Many central air conditioning systems have 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 handler, as well as the air that passes through the return grilles. This configuration exists for several reasons, such as the service life of the filters being different and needing to be replaced at different times. The intake vents draw air out of the room and the ventilation grilles in the ducts push the air conditioner into the room.
You can usually tell the difference visually, as the intake grilles are larger and fewer in number than the duct ventilation grilles, which are often found in every room in your home. If you live in a large home, your system will most likely require more than one return air duct, and each duct requires a filter to stop particles before they reach the blower motor. Or, you have a system that has several air conditioning units. It may be that the cabinet is configured to accept a one- or two-inch filter.
A single good quality Merv 8 filter is sufficient for most systems. However, some people may think that doubling the filter will increase filtration or extend lifespan. If you're concerned about indoor air quality and want to increase filtration, it's best to get a media filter that's the right size for your equipment. Otherwise, it will shorten the life of your system.
Most central air conditioning units have two or more filters. These filters clean the air and ensure good air quality as the unit circulates between indoor and outdoor air. There are several types of HVAC units and each may require the filter to be installed differently. If you have more than one HVAC system, you'll usually have more than one roof air filter.
Commonly used in HVAC — R, these orange polyester antimicrobial rolls provide a more durable solution you can use to replace filters over time and excellent mold prevention. It's important to note that some HVAC systems, especially larger ones, may have more than one air filter and may even require different sizes in different locations.