Method, systems, and devices for determining instantaneous heat exchange between an airflow and a heat exchange element of an HVAC system are disclosed. In some embodiments, methods and systems include receiving or otherwise obtaining airflow data, temperature and humidity data. The airflow data may be used to calculate the volumetric airflow rate, and the temperature and humidity data may be processed with enthalpy data to determine the change in enthalpy (Δh) of the airflow after encountering a heat exchange element. The instantaneous rate of heat exchange (E) between the airflow and the heat exchange element may then be determined from the volumetric airflow rate and change in enthalpy.