Many central air units have at least two filters, although you can find systems that only have one. The reason there are a pair of filters is because it is not only necessary to filter the air that travels to the air controller, but also the air that passes through the return vents. Usually, a house will have 2 air filters on its intake grilles. In some cases, there may be more or less depending on the square footage of the house or apartment and the number of floors the central air system must support.
The general rule is a filter for each air controller. Most homes, especially large ones with multiple HVAC systems, have more than one air cleaner installed. They 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 system will normally have at least one air filter. Therefore, your home may have air filters located on the air controller AND on 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. That was probably easy at the time of installation. A single good quality Merv 8 filter is sufficient. Understanding the parts of your central air system is important because it helps explain the cases where 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 because they have 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. Asthma, skin and eye irritation, and allergic rhinitis are some of the many health problems caused by clogged air filters.
The reason for replacing filters in central air systems is because they become too restrictive on air flow (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. Let's say you just moved to a new house 3 months ago and it's time to change your air filters. Having multiple return grilles solves this problem, but it 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 the pre-filtered air with whatever is left over to the more expensive staged filter. It's questionable whether it works the same way as a 2- or 4-inch air filter in terms of filtration effectiveness 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. It seems that there are different opinions among HVAC contractors about what type of filter is best to use in a forced air system. .