Expressions of Ventilation

Oct 11, 2010

Several expressions can be used when quantifying a building’s ventilation. You may talk to an on-site building engineer that expresses it in percent outdoor air. Or, you may talk to the mechanical engineer that designed the system, and he may express it as cubic feet per minute (cfm) per person. Heaven forbid you bump into a building scientist who likes to express ventilation in terms of air changes per hour!

In this post, I would like to describe these three common expressions for ventilation.

1. Percent Outdoor Air

In most heating, ventilating and air conditioning (HVAC) systems, the supply air is a combination of recirculated and outdoor air.  When the ventilation is brought in mechanically through an HVAC system, we can divide the outdoor air amount by the total supply air (which includes the recirculated air) to get our number.  For example, a commercial system may be supplying 10,000 cfm, with 2,000 cfm representing outdoor air and 8,000 cfm from recirculated air.  The percent outdoor air is 20% in this example, calculated as follows: (2,000/10,000)x100.  A minimum setting commonly found in commercial buildings is 20% outdoor air (O.A.).

2. Ventilation per Occupant

The majority of codes and standards use a ventilation rate per occupant such as cfm/person.  This expression is greatly affected by the building’s occupancy.  An individual office may be getting 300 cfm of total supply air and 60 cfm of outdoor air (assuming 20% O.A.).  If one person works in the office, the ventilation rate is 60 cfm/person.  However, if two people share the office, the ventilation rate is 30 cfm/person.  A common minimum ventilation rate used is 15 cfm/person.

The most current ventilation standard from the American Society of Heating, Ventilating and Air-Conditioning Engineers (ASHRAE Standard 62.1-2010) adds a ventilation rate per square foot to the per person number.

3. Air Changes per Hour

An air change per hour (ACH) is the theoretical amount of times the air is replaced in a space over a 60 minute time period.  This expression is often used in naturally ventilated buildings or by building scientists calculating air infiltration.  This expression will measure air changed out by mechanical ventilation, natural ventilation and even unwanted air infiltration.   A tracer gas should be used to accurately measure air changes per hour. A minimum ventilation target for homes often quoted is 0.35 ACH.


If you are called out to perform an indoor air quality assessment, you should know the building’s ventilation code requirement.  The code may be expressed with any of the three terms above.  Calculate the building’s minimum ventilation and check to see if it meets code. It is also recommended to compare ventilation rates to those is ANSI/ASHRAE 62.1 even though the rates may not be required by code.

And remember, ventilation is just one strategy to improve the indoor air quality of buildings.