Which Way Do Furnace Filters Go?

When installing a new furnace filter or air conditioner filter it is important to know which way they are 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 type filters it does not really matter and with some it makes a big difference. With most pleated air filters, electrostatic air filters, HEPA filters, fibreglass panel filters and even electronic air filters the direction of air flow is very important. Installing furnace filters the wrong way can have a very negative impact on air flow which reduces the efficiency of the equipment and can also reduce the usable life of the filters.

Usually there are arrows on the frame of the filter that indicates which direction the filter should go. This arrow points in the direction the air is flowing through the duct system. If you are unsure which direction the air will be flowing there are a couple of other points of reference that you can use. You can install the furnace filter so that the arrow is pointing toward the evaporator or if installing in a return grill point the arrow toward the air handler. The arrow always points in the direction of the air handler.

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Sometimes, for some reason arrows don't get put on filters or the type of filter, media air filters for example, are physically imposable to put direction of air flow arrows on. In these cases there are usually other ways to determine the proper way for the furnace filters to go.

Pleated Air Filters - Sometimes 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 a wire mesh is added to one side or the cardboard frame may have cross pieces that make that side appear stronger 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.

Fibreglass Air Filters - Fibreglass panel filters are really not a good filter to use in air conditioning or heating system but if you use them this is what to look for. Usually one site of these filters are 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.

Media Air Filters - With media filters you can use the same rules as for the fibreglass panel filters. One side of the filter is often a little denser or the fibres are more tightly woven than the other. Often, depending on the manufacturer, there is no difference between the two sides and it won't matter which way it is installed.

If you make a mistake installing your new furnace filter, air flow can be drastically reduced within a couple of weeks because dust will collect in a way that will guickly reduce proper air flow through the filter.