Installing an air filter in your home is an important part of keeping the air clean and healthy. It's important to know which side of the filter should face up, and how to determine the size and MERV rating you need. When installing a new air cleaner in the AC return duct, the arrows should point inward to indicate the direction of air flow. This is because the return duct is extracting air.
An HVAC air filter is manufactured with one side having a filter material and the other side having material to reinforce it. Almost every filter has arrows printed on the sides that show how they are supposed to be installed. To determine the size and MERV rating you need, you can use an AC air filter size chart guide. A clue that tells you when it's time to replace filters is when you can no longer see through the filter.
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. If you install a smaller air filter, dust and dirt particles will travel around the filter and go directly to the air conditioning unit. Dirt, dust and animal hair surrounding the filter will adhere to the internal parts of the air conditioning unit or oven. If the filter is facing backwards, the actual filter parts can come off and be absorbed by the unit and damage the fan blades. For any air filter, whether for an air conditioner or an oven, the arrows always point in the direction of the air flow.
This means that away from the supply ducts and (normally) to the blower. When changing your oven filter, make sure that you have calculated the MERV size and rating correctly.