When manufacturer specifications are unavailable, how is CFM through a coil best calculated?

Using temperature rise on emergency heat mode to calculate CFM through a coil provides a direct relationship between airflow and temperature change. When a heat pump operates in emergency heat mode, it typically uses resistance heating elements for additional heating. By measuring the temperature rise of the air as it passes through these elements, one can effectively gauge the airflow rate.

This method relies on the principle of heat transfer, where the increase in temperature can be correlated with the volume of air moving through the coil. The greater the airflow, the less the temperature rise will be for a given amount of energy input; conversely, a lower airflow results in a higher temperature rise. This relationship allows for a calculation of CFM based on observed temperature changes, making it a practical approach when manufacturer specifications are not accessible.

In this scenario, other methods like using static pressure readings or calculating external static pressure, while useful in some contexts for airflow estimation, are less direct and can be influenced by various factors such as duct design or blockages. Averaging velocity at multiple locations, although useful in some cases for improving accuracy, does not directly utilize the relationship between heat transfer and airflow that is present when measuring temperature rise.