DEV Community

Cover image for Quick Tip: CFM Per Ton — The Fan Speed Setting That Changes Humidity by 13%
Evgenii Konkin
Evgenii Konkin

Posted on

Quick Tip: CFM Per Ton — The Fan Speed Setting That Changes Humidity by 13%

#hvac #engineering #quicktip #energy

The standard residential airflow is 400 CFM per ton of cooling. But that number is a design compromise — and adjusting it up or down changes humidity control more than most people expect.

The tradeoff 

Lower airflow (350 CFM/ton):
  Coil surface gets colder
  Air spends more time on the coil
  More moisture condenses and drains
  Sensible capacity drops ~8%
  → Best for humid climates

Standard airflow (400 CFM/ton):
  Balanced sensible + latent
  Equipment runs at rated capacity
  → Default for most applications

Higher airflow (450 CFM/ton):
  Coil surface stays warmer
  Air passes through faster
  Less moisture removed
  Sensible capacity increases 5-8%
  → Best for dry climates only
Enter fullscreen mode Exit fullscreen mode

The numbers 

3-ton system (36,000 BTU/hr rated):

350 CFM/ton = 1,050 CFM:
  Total capacity: 33,200 BTU/hr (-8%)
  SHR: 0.72 (28% of cooling = moisture removal)
  Room humidity: ~45%

400 CFM/ton = 1,200 CFM:
  Total capacity: 36,000 BTU/hr (rated)
  SHR: 0.78 (22% = moisture removal)
  Room humidity: ~50%

450 CFM/ton = 1,350 CFM:
  Total capacity: 38,800 BTU/hr (+8%)
  SHR: 0.85 (15% = moisture removal)
  Room humidity: ~58%
Enter fullscreen mode Exit fullscreen mode

Same equipment. Same tonnage. 13 percentage points of humidity difference from fan speed alone.

Why lower airflow dehumidifies better 

At 350 CFM/ton:
  Air velocity across coil: lower
  Contact time: longer
  Coil surface temp: colder (closer to refrigerant temp)
  More air reaches dew point on the coil surface
  More moisture condenses per CFM

At 450 CFM/ton:
  Air velocity: higher
  Contact time: shorter
  Coil surface temp: warmer (air warms it faster)
  Less air reaches dew point
  Less moisture per CFM — but more total air moved
Enter fullscreen mode Exit fullscreen mode

When to adjust 

Climate              Recommendation    Why
──────────────────────────────────────────────────────
Houston, Miami       350 CFM/ton       Humidity is the problem
Atlanta, Charlotte   375 CFM/ton       Moderate humidity
Most US cities       400 CFM/ton       Standard
Denver, Phoenix      425-450 CFM/ton   Maximize sensible capacity
                                       humidity is never an issue
Enter fullscreen mode Exit fullscreen mode

How to change it 

Residential systems:
  Most air handlers have multiple fan speed taps
  (low, med-low, med, med-high, high)
  Installer selects the tap during commissioning
  Each tap changes CFM by ~50-75 per step

Commercial systems:
  VFD on the supply fan
  BAS controls fan speed
  Can adjust dynamically by season or humidity
Enter fullscreen mode Exit fullscreen mode

The warning 

Below 300 CFM/ton:
  Coil surface drops below 32F
  → Ice forms on the coil
  → Airflow drops further
  → More ice → complete freeze-up

Above 500 CFM/ton:
  Coil cannot cool the air enough
  → Supply air temp too high
  → Delta-T drops below useful range
  → System runs but doesn't cool effectively
Enter fullscreen mode Exit fullscreen mode

350-450 CFM/ton is the operating range. Below or above causes equipment damage or failure.

Calculate the CFM for your system with the CFM calculator on CalcEngineer — then decide where in the 350-450 range your climate needs to be.

Top comments (0)