Methods and related systems for operating a climate control system for an indoor space are disclosed. In an embodiment, the method includes increasing a speed of a fan of the climate control system, and determining a first fitted external static pressure (ESP) function from a first plurality of airflow values and a first plurality of ESP values. Additionally, the method includes obtaining a baseline ESP from the first fitted ESP function, and determining a second fitted ESP function from a second plurality of airflow values and a second plurality of ESP values collected at least one week after the first plurality of airflow values and the first plurality of ESP values are collected. Further, the method includes comparing the first calculated ESP obtained from the second fitted ESP function to the baseline ESP to determine a condition of an air filter of the climate control system.