Comfort & Airflow
Airflow measurement (CFM), duct sizing principles, humidity control, temperature differential targets, and the relationship between airflow and system performance.
- Calculate required CFM based on system tonnage and apply the 400 CFM per ton rule
- Explain how duct sizing affects static pressure, velocity, and system performance
- Describe the relationship between humidity, dew point, and comfort
- Measure and interpret supply-to-return temperature differential for cooling and heating
Lesson 1
CFM - The Airflow Foundation of HVAC Performance
Why Airflow Matters More Than You Think
Airflow is the single most overlooked factor in HVAC system performance. A system with perfect refrigerant charge but incorrect airflow will still deliver poor comfort, high energy bills, and premature equipment failure. The CHP-5 exam tests your understanding of airflow requirements and how to verify them.
The standard rule for residential cooling is 400 CFM per ton of cooling capacity. A 3-ton system needs 1,200 CFM of air moving across the evaporator coil. This is not an arbitrary number - it is the airflow rate at which the evaporator coil produces the correct temperature drop and moisture removal.
What Happens When Airflow Is Wrong
Low airflow (less than 350 CFM per ton) causes the evaporator coil to get too cold. The coil temperature drops below 32 degrees F and ice begins to form on the fins. Ice further blocks airflow, making the problem worse in a feedback loop. Low airflow also causes low suction pressure, low superheat, and can send liquid refrigerant back to the compressor.
High airflow (more than 450 CFM per ton) prevents the evaporator from removing enough moisture. The air passes across the coil too quickly to cool below its dew point, so humidity stays high even though the temperature drops. Occupants feel clammy. High airflow also causes high suction pressure and high superheat.
Low Airflow (Below 350 CFM/ton)
Evaporator temp: Drops below 32 F - ice forms
Suction pressure: Lower than normal
Superheat: Drops - risk of liquid slugging
Temperature split: Higher than 22 F
Causes: Dirty filter, collapsed duct, closed registers, undersized duct
High Airflow (Above 450 CFM/ton)
Evaporator temp: Stays above dew point - poor dehumidification
Suction pressure: Higher than normal
Superheat: Rises above normal
Temperature split: Lower than 18 F
Causes: Oversized ductwork, blower speed set too high
Measuring Airflow
There are several methods for measuring airflow in the field:
- Temperature rise / drop method: Measure supply and return air temperatures, then use the formula: CFM = (BTU/h output) / (1.08 x temperature difference). This works for both heating and cooling.
- Flow hood (balometer): Place over a register to directly measure CFM at each outlet. Add up all supply register readings for total system CFM.
- Static pressure and fan curve: Measure total external static pressure, then reference the blower manufacturer's fan performance table to find CFM at that static pressure and speed setting.
The standard is 400 CFM per ton of cooling. Low airflow causes coil icing, low suction pressure, and liquid slugging risk. High airflow causes poor dehumidification and high superheat. Normal cooling temperature split across the evaporator is 18-22 degrees F.