combustion air from outside. Additionally, if there are hallways or other rooms connected to the mechanical room without a door, we can factor in these areas as well.
It is important to assess all potential sources of combustion air until reaching a doorway with a door. The regulations specify that even if the door is louvered, calculations should cease at the door. If the utility room has a door, even if it remains open, we stop at that point. The formula utilizes BTUs in MBH (thousand BTUs per hour), meaning the last three zeros are omitted. For instance, 130,000 BTUs would be represented as 130. Now, let us consider a mechanical room that includes a door.

If we did 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, let's 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.

It is evident that despite having multiple areas, we still require combustion air to comply with the national code. This calculation presumes that there is no door on the mechanical room. If a louvered door is present, can it provide sufficient air from these other areas? Given that we are assuming doors are located at both ends of the hallway, we would still fall short. A more effective solution would be to source combustion air from outside. When drawing air from outdoors, you only need to account for the additional air required. In direct connection with the outside, you need one square inch of free space for every 4,000 BTUs. This necessitates cutting a hole in the wall and installing a grille. Refer to the calculations below for further details. To determine the total appliance input, divide this figure by 4,000 to ascertain the required square inches of free area. In our example, a total input of 170,000 BTUs divided by 4,000 results in 42.5 square inches of free area. For simplicity, we will round this up to 50 square inches. While it may seem straightforward to determine the grille size, such as a 10" x 5" grille equating to 50 square inches, it is more complex. We must consider the free area of the grille face. Manufacturers provide charts detailing the free area of their grilles.

When comparing the free area of metal grilles to wooden grilles, as well as some decorative options, there can be significant variations. The grilles may lose between 15% to 70% of their 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.

Applications	BTU's Per Square Inch
Direct Communication to Outside (Grilles)	4000 BTUs per 1 sq. in.
Vertical Duct	4000 BTUs per 1 sq. in.
Horizontal Duct	2000 BTUs 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.