When installing a new filter in your heating or air conditioning system it is important to know which way it is supposed to go. The reason this is so important is because most air filters are designed to work best when air flow is in a specific direction. With some media pad filters it doesn’t really matter but with pleated air filters it can make a big difference.
The Arrow Always Points Toward The Air Handling Unit
Usually there are arrows on the frame of the filter that indicates which direction the filter should go. If you enlarge the image above and look at the filter you will see an arrow pointing toward the air handler. It also points in the direction the air is flowing through the duct system and the air handler. If you are unsure which direction the air will be flowing there are a couple of other points of reference that you can use.
Install the furnace filter so the arrow is pointing toward the evaporator coil, blower motor and fan, which are located inside of the air handler. If the filter is being installed in a return air grill then the arrow will point toward the air handler.
If there is no arrow to indicate the direction of air flow through the filter look at both sides of the filter and see if one side is made a little sturdier than the other. Sometimes wire mesh is added to one side or the cardboard frame may have cross pieces that make that side appear more durable than the other. Which ever side appears to be sturdier is the back of the filter. You want the air to flow into the filter through the other side first.
Fiberglass panel filters are really not a good filter to use in air conditioning equipment but if you do use them and there is no arrow to indicate which way it should be installed this is what to look for.
Similar to media pads, one side is usually a bit denser than the other. The denser side is sometimes even a different color and this side will face in the direction of air flow. In other words, install it so that air will pass through the less dense side first and then through the denser side.
If you make a mistake installing your new filter, it can have a drastic impact on air flow very quickly because within a couple of weeks dust will accumulate quickly reducing air flow and have a negative impact on operating efficiency.