NATE Air Conditioning and Heat Pumps Practice Exam

Use the collected service data to calculate the air flow volume for a residential electric furnace: Voltage = 240 V ac, Current draw = 40 A, Entering air temperature = 70°F, Leaving air temperature = 105°F.

1. 798 cfm

2. 867 cfm

3. 936 cfm

4. 1,100 cfm

The correct answer is: 867 cfm

To determine the airflow volume for the residential electric furnace, you need to first calculate the heat output of the furnace in BTUs. This can be done using the formula: \[ \text{BTUs} = \text{Voltage} \times \text{Current} \times 1.732 \] In this case, the voltage is 240 V and the current draw is 40 A: \[ \text{BTUs} = 240 \, \text{V} \times 40 \, \text{A} \times 1.732 \approx 16,638 \, \text{BTUs} \] Next, with the entering air temperature and the leaving air temperature, we can find the temperature rise: \[ \text{Temperature Rise} = \text{Leaving Temperature} - \text{Entering Temperature} = 105°F - 70°F = 35°F \] After obtaining the BTU output and the temperature rise, the airflow volume can be calculated using the formula: \[ \text{CFM} = \frac{\text{BTUs}}{1.08 \times \text{Temperature Rise}} \] Plugging in the values: \[ \text{CFM}