400 CFM per ton: when to use more or less

Almost every HVAC course teaches that a central air conditioner should move about 400 cubic feet of air per minute (CFM) for each ton of cooling. So a 3-ton system gets 1,200 CFM. The number is a useful starting point. But the right airflow for a particular house depends on the climate, the equipment and the job you need the system to do.

Why airflow matters

An air conditioner's indoor coil does two jobs at once:

1. Sensible cooling lowers the air temperature.
2. Latent cooling removes moisture. Water vapour condenses on the cold coil and drains away.

Airflow decides how the coil's capacity is split between the two. Air that moves slowly across the coil spends more time in contact with it, and the coil runs colder. A colder coil condenses more water. Faster air keeps the coil warmer and puts more of the capacity into lowering the temperature.

The share that goes to temperature is the sensible heat ratio (SHR). An SHR of 0.75 means 75% of the capacity lowers the temperature and 25% removes moisture.

What the numbers look like

In an ACCA blog article, Matt Akins worked through a 2.5-ton example. As airflow rose, the SHR went from:

• 0.52 at 350 CFM per ton
• 0.58 at 400 CFM per ton
• 0.61 at 450 CFM per ton

So the same equipment removed noticeably more moisture at lower airflow, and lowered the temperature more at higher airflow. Those exact values depend on the equipment and the conditions, but the direction is consistent.

Adjusting for climate

Trade guidance groups the common targets like this:

• Humid climates: about 300–350 CFM per ton. Lower airflow improves moisture removal, which is often the real comfort problem in places like the U.S. Southeast.
• Typical conditions: about 400 CFM per ton. The nominal design value.
• Dry climates: about 450 CFM per ton, sometimes up to 550. With little moisture to remove, the capacity is better spent on lowering the temperature.

These are guides, not hard rules. The equipment manufacturer's airflow table always comes first. It gives the allowable range for that coil and blower, and the capacity at each setting.

When low airflow becomes a problem

Lower airflow helps with humidity only up to a point. When airflow drops too far, below roughly 300 CFM per ton, the coil can get cold enough to freeze. Ice blocks the coil, capacity collapses, and the system can be damaged.

Unintended low airflow is common. Usual causes include:

• a dirty filter or a filter that is too restrictive;
• a dirty indoor coil;
• undersized or crushed ducts, or too few returns;
• a blower speed set too low for the equipment.

If you measure low airflow, look for these before treating it as a humidity setting.

Working it out

Target airflow is the system size multiplied by the CFM per ton you are aiming for. For a 3-ton system:

• at 400 CFM per ton: 3 × 400 = 1,200 CFM;
• at 350 CFM per ton: 3 × 350 = 1,050 CFM;
• at 450 CFM per ton: 3 × 450 = 1,350 CFM.

To check an existing system, divide the measured airflow by the nominal tons. Measuring real airflow takes a proper method, such as a flow hood, a duct traverse, or static pressure read against the manufacturer's fan table. Our CFM per ton calculator does both directions and flags values that are risky.

Relating airflow to temperature drop

For sensible heat, the standard-air equation is:

Q = 1.08 × CFM × ΔT

Q is in BTU per hour and ΔT is the temperature drop across the coil in °F. The constant 1.08 comes from the density and specific heat of standard air multiplied by 60 minutes. At high altitude the air is thinner and the constant is smaller. The equation shows why airflow and temperature split move in opposite directions: for the same sensible capacity, less air means a bigger drop. The CFM calculator solves it for any of the three values, and the temperature split calculator helps interpret a measured drop.

Key points

• 400 CFM per ton is a nominal starting point, not a universal target.
• Lower airflow removes more moisture; higher airflow lowers the temperature more.
• Humid climates often use about 350 CFM per ton, dry climates about 450.
• Below roughly 300 CFM per ton, the coil can freeze.
• The manufacturer's airflow table always takes priority over any rule of thumb.

Sources

• 400 CFM per Ton, or Is It?, ACCA HVAC Blog (Matt Akins).
• Getting the Right Airflow for an Air Conditioning System, Green Building Advisor (Allison Bailes III, PhD).
• Regional Line Card — air-side formulas, Trane, for Q = 1.08 × CFM × ΔT.