by 
The Curious Bag
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<h4 class="dechead">On One Hand: Proper Sizes for Furnaces
Forced air furnaces or electric furnaces require a filter in order to function properly. This reduces the dust in the air and helps your furnace function more effeciently. Most furnaces will have a label that states what size filter should be used. It is vital to purchase the right size to ensure that your furnace will run properly.
On the Other: Size Variations
If your furnace does not have a label, you must measure the existing furnace filter. Keep in mind that "nominal sizing" is used in this industry. That means that if your filter measures 13.75 x 13.75 inches, purchase a 14 x 14 filter. Also measure the depth of the filter, which is typically 1 inch.
Bottom Line
When in doubt, measure the entrance for the filter on the furnace. You can then purchase several filters with the correct width and different depths. Try each one out until you find the right fit for your furnace.
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