Which of the following is a requirement for conducting an airtightness test?

Using a well-calibrated fan is essential for conducting an airtightness test because the accuracy of the test results relies heavily on the fan's performance. A calibrated fan ensures that the airflow measurements taken during the test are precise and reliable, allowing for a correct assessment of the building's airtightness. Proper calibration accounts for any system or environmental variables, ensuring that airflow rates are measured correctly to provide an accurate evaluation of the building envelope's integrity.

In addition to this, other factors may be important in various contexts of testing, but none are as critical to the fundamental process of measuring air leakage as a well-calibrated fan. For instance, deploying heaters, having a secondary power source, or using a larger diameter duct might be relevant in specific scenarios or for enhancing comfort or convenience, but they do not directly affect the primary requirement of obtaining accurate airflow measurements in airtightness testing.