Airflow Measurement
Airflow measurement techniques including the temperature rise method, static pressure measurement, flow hood use, Tru-Flow grid, and the relationship between airflow and system performance.
- Calculate airflow using the temperature rise/drop method
- Measure and interpret total external static pressure
- Use flow measurement devices to verify room-by-room airflow delivery
- Diagnose common airflow problems and their causes
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The Temperature Rise/Drop Method
Why Airflow Is the Most Important Measurement
Airflow is the single most critical factor in AC and heat pump system performance. Without correct airflow, refrigerant charge readings are meaningless, capacity is reduced, humidity control fails, and energy efficiency drops. BPI considers airflow verification the first step in any HVAC performance evaluation.
The target airflow for cooling is 400 CFM per ton (+/- 10%, giving an acceptable range of 350-450 CFM per ton). For a 3-ton system, the target is 1,200 CFM (acceptable range: 1,050-1,350 CFM).
The Sensible Heat Equation
The temperature rise/drop method calculates airflow from the sensible heat equation:
CFM = Sensible BTU/h / (1.08 x Delta-T)
Where:
- Sensible BTU/h = the equipment's sensible cooling or heating output at current conditions
- 1.08 = the sensible heat constant (derived from 60 min/hr x 0.075 lb/ft3 air density x 0.24 BTU/lb-F specific heat)
- Delta-T = temperature difference between return air and supply air (degrees F)
Cooling Mode Calculation
In cooling mode, measure the return and supply air temperatures with the system running at steady state (15+ minutes):
Example:
- 3-ton system, manufacturer's sensible capacity at current conditions: 25,000 BTU/h
- Return air temperature: 76 F
- Supply air temperature: 57 F
- Delta-T: 76 - 57 = 19 F
- CFM = 25,000 / (1.08 x 19) = 25,000 / 20.52 = 1,218 CFM
- Per ton: 1,218 / 3 = 406 CFM/ton (within the acceptable range)
Heating Mode Calculation (Temperature Rise)
For heating systems (furnaces), the temperature rise method is the standard airflow measurement technique. Measure return and supply temperatures with the burner firing at full capacity:
Example:
- 80,000 BTU/h input furnace at 80% AFUE = 64,000 BTU/h output
- Return air: 68 F
- Supply air: 118 F
- Delta-T (temperature rise): 118 - 68 = 50 F
- CFM = 64,000 / (1.08 x 50) = 64,000 / 54 = 1,185 CFM
Compare this to the furnace nameplate temperature rise range (e.g., 35-65 F rise). If the measured rise exceeds the maximum, airflow is too low. If below the minimum, airflow is too high.
Temperature Rise Range Is on the Nameplate
Every furnace has a temperature rise range on its nameplate (e.g., "35-65 F Rise"). If the measured rise exceeds the maximum, airflow is too low and the heat exchanger may overheat, triggering the high-limit switch. If below the minimum, airflow is too high and comfort suffers. This is the quickest field check for furnace airflow.
Limitations of the Temperature Method
The temperature rise/drop method has several limitations:
- Requires knowing the equipment's actual sensible output at current conditions (not just the nameplate rating)
- In cooling mode, the sensible capacity varies with outdoor temperature and indoor wet-bulb
- Temperature measurements must be accurate to +/- 1 F for reasonable accuracy
- The method measures total system airflow, not individual room delivery
The temperature rise/drop method calculates airflow from the sensible heat equation: CFM = Sensible BTU/h / (1.08 x Delta-T). For cooling, a normal delta-T is 14-22 F at 400 CFM/ton. For heating, the temperature rise must fall within the nameplate range. This method is quick but requires knowing the equipment's actual sensible output and having accurate temperature measurements.