When it comes to air filters, it's important to understand the system and the parts that make it up. In most homes, two air filters are placed on the intake grilles. However, it's not recommended to stack two pleated filters together as this will increase static and affect system performance. Using a two-fold filter is a better option than stacking two disposable filters.
This is because the system was not designed to handle two stacked filters. It's best to use standard pleated filters, not the ultra-restrictive ones found in hardware stores, as these will make the fan work harder and reduce efficiency. It's also important to note that just because two filters are available doesn't mean they can be stacked together. There is a difference between a 2x1 stack and a 1x2 stack, as the former contains two square inches of a single layer while the latter represents two layers that are filtered twice.
Having multiple return grilles is an effective way to recirculate air throughout the house, but it also requires air filters in each vent to properly filter the air and prevent particles from entering the fan motor. If you find two cheap glass filters, it would be better than stacking two filters together. Stacking filters is likely to cause the system to malfunction, wear out more quickly, or work less efficiently. High-efficiency filters can also be harmful to the fan motor as a dirty filter can be.
It's important to change your filters regularly if you have more people, pets, or clutter than most homes. Where they are placed may be irrelevant, but make sure they don't reduce the required airflow that was designed in your system. Understanding the parts of your central air system is essential for explaining cases where multiple air filters are used in several places. The most commonly sized filter for return vents is one inch deep and is designed to hold one-inch air filters.