The air filter in an HVAC system is typically located in a slot above or below the unit. In other systems, it can be found behind the return air grille on the wall, ceiling, or closet door. It may also be located inside the oven itself, next to the blower motor, or in a central filter grid installed in a wall or ceiling. An extremely dirty filter will block the flow of air and cause the system to work harder and consume more energy.
It is important to change the air filters every month when the air conditioning system is operating. If the building's duct system is so dirty that people are installing filters, it may be preferable to have it professionally cleaned. Someone may have added an air intake to improve the total air flow through the system, but drawing return air from near the oil or gas burner in a furnace can be dangerous. The cantilever size of the top-mounted cooling section suggests a poor design: the sizes of the air movement sections do not match.
Air handlers are installed horizontally (in an attic or low space) or vertically (in a basement, high attic, or other area of the building). Opening the blower compartment door will expose one or more filters that are simply placed in place in a frame. If the system has been operating in heating mode, the supply or outlet air ends of the ducts connected to the air controller will be warm to the touch and the inlet or return air ducts will be cooler. We have seen a 100% improvement in air conditioning or heating flow when cleaning a heavily soiled fan.
Leaving things at the top of the return air intake can result in a small reduction in airflow and a little more in home heating costs. It is important to check for an arrow on an air filter; almost every filter has one.