Understanding the HVAC Temp Rise Formula for Measuring Airflow CFM

As an HVAC technician, one of the essential tasks is to measure the airflow CFM (Cubic Feet per Minute) of an air handling system. This is crucial for ensuring the system operates efficiently and effectively. In this article, we will delve into the Temp Rise Formula used to measure airflow CFM in air handlers with electric resistance strip heating.

The Temp Rise Formula

The Temp Rise Formula is a fundamental tool for HVAC technicians when it comes to measuring airflow. It involves taking measurements of voltage, amperage on the electric strip heater, and the temperature rise from the return duct to the supply duct. These measurements are then used to calculate the airflow CFM.

Steps to Measure Airflow CFM

  1. Preparation: Before taking measurements, it’s important to ensure that the blower motor compartment is covered to isolate the electric strip heater. This can be achieved using cardboard or metal, securely fastened in place.
  2. Temperature Measurement: B-type temperature sensors are used to measure the temperature differentials in the return and supply ducts. These sensors are inserted into the ducts using quarter-inch sheet metal screws.
  3. Voltage and Amperage Measurement: The voltage and amperage on the electric strip heater are measured using appropriate tools and equipment.
  4. Calculation: The Temp Rise Formula involves multiplying the volts, amps, and a constant factor, and then dividing the result by the temperature differential. This calculation yields the airflow CFM.

Interpreting the Results

Once the Temp Rise Formula is applied, the calculated CFM provides valuable insights into the system’s performance. It’s important to note that for every ton of capacity, around 400 CFM of airflow is recommended for a cooling system. Deviations from this value can indicate potential issues such as duct restrictions or inadequate blower motor speed.

Troubleshooting and Considerations

In the process of measuring airflow CFM, it’s crucial to troubleshoot any anomalies. For instance, if the temperature differential continues to rise, it may indicate issues with blower motor speed or airflow restrictions. Additionally, using the correct tools and ensuring accurate measurements are essential for reliable results.


The Temp Rise Formula is a powerful tool for HVAC technicians to accurately measure airflow CFM in air handling systems with electric resistance strip heating. By following the proper procedures and considering various factors, technicians can ensure that the system operates optimally, delivering efficient and effective performance.

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