Some rooms present a real challenge when it comes to determining how much air to deliver. The problem is that heating or cooling load airflow requirements are sometimes very different from ventilation airflow needs. When you find yourself wrestling with this scenario, use the air changes per hour method as a check to be sure you’re doing the right thing.
What this calculation really does, is determine how many times a room would fill up with the air from the registers in one hour (compare it to the number of times a hose would fill the room with water in an hour). Then you compare that number to our table of industry air changes. If it’s in the range, you can proceed to design, or balance, the airflow and have an additional assurance that you are doing the right thing. If it’s way out of range, you’d better take another look.
Here’s the formula:
In plain English, we’re changing CFM into Cubic Feet per Hour (CFH). Then we calculate the volume of the room by multiplying the room height times the width times the length. Then we simply divide the CFH by the volume of the room.
Here’s how a full formula works:
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 it’s a lunch or break room that requires seven to eight air changes per hour, you’re right on target. If it’s a bar that needs 15-20 air changes per hour, it’s time to reconsider.
Let’s 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. Let’s say it’s 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.
Here’s 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.
To download a pdf of the Air Changes per Hour chart, click here.
Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a free sample 30-min. in-house training lesson you can use in your company, contact Doc at email@example.com or call him at 800/633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.