When it comes to air filters, size is an important factor. A larger filter can contain more dust, and a typical 20 x 25 x 1 filter has almost 30% more surface area than a 16 x 25 x 1 filter, and therefore has 30% more dust holding capacity. If you try to use a 16x20x1 filter in place of a 16x25x1 filter, the air will not be able to pass through the filter properly, resulting in an inadequate air flow. It is not recommended to use two filters instead of one, as two filters have twice the surface area of 1 filter and the filter material itself is the same thickness.
If you're going to choose a smaller filter or nothing at all, at least do something like taping or taping a piece of cardboard filling to the filter to get the right size. You can easily check the size of your old filter when it's in place; normally, the size is shown on the cardboard filter frame. Using the wrong size air filter won't necessarily damage the air conditioning system, at least not immediately. However, two filters in good condition can block airflow enough to cause problems, so if you use two filters together, make sure one is cheaper than the other. Newer central air conditioning systems and heat pump systems are often designed so that the heating unit and the cooling unit share the same air controller and the same intake air filters.
When it comes to air conditioning filters and furnace filters, they are not identical, but they are not completely separate products either. My sister can get 3M oven filters quite cheaply, but if you want to replace your furnace filter, make sure you get one that is the correct size for your system. Otherwise, it won't fit and you'll spend time and money trying to return the filter you bought. Learn how often to clean your washable air filter to keep your heating and cooling system working efficiently and effectively.
