First, what  the filter does  can be summed up simply by stating this: The filter keeps your Evaporator coil from becoming the filter. Two things are happening when your system is running. Air is being sucked into the system, cooled, and then being thrown back out. What stops the stuff being sucked up from sticking to the Evaporator Coil? You guessed it, the filter. Your filter keeps the coil from becoming the filter. Now there is much more going on then just that, but I want to keep it simple as pertains to the question. There are a couple follow up questions as well. They go like this:

• “How often should I change my AC filter?” The simple answer is, when its dirty. If you have a really clean house and you go check your filter on day 45 and it is clean, don’t change it. However, if you go to your filter on day 90 and it’s clean, I would be worried. This probably means something is getting passed your system and the filter isn’t catching it.
• “What is the 30 day rule?” Checking your filter every 30 days is a good rule of thumb. If you check it every 30 days, then you will never let it get too dirty to ruin your system
• “How bad can it get if I don’t change my filter?” Oh boy. You don’t want to know. This little $7-$15 item if not checked and changed can cause massive massive damage to the AC system. Here is a short list: Cracks in the copper from freezing and thawing as the AC is starved of the proper air flow, over-heated blower motor, over heated condenser motor, burnt out compressor, and water leaks from freezing and thawing. 

I addressed this question because this is probably the single most commonly asked question by those who live in the Keller, Southlake, Watauga, Haltom City, Saginaw, North Richland Hills, Fort Worth & DFW and Mid-Cities area.