In HVAC systems, we always have to think about airflow (measured in CFMs). One of the most common CFM-related questions we get is this:
“How many CFM per ton of cooling?”
In most situations, you would need a 400 CFM per ton of cooling. This is the standard HVAC rule of thumb, but it’s a bit antiquated and we will explain when you will need more than 400 CFM per ton (up to 600 CFM per ton) or less than 400 CFM per ton (down to 325 CFM per ton). This has mostly to do with relative humidity levels.
Here is the general rule of thumb of how many CFM you need for certain tonnage AC units:
How Many CFM For 1 – 6 Ton Units? (Chart)
|Unit Tonnage:||CFMs Required:|
|1 Ton Unit||400 CFM|
|1.5 Ton Unit||600 CFM|
|2 Ton Unit||800 CFM|
|2.5 Ton Unit||1,000 CFM|
|3 Ton Unit||1,200 CFM|
|3.5 Ton Unit||1,400 CFM|
|4 Ton Unit||1,600 CFM|
|4.5 Ton Unit||1,800 CFM|
|5 Ton Unit||2,000 CFM|
|5.5 Ton Unit||2,200 CFM|
|6 Ton Unit||2,400 CFM|
This chart is quite simple to read. Example: A 5-ton unit requires 2,000 CFM airflow. A 3-ton unit requires 1,200 CFM airflow.
Now, it is important to understand when the 400 CFM per ton rule of thumb should not be applied. We will look at when the CFMs per BTU of cooling should be higher and lower:
When CFM Per Ton Should Be Higher Than 400 CFM
In general, you would use more than 400 CFM when you have to remove more-than-average sensible heat. In most cases, this means that you are trying to cool space will low humidity levels (below 50% relative humidity).
At lower humidity levels, the latent heat is higher.
At below 50% relative humidity, you may use 500 CFM per ton.
At even lower humidity levels (below 35%), you could also use up to 600 CFM per ton.
This higher airflow will provide a sufficient airflow needed to homogeneously cool a space. Since the air conditioner doesn’t need to remove access moisture from the air, it can cool much better and more efficiently.
However, if you have a high above 60% humidity levels, the airflow should be below 400 CFM per ton:
When CFM Per Ton Should Be Lower Than 400 CFM
As the humidity levels increase, the sensible heat in the environment decreases. You will have to use a lower airflow output in order to increase dehumidification rates.
If you use a 400 CFM per ton at above 60% relative humidity levels, the cooling coil is not sufficiently cold to condense all that moisture from the incoming air.
If you lower the CFMs per ton of cooling, the cold coils will be colder, more water will condense on the coils, and you will remove the moisture in the air quicker.
That’s why at above 60% relative humidity levels you are recommended to use 350 CFM per ton. If the humidity levels are excessive (80% or more), using a 325 CFM per ton rule of thumb comes into play as well.
With these measures, you can determine how many CFM per ton of cooling you need more precisely than with the simplified 400 CFM per ton rule of thumb.