Does it matter what size filter I put in my furnace?

The Right Size Filter Matters

The filter size is based on the cabinet size used during the installation of your equipment, the model of you’re a/c or furnace is generally irrelevant to the size of the filter.

It is important that you use the correct size filter that was designed for the filter box. If your furnace has an existing filter the size is printed on the side of the filter.

The most common filter is a 1” thick media that comes in a wide range of sizes, such as 20X20X1. These filters need to be changed monthly as the surface area and thickness of the media is minimal.  It is standard business practice for Area Heating & Cooling to install an upgraded 4” Merv13 media filter. It is a better quality product and will need to be changed approximately every six months.

If the air filter is too big, DO NOT cut it to fit.

Cutting a filter to fit can create air gaps and contaminants can pass through into your air.

If the filter is too small, it too will have air gaps and contaminants can pass through into your air. Also, as the furnace is operating the air suction or flow will likely cause the filter to flop around rendering it useless.

DO NOT stack filters on top of one another. If you have a 4” or 5” thick filter box, do not purchase four 1” filters and stack them on top of each other. All this is doing is causing air restriction, and not filtering correctly.

The deep pleats on the 4” filter collect dust, dander, pet hair, and allergens because it has more surface area to do so. It also allows the air to flow properly through the filter, for correct airflow. Stacking 1” filters defeats that purpose entirely.

If you are unable to locate the filter size on the existing filter, simply measure the filter itself and round up to the nearest whole number.

If you are unsure where your filter is located, or you think there is not a filter currently in your furnace, here are a few things to try:

  • Most filters are located at the furnace, the filter box may be located above the main blower panel of the unit, or in some cases it is along the side of the furnace.
  • If your filter is in an attic or crawlspace you may have a filter grill that is located in the home at the return grill. The return grill is usually in the ceiling. The grill will have nobs to release the grill to access the filter.

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