 Airflow Calculation

by Ron Walker It seems like airflow is the lost component of system evaluation and troubleshooting.

To accurately measure the performance of a system or use the charging data accurately, you must measure the amount of air passing across the evaporator coil. The airflow is measured in cubic feet per minute (CFM). 350 to 400 CFM per ton of cooling is required for proper air conditioning  system operation. For example, if you are checking a 3 ton system, the air flow required is between 1050 and 1200 CFM.

Too little airflow, and you will be unable to charge the system properly. Low air flow may ice up the coil and allows liquid refrigerant to flood the compressor. Too much airflow and the system and high humidity levels may be a problem in the home. Both of these conditions drastically affect system performance and may damage the compressor.

Measuring a system’s airflow using the temperature rise method does not require any expensive airflow measurement tools, just a thermometer, voltmeter, clamp-on ammeter, and a calculator. This method of airflow measurement can be used with either a gas fired furnace or an AC/heat pump system with electric strip heat. In this procedure, a mathematical formula and the temperature difference between the supply air and the return air (Delta-T) are used to establish the CFM volume of the system.

Both the gas fired furnace and electric strip heat systems are required to be running in the heating mode, but the methods used are different for each.

Temperature Rise Method – Gas Heat

Tools:

• Thermometer
• Calculator

Steps:

1. Disconnect power.
2. Set the furnace to run on HIGH speed in the heating mode. High speed is what the system usually runs in, when in the cooling mode.
3. Reconnect power.
4. Set the thermostat to call for heat. If it is a two-stage system, it must be firing on the second stage.
5. Check the furnace data plate and determine the BTU output of the furnace and record the value.
6. Allow time for the system temperatures to stabilize.
7. Measure and record the return air temperature at the filter rack.
8. *Locate a duct approximately 36″ away from the supply air plenum. Measure and record the supply air temperature at this point.
9. Disconnect power and reset the blower speed back to its original setting.
10. Determine the Delta-T by subtracting the return air temperature from the supply air temperature.
11. **Multiply the Delta-T value by 1.08. Record this value.
12. Divide the BTU rating of the gas fired furnace by the value determined in step 7. The answer is your airflow in CFM.

Example:

BTUH: 100,000
Supply Air Temperature: 120F
Return Air Temperature: 70F

• 102F – 70F = 50F (Delta-T)
• 100,000 / 1.08 x 50
• 100,000 / 54 = 1852 CFM

Temperature Rise Method – Electric Heat (air conditioning and heat pumps)

Tools:

• Thermometer
• Calculator
• Voltmeter
• Ammeter

Steps:

1. Set the thermostat to call for heat. Wait for all stages of heat to energize. If this is a heat pump, make sure ONLY the electric heat is operating for this test.
2. Using your voltmeter, measure and record the supply voltage to the air handler.
3. Using your ammeter, measure and record the total amperage drawn by the air handler.
4. Multiply the recorded supply voltage by the amp draw. This gives you the power in Watts.
5. Multiply the answer derived in step 4 by 3.414. This converts Watts to the BTU output of the electric heat.
6. Allow time for the system temperatures to stabilize.
7. Measure and record the return air temperature at the filter rack.
8. *Locate a duct approximately 36″ away from the supply air plenum. Measure and record the supply air temperature at this point.
9. Determine the Delta-T by subtracting the return air temperature from the supply air temperature.
10. **Multiply the Delta-T by 1.08. Record this value.
11. Divide you BTU output determined in step 5 by the value obtained in step 10 to determine the actual CFM

Example:

Air Handler Supply Voltage: 235 Volts
Air Handler Amp Draw: 75 Amps
Supply Air Temp: 110F
Return Air Temp: 74F

• 235 V x 75 A x 3.414 = 60171 BTUH output
• 110F -74F = 36F Detla-T
• 60171 / 1.08 x 36
• 60171 / 38.88 = 1548 CFM

It seems, to most technicians, that measuring airflow is difficult and time consuming. Additionally, airflow measurement is not a troubleshooting technique usually taught to service technicians. Knowing to calculate the airflow of a system in CFM is a useful tool when troubleshooting a problem air conditioning system.

*The key is to be out of the ‘line of sight’ of the heat exchanger or electric strip heat. Radiant heat causes erroneous temperature readings, throwing off the airflow calculation.

**The constant 1.08 is derived from the product of the  specific heat of 70F air (.24 btu) times its density at sea level (.075 lbs cubic foot) times the number of minutes per hour (60 min\hour).