Section 126.96.36.199 of ASHRAE Standard 111-2008 describes the Flow Rate Approximation by Temperature Ratio method for determining outdoor airflow when the outdoor airflow itself cannot be directly measured. The method involves taking temperature measurements of the outside air, the return air, the supply air (mixed air), and the supply air volume. See the standard for more details and note that to maintain an acceptable level of accuracy, a temperature difference of 25°F to 50°F (14°C to 28°C) must be present between the outdoor air and return air temperatures.
The outdoor air intake for our ventilation system is not accessible, and it’s not possible to measure the outdoor airflow directly. How can we determine the outdoor airflow for this system?