Many central air systems come with at least two filters, although you can find some with only one. The reason for this is that it is necessary to filter the air that travels to the air controller, as well as the air that passes through the return vents. Generally, a house will have two air filters on its intake grilles. Depending on the size of the house or apartment and the number of floors the central air system must support, there may be more or less. The rule of thumb is one filter for each air controller.
Most homes, especially those with multiple HVAC systems, have more than one air cleaner installed. These are usually placed near the oven or manipulator and the return grilles of the air conditioning system. Larger homes usually have more than one climate control system, each of which will normally have at least one air filter. Therefore, your home may have air filters located on both the air controller and the returns. You should check every possible location to ensure that you have found all your air filters.
The cabinet may be configured to accept a one- or two-inch filter; you would have to remove the side tracks to insert a two-inch size. A single good quality Merv 8 filter is sufficient. Understanding the parts of your central air system is important because it helps explain why multiple air filters are used in several places. The lifespan of filters varies, so the number of times it is necessary to replace the air filters in the return duct compared to the air controller varies due to their different sizes. There is usually not enough volume to keep dust in the air long enough for the air conditioner inlet to act as an air purifier. While central air intakes also help filter contaminants to provide clean air, filters offer additional removal.
Clogged air filters can cause health problems such as asthma, skin and eye irritation, and allergic rhinitis. The reason for replacing filters in central air systems is because they become too restrictive on airflow (CFM) or start to not work as well by letting in the same things they are trying to filter, if not replaced over time. The air controller draws air from the house through the return duct system and then blows it through the heating or cooling system and returns to the house through the duct system. If you just moved into a new house three months ago and it's time to change your air filters, having multiple return grilles solves this problem but also requires air filters in each vent to properly filter the air and prevent particles from entering the fan motor. Electrostatic filters clean the air by creating static that acts like magnets that adhere dust and contaminants to the film. Because HVAC units are often hidden in smaller, less convenient spaces, some homes are configured to place air filters in return vents.
Multi-stage filtering is not a bad idea, and is done in many environments (especially commercial) for the same reason you requested, taking only pre-filtered air with whatever is left over to more expensive staged filters. It's questionable whether it works as effectively as a two- or four-inch filter when working with your central air system. In this case, your air filter is most likely located right next to the boiler air controller or air conditioning system. There are different opinions among HVAC contractors about what type of filter is best to use in a forced-air system.