Do all air conditioners have filters? The answer is yes! Most central air conditioning units have two or more filters that clean the air and ensure good air quality as the unit circulates between indoor and outdoor air. The air cleaner is usually close to your home thermostat, in a return duct or right next to the air handler of your air conditioning system. If you have a large system with multiple air handlers and return ducts, finding the filters can be a challenge. If you can't find the filters for your air conditioning system, here are some places to look. Other vertical air handlers may be a downflow unit having return air entering the top of the air handler or blower and air conditioner passing into the supply ducts connected at the bottom of the unit.
For the Ruud achiever 80-foot super quiet air controller, there is a molded permanent air filter that slides down the back of the unit when you look at it from the burner side. In this case, the cantilever size of the top-mounted cooling section suggests a poor design: the sizes of the air movement sections do not match. There should be a metal divider that prevents significant air intake if you only use a 1-inch filter. Normally, we would put the air cleaner on the return side, either on an intake grille or possibly your previous photo below shows a holder for an air cleaner in the return air chamber in what I consider to be the bottom of the air handler. We have seen a 100% improvement in air conditioning or heating flow when cleaning a heavily soiled fan.
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. Also, keep in mind that you may want filters that don't just keep dust out of the air conditioner. If both heating and air conditioning systems share the same ducts, it is also possible that the air cleaner for both systems is located in or near the blower system of the furnace. These systems move air from a return chamber (connected to the end of the return duct of the system) through a cooling coil (for air conditioning) or a heat exchanger (for heating systems). 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. The filters of the air handler may be located between the return air chamber and the rest of the air handler containing a cooling coil for an air conditioner or a heat exchanger for heating systems, and also a blower fan that causes air to move through these components and go out to the supply lines.
Probably someone added this air intake (yellow arrows) to improve total airflow through the system, but drawing return air from near an oil or gas burner in a furnace can be dangerous. Once you have located your HVAC system's filters, document their location for future building owners, occupants, or repairers. The filter should be installed so that an arrow (and airflow) shows it moving through to either your duct system (on a central return grill) or to your air handler (for filters installed in your blower unit).