Calculating Combustion Air

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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 replacing our windows and doors, adding weatherstripping and caulking, and installing insulation. When I first started in this field, there were no double-pane windows in basements, let alone steel-insulated doors leading outside. Instead, there were just old single-pane glass windows and wooden doors, often with gaps where light could come through the frames.
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 significant, they ultimately reduce the combustion air that's crucial for a safe burning process. If your heating system pulls air from inside your home for combustion, monitor the amount of air needed to ensure safety.

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 inside the home is used for combustion, it reduces the home's internal pressure. This pressure reduction draws cold air through gaps and openings, lowering the home's temperature and requiring heating to recover. Supplying outdoor air to the appliance can prevent this issue. Insufficient combustion air may cause the appliance to draw air down the chimney, resulting 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 occurs when air escapes from the home. Furthermore, the building's height can enhance thermal circulation, leading to greater 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 remain increasingly vigilant about this issue as we improve our homes' insulation and use appliances that rely on indoor air for combustion.

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

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. Federal regulations require 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 appliance's input 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 point where the door meets the wall. If the utility room has a door, even if it's open, we stop there. 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 were no doors 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 assumes there is no door in 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 the building. 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 with that of wooden grilles, as well as decorative options, significant differences can be observed. 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 BTUs input
*Vertical duct - 1 sq in free area per 4000 BTUs input
*Horizontal duct - 1 sq in per 2000 BTU input
Indoor air grille through interior wall - 1 sq inch per 1000 BTUs
*Cross section of duct must equal opening free area
If you want to use a 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 BTUs (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 the exterior of the home

Some states have increased the 50 cubic feet per 100 BTUs per 1000 BTUs. This is a safer number because homes are more airtight today, and the air required for combustion will enter more slowly. You can change the cu. Ft. per thousand BTUs 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 BTUs (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 the 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.

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