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Buying furnace filters or replacement filters for an air handling unit can be an easy task, but watch out for filters that are too cheap or that don't meet your units specifications! Make sure that the filter you're buying meets your specifications, and when in doubt, go with a mid level MERV rating (MERV 8 or 11) and a high capacity air flow rating.

Some things to consider when comparing filters:

* Filter Pleats (these are the V shapes of the filter media) are an easy visual indicator to compare surface area. Most of our 1, 2 and 4 inch filter images are of the standard capacity filters. The high capacity option, where available, increases the number of pleats in the filter media. Check our detailed specs page where available for more information on filter media surface area.

* The MERV rating along with the surface area of the filter media factor into the airflow capacity and the resistance of the filter. Check our detailed spec tabs on the product pages for these ratings. When comparing filter prices, these numbers (MERV, Airflow capacity, Initial resistance) really determine the comparability of different filters.

* You're always better off buying a higher capacity filter if the option is available. These filters offer a lower initial resistance and will make up for their extra cost in performance. To understand how this cost savings is achieved consider how a heating or cooling system works. Airflow across a heat exchanger transfers heat and the more airflow that moves across the exchanger the more heat is exchanged. In a gas furnace system this airflow is critical to the effectiveness, efficeincy and life expectancy of the system. The heat exchanger relies on airflow to keep the metal cool. Also, the more air that moves across the heat exchanger, the more heat that is removed, and the cooler the exhaust air is. Less airflow results in a higher exhaust temperature, which is waster energy and also a higher operating temperature on the heat exchanger which can reduce the life of the system.

* The Bottom line is that when it comes to an expensive heating or cooling system, the cheapest filter on the market isn't always the best option.