Air filters are designed with arrows printed on the sides to indicate the direction of airflow. These arrows should point away from the supply ducts and towards the blower, which is the direction in which air flows through the system. If the filter is in the ceiling, the arrow should face up, and if it's on the wall, it should face out. Most air filters are designed to fit only one way, so you can easily tell if you're installing it correctly.
Look for the arrows printed on the side of the filter that indicate which way it should be inserted. This is illustrated in the image below. If you're dealing with an oven filter, look for an arrow that shows the correct direction of airflow. The arrow should always point towards the furnace and away from the return duct that carries air that needs to be heated or cooled.
Both sides of the air cleaner look similar, but they can't be used interchangeably. Some filters don't have arrows, so if you come across one of these, double-check it before installation. Installing an air filter incorrectly can reduce its efficiency and cause a variety of maintenance issues. Dirty air filters also force your HVAC system to work harder than necessary, shortening its lifespan and increasing your energy bills.
All air filters have specific performance ratings that indicate how well they can remove large and small particles from the air. When you insert an air cleaner incorrectly, you won't be able to filter your home's air efficiently. Some downflow ovens have a V-shaped filter system and require two filters placed in a V-shaped angle in the compartment. In all types of ovens, however, the filter is always located near the intake air blower fan and positioned so that air flows through it.
Some filters can even remove small particles like allergens and provide clean air for your living space. With an upflow oven, the filter will be located near the bottom of the unit in the blower compartment next to the cold air return. Be careful when removing old air filters as they may have accumulated a lot of dust and other particles. This means that you want spent air that is sucked in to flow through the filter before being blown back into your home.