Airflow Genius- How Many CFM Per Ton?

Is 400 CFM per ton a good Rule. We answer how many CFM per Ton here!

To many industry insiders the answer is simple. How many cfm per ton? Easy! 400. Done deal. No need for further explanation. However there are a lot more factors that are involved in how much airflow you need.

You may have heard a rule of thumb that says "That system needs 400 CFM per ton!" But do you know that this rule is not always true? 

The ideal CFM per ton for a system depends on many factors. For example, 

• How humid is the air? 

• How hot or cold is the dry-bulb temperature? 

• How wet or dry is the wet-bulb temperature? 

• How dense is the air? 

• How fast is the air moving? 

• How high is the elevation? 

Understand the physics

A ton of cooling is a unit of measurement that describes how much heat an HVAC system can remove from a space in one hour. One ton of cooling is equivalent to 12,000 BTU's (British thermal units) of cooling capacity. The term comes from the historical use of ice to cool buildings before the invention of refrigeration. A ton of ice would melt in 24 hours and provide 12,000 BTU's of cooling per hour.

The physical transfer of energy that takes place depends on factors beyond just the airflow. Temperature of the coil, transfer coefficient of the coil material, coil design, return air temperature, and many more have an impact on the performance.

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Where do I find coil performance data?

Usually you can go right to the manufactures website and track down this information. If it's for a simple residential system it'll likely be in the IOM. If it's for a commercial piece of equipment this may be something you will have to ask the OEM(original equipment manufacturer) about. The system could be semi-custom where the rating is specific to that unit rather than a class of units. Sometimes I have to dig a little to find the data I need, but it's worth it to make sure I get the most accurate information. After all, I don't want to end up with a ton of trouble instead of a ton of cooling. Ha, see what I did there? 

So should I use the Rule?

Now, you might be wondering how to figure out the right CFM per ton for your system. Some people use a rule of thumb, like 400 CFM per ton for high humidity areas, or 450 CFM per ton for low humidity areas. This can be okay in a pinch, but it's not very accurate or reliable. The best way to determine the optimal CFM per ton is to look at the coil performance data and see how different airflows affect the sensible and latent capacity of the system. Sensible capacity is the amount of heat that the system can remove from the air without changing its moisture level. Latent capacity is the amount of moisture that the system can remove from the air by lowering its temperature and causing condensation. You want to find the right balance between these two capacities, depending on the conditions of your space and your climate. By using the coil performance data, you can make sure that your system operates as efficiently and effectively as possible, and that you get the comfort and quality that you and your clients deserve.