The general rule of thumb is that a house should have one air filter for each air handler. This is especially true for larger homes with multiple HVAC systems, which may have more than one air filter installed. These filters are usually located near the oven or manipulator of the air conditioning system and return grilles. It's important to check every possible location to make sure all air filters are accounted for.
If a single 1-inch filter isn't doing an adequate job of filtering particles out of the air, then it may be possible to upgrade to a thicker option, depending on the size of the filter slot. It's also a good practice to regularly change dirty air filters with new, clean ones during times of the year when the HVAC system is used most often. If your home is well-sealed, pet-free, and has dust-prone furniture such as carpets and cloth-covered furniture, and you clean the dust and vacuum regularly, your air filters will have fewer airborne particles. In addition to the MERV rating system for air filters, they can be manufactured with a variety of materials and in a variety of styles.
Air filters can usually be found near manipulators and ventilation grilles, but they can sometimes be found elsewhere. For example, an HVAC air filter location may be behind a return air grille for easy access when it comes time to change the filter. It's not recommended to replace a filter with a smaller size filter because unfiltered air will flow around the filter and cause dust, allergens, and debris to accumulate on the coil. Return ventilation filters are recommended as they not only protect HVAC equipment but also make the air you breathe cleaner by filtering various particles.
Selective filtering is one of the main disadvantages of most filters such as HEPA, Spun, Media, Washable and Electrostatic, since the first three cannot filter odors and fumes, while the latter two do not guarantee mold and mildew blockage.