There are three reasons you would need to place an air filter. You might have bought a new filter or HVAC system, and need to place the filters correctly. You might have microfilters that have reached the end of their useful lives and need to be replaced by new filters. Or you might use electrostatic filters that need to be regularly taken out, cleaned and put back into the air system. In every case, it is important to place the filters in the ventilation system correctly.
Air filters remove dust and other unwanted particles from the air as it circulates through the air conditioning system. There are many different types of filters, but most work in one of two ways. The first and traditional method uses microfilters that are made out of some type of filtering material, such as fiberglass. These filters have very small holes that only let air through, while stopping other particles. These filters tend to clog over time and must eventually be replaced.
The other type of filter is electrostatic, and it creates an static charge on metal plates that natural attracts particles in the air. The particles attach to the metal plates of the filter and stay there, waiting to be cleaned off. While electrostatic filters are not quite as efficient as microfilters, they do not have to be replaced. In both cases, the filters must be placed in the air conditioning system correctly, and a backwards filter can hamper air flow or severely hinder the efficiency of the filter itself.
Most filters, especially microfilters, come with arrows on the edges that are indicators of how the filter should be placed. These arrows should always point in the direction that the air is flowing. If the duct is channeling air into the air conditioning unit, then the air should point toward the unit, usually toward the evaporator coils or other devices. If the duct carries air away from the unit, then the filter should be placed facing away from the unit and pointing away from the fan, toward the room being cooled or heated.
If there are no arrows on your filter frame, you can often tell which way the air is supposed to flow through by feeling of the filter itself. Filters are generally softer on the side that should be facing air flow, and tougher or harder on the side that should be facing the direction the air is flowing. Some filters, like electrostatic versions, do not need to be placed in a certain direction.