In general the furnace filter is located inside the blower compartment.
This will vary with the installation of the Furnace. The HVAC (Heating Ventilating and Air Conditioning) contractor can install an Air Cleaner Box, or cut a slot in the return air duct. There is a possibility that you're Furnace or Air Handler with have filters in the central return Grill. You do not want to install in both places. When changing the filters in the furnace compartment, ALWAYS turn the power off at the main circuit breaker.
This means the air is moving upwards. This filter is located in the blower compartment (bottom door) adjacent to the cold air return. You have to remove both the top door and then the bottom door. Make sure that when you re-install the bottom door, it goes over the furnace frame so that it engages the safety switch. If not the furnace will not run.
This means the air is moving downward in direction. This filter is located in the upper blower compartment. Most furnaces will have a V shape filter system; the system will take 2 filters. The filters go up into the plenum in a V shape. Make sure that when you re-install the top door, it goes over the furnace frame so it engages the safety switch. If not the furnace will not run.
Most Horizontal Furnaces will have a slide in Furnace rack, built into the furnace.
In warmer climates some homeowners do not have a traditional furnace but rather air mover for air conditioning. Most Air Handlers will have a built in filter rack in the unit. Note most Air Handlers will have central returns, which have built in filter grills to accommodate a filter. Check to see that you DO NOT HAVE a filter in both the Air Handle and the filter grill. This will cause you reduced air flow and higher utility bills.
Note: Many customers us plastic deflectors to keep their drapes away from the floor vent and keep the warm air going across the floor. (Remember) warm air raises and cold air fall. REMOVE in the cooling season to have better cooling and lower utility bills.
Changing your HEPA style filter is almost as simple as the others. All you need to do is open the front of the media housing and slide the old one out and slide the new one in. Some models require pleat spacers. Don't forget to order them if yours are damaged.
Changing your common style filter is simple. All you need to do is slide the old one out and slide the new one in. Be certain that the air flow arrow points toward the furnace or blower motor. Please remember, .99 cent filters do not work effectively. We carry an efficient array of Furnace Filters for your home.