The frequency to change filters depends on a number of factors like how much traffic you have in your home, type of flooring, whether or not you have pets and how many, type of filter your unit requires, general air quality in the area which you live, and how often your unit runs. We recommend checking your filter once a month. Programmable thermostats can be programmed to remind you to do this, or another simple way to remember is to associate your electric bill with the task, then just check your filter whenever you get an electric bill. Change your filter when it is dirty.