Air filters have arrows printed on the sides that show how they are supposed to be installed. These arrows should point in the direction in which the air flows through the system, which is away from the supply ducts and (normally) to the blower. Fortunately, most air filters are designed to not fit properly unless they are inserted correctly. Look for the airflow arrows printed on the side of the air cleaner that indicate in which direction the filter should be inserted.
It is shown in the image below. When air filters become clogged with dust, dirt, and debris, they can no longer do their job of improving the indoor air you breathe. HVAC systems expel hot or cold air and extract spent air, circulating air throughout the house. A horizontal oven draws in air on one side, heats it or removes heat to cool it, and blows the treated air into the duct network on the opposite side.
If you're not sure when to change the filter, invest in an electronic air cleaner monitor that will tell you when it's time to change it. Dirty air filters also force your HVAC system to work harder than it needs to, reducing equipment life and increasing your energy bills. My problem stems from the fact that the last batch of filters I rescued doesn't have airflow arrow indicators or a wire mesh, it's multi-stage with a removable fiberglass (velcro) layer, an activated carbon honeycomb layer, and finally a pleated paper layer with a VERY thin fiberglass layer attached. Once you've located where your filters are, you'll need to find the size you need and determine which MERV rating best suits your needs.
This allows the filter to effectively trap particles in the air and, at the same time, minimizes the chances of slowing down the airflow of the system. It's important to routinely monitor your air filters to prevent them from becoming clogged with dirt, dust, or debris, and to keep them filtering the air you and your family breathe. You may need to change or wash filters beforehand if you have furry pets, if you use the system most of the time, or open windows frequently. The arrows on the filter should point toward the oven so that those arrows point to the direction of air flow through the ducts and into the oven.
In this case, the arrow would point upward as the air travels up through the vent before being pumped to the HVAC unit.