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Outside Air

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If you are going to introduce outside air into a conditioned space, it is going to have a cost (in Btuh) associated with that amount of air. But before we get into the math that will produce that Btuh value, let’s define some of the terms.


Infiltration is the uncontrolled outdoor air leakage into a conditioned space through cracks and openings in the exposed surfaces or leakage through an attic ceiling, a leaky crawl space or a leaky basement, etc.

The key words here are uncontrolled and leakage. Infiltration is a function of the tightness of the building or how well it was built.

Infiltration can be caused by a collection of pressure drivers such as: wind, the stack effect, vents, chimneys, exhaust fans and duct leakage.

Duct leakage is something over which our industry has control. Properly sealing ducts can have a significant effect on system operating costs, as well as the comfort level within the home.

Ideally, we shoot for a neutral pressure within a residence. This can be accomplished by properly sealing all ducts. This, of course, assumes that you can get to all the ducts to seal them. This can be a frustrating undertaking in an existing residence. However, in new construction,access should not be an issue.

If your duct system is located in an unconditioned space, and if your return duct system is tight but you have a leaky supply duct system, then you will create a negative pressure in the home.

Conversely, if you have a tight supply side and a leaky return side, then you will induce a positive pressure into the residence.


Ventilation is the controlled (engineered)movement of air from the outdoors, through the conditioned space, to the outdoors again. The key words here are controlled and engineered. If, for instance,you were to install a 200 cfm exhaust fan that operates 24/7.

The flow of air may exit through cracks and penetrations in the thermal envelope, relief openings, ancillary exhaust systems, dedicated exhaust systems, heat recovery equipment, chimney or vents.


There are three common ways to calculate the amount of outside air that is entering a residential structure:the Simplified Method, the Detailed Method,and the Blower Door Method.

The Detailed Method is also known as the Component Leakage Area Method. This methodology requires that each building component (i.e., walls, ceilings, floors, door, windows,etc.) be evaluated separately and individually. The detailed method should be used where the building components have different construction characteristics such as an old, leaky building that has new, tight windows installed. The Detailed Method is a time-consuming process.

The Blower Door Method can only be used on existing buildings.Therefore, the most commonly applied method is the Simplified Method(or Table Method), especially in new construction.

The Math

To determine the amount of outside air that will enter a building (in cfm), use the following equation:

cfmInfl= AC/HR x ft3 of above grade volume x 0.0167

The air change per hour (AC/HR)rate is taken from Table 5A in Manual J.

To determine the additional heat loss associated with bringing that amount of outside air into a conditioned space, use this equation:

Btuhsensible loss = cfmInfl x 1.1x ΔT

Example: A residence 60’ x 40’ with 8’ ceilings and average construction with one average fireplace located in Chicago.

.32 AC/HR x (60 x 40 x 8 = 19,200 ft3) x 0.0167 = 103 cfm

103 cfmInfl+ 20 cfmFireplace = 123 cfmTotal

123 cfm x 1.1 x 66ΔT = 8930Btuh

The definitions for tight, average, loose, etc. are given at the bottom of Table 5B in Manual J. Be sure to read them as it is important for you to understand what these terms mean when using a table that employs them.

Notice that we just calculated almost 9,000 Btuh for a 2400 square foot, average residence in Chicago. Furnace and boiler sizing is typically done in 20,000 Btuh increments. You can’t ignore infiltration and ventilation loads and properly size heating and cooling equipment.

Jack Rise
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Posted In: Technical Tips

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