Some HVAC systems have filters located in the grills that can be seen from inside the home. These filters are usually 1” thick and should be changed once per month in almost all climate regions. Other systems will have a filter located at the equipment where the duct connects to the system. These may also be filters that need to be changed once per month or could last up to a year in some cases. I recommend all filters to be pleated style. If a duct system is too restrictive a fiberglass filter might be used to allow more air flow. I recommend changing these filters every week. No system should be used with out a filter or air cleaner in place. If the system is ran for a significant time without a filter or air cleaner damage can occur.