Air changes per hour is a measure of how many times the air within a defined space (normally a room or house) is replaced. Air changes in a confined space are important for a variety of reasons, mainly though, we need fresh air to live. Without sufficient fresh air exchange, moisture is trapped in a room/home/building, molds can feed, and other allergens and excessive dangerous gases (e.g. Carbon monoxide, Carbon Dioxide, urea formaldehyde), can remain in the home. "Stale" air is unhealthy and, since humans and pets add to it by breathing, sweating, washing, showering and drying, we need to ventilate the home, increasing the number of times the air 'exchanges' in the home with outside fresh air. Number of 'air changes per hour' were less of a problem before 'air sealing' came into play, because construction practices and products were not geared to energy efficiency (in the USA, with lower energy costs). With a new focus on energy efficiency, reducing carbon footprints, and reducing dependence on fossil fuels, consumers try to seal their homes from air transfer in and out of their homes in winter and summer. Double edged-sword. The importance of fresh air intake cannot be understated.
An air change does not represent a complete change of all air in the enclosure or structure unless it can be considered plug flow. The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH.  In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same.
· 1 Measure the length, width and height of your room.
· 2 Multiply the length times the width times the height to calculate the volume of the room. For example, if your room is 10 feet wide, 15 feet long and eight feet high, the volume would be 1,200 cubic feet.
· 3 Check your owner's manual to determine the capacity of the fan in cubic feet per minute.
· 4 Multiply the capacity of your fan in cubic feet per minute by 60 to convert it to cubic feet per hour. For example, if your fan were capable of 200 cubic feet per minute, you would multiply it by 60 to get 12,000 cubic feet per hour.
· 5 Divide the result from Step 4 by the volume of the room from Step 2 to calculate the number of air changes per hour. Finishing the example, divide 12,000 by 1,200 to find that you would get 10 air changes per hour.
Now, compare the air changes in the room to the required air changes for the type of room it is on the Air Changes per Hour Download accompanying this article. If itís a lunch or break room that requires seven to eight air changes per hour, youíre right on target. If itís a bar that needs 15-20 air changes per hour, itís time to reconsider.
Letís look at this engineering formula differently. When airflow is unknown and you need to calculate the required CFM for a room, first you look at the Air Changes per Hour Chart and identify the required air changes needed for the use of the room. Letís say itís a conference room requiring 10 air changes per hour. Next calculate the volume of the room (L x W x H). Then divide by the required air changes per hour to get required CFM.
Hereís an example of how to work the formula:
When designing or balancing a system requiring additional airflow for ventilation purposes, remember this room will normally demand constant fan operation when occupied. This may present a problem for other rooms on the same zone, so take that into consideration.
Also many of these rooms require a significant amount of fresh or outdoor air. The BTU content of this air has to be included in the heat gain or heat loss of the building when determining the size of the heating or cooling equipment
Practice these calculations several times in the shop or office. Then do the calculations in the field several times over the next week to check airflow in rooms with uncommon ventilation requirements. Study the Air Changes per Hour Tables to become familiar with the rooms that need more ventilation than heating or cooling load requires.
Read more: How to Calculate Air Changes Per Hour | eHow.com