In most homes, you'll find the air cleaner housing located right next to the air handler of your heating or cooling system. This is a large metal box with a fan motor and a fan inside. Most air handlers are installed in the basement, attic, mezzanine, or garage. If you live in a smaller space such as an apartment, your unit is likely in a closet.
The air filter can be difficult to locate in some HVAC systems, but it is usually close to your home thermostat, in a return duct or right next to the air handler of your air conditioning system. It may be intended to hold an air cleaner against the inside face of that grill return air chamber door-cover at the bottom of the air handler. Every month during the heating or cooling season, inspect the type and condition of the air conditioner filter. I like a filter on the air inlets, as it helps keep the return ducts and fan clean; however, if you already have a proper air filter in or on the air handler that should protect the fan, you would be careful to add more layers of filtration without talking to your HVAC company about air requirements your system's flow rate and restricting additional filters to ensure that you don't slow down the air supply through the system.
We have seen a 100% improvement in air conditioning or heating flow when cleaning a heavily soiled fan. Getting inside a blower cab when the fan is running is very dangerous, as you can lose fingers on the rotating fan of the air conditioner blower assembly or its drive motor or belt. Of course, a heavily soiled air filter will also slow down air movement and increase the operating cost in an air conditioning or heating system. In almost all central air cooling or heating systems, the blower is located at the return air end of the system.
For safety reasons, turn off power to the unit before opening the fan compartment to inspect and change a filter. I have a new house and I can't find the filter in the attic air conditioner unit, there are no filters in the roof suction tubes. We will replace the air filter in your air conditioner, clean your ducts, fix any other air conditioning problems you may have, and answer all your questions. Because the vent pushes air in, it is more efficient for particles to collide with the smooth side of the fibers. All central heating and cooling systems must have an air filter, but it may be more difficult to locate in some HVAC units than others.