Air Distribution & Duct Systems
Airflow measurement, CFM calculations, static pressure, duct design, filter selection, and blower performance.
- Apply the 400 CFM per ton rule of thumb for cooling airflow
- Measure total external static pressure and interpret results
- Calculate CFM using the temperature rise method on gas furnaces
- Explain the effects of duct leakage on system performance
- Select appropriate filters and understand their impact on airflow
Leçon 1
CFM, Airflow Rules of Thumb & Calculations
What Is CFM?
CFM stands for cubic feet per minute - it is the standard unit for measuring airflow volume in HVAC systems. Proper airflow is essential for both cooling and heating performance. Too little airflow causes coil icing, poor dehumidification, and high temperature rise. Too much airflow reduces latent heat removal and causes comfort complaints.
Cooling Airflow
The rule of thumb for cooling airflow is approximately 400 CFM per ton of cooling capacity. This can be adjusted based on climate and latent load:
- Dry climates - may increase to 450 CFM/ton (more sensible cooling)
- Humid climates - may decrease to 350 CFM/ton (more latent removal)
For a 3-ton system: 3 x 400 = 1,200 CFM
Gas Heating Airflow
For gas furnaces, airflow is approximately 130-150 CFM per 10,000 BTU of output. The manufacturer specifies a temperature rise range on the nameplate (typically 30-60 degrees F). If the measured temperature rise is outside this range, airflow must be adjusted.
Calculating CFM from Temperature Rise
The CFM formula using furnace output and measured temperature rise:
CFM = Output BTU/hr / (1.08 x delta-T)
Example Calculation
A furnace with 80,000 BTU/hr output and 45 degrees F temperature rise: CFM = 80,000 / (1.08 x 45) = 80,000 / 48.6 = 1,646 CFM. If the nameplate says 35-65 degrees F rise and you measure 45 degrees F, airflow is within range.
Why Airflow Must Be Verified After Changeout
When replacing equipment, the new unit may have different airflow requirements than the old one. Verifying airflow ensures:
- Design CFM matches the new equipment's capacity
- Proper latent heat removal for humidity control
- Furnace temperature rise stays within the nameplate range
- System operates at rated efficiency
The cooling airflow rule of thumb is approximately 400 CFM per ton. For gas furnaces, use the formula CFM = Output BTU/hr / (1.08 x delta-T) and verify the temperature rise is within the nameplate range (typically 30-60 degrees F). Always verify airflow after equipment changeout.