When it comes to air conditioning and oven filters, it's essential to understand the size you need. Air filters are labeled with three different dimensions: the length, width, and thickness or depth. AC filters come in many sizes, so it can be confusing if you don't know what size you need. You also don't want to put a filter that's too small in a slot because the air will take the path of least resistance and flow around the filter instead of through it.
But when it comes time to replace it, does it really matter what type of filter you choose? Aren't all filters basically the same? When making your purchase, you should order the filter by its nominal size, rather than its actual size. If an air filter is too small, this will create air gaps and contaminants will avoid filter material. Installing several 1-in-a-slot air filters can cause costly damage to your air conditioning unit, as residential ovens are not equipped to handle this level of airflow restriction. To ensure that the next filter you purchase is the correct size for your system, you should measure the inside dimensions of the air cleaner inlet.
The easiest way to identify the size of the AC filter is to remove the existing filter from its slot and examine the filter frame. Note that the size shown in bold type on the filter frame is the nominal size of the filter, which is likely to differ from its actual size. The size of the vent filter must match a size slightly smaller than the measured outer length and width of the frame around that opening. Note that the sizes of the air conditioner and oven filters must be smaller than the dimensions of the filter compartment.
While you should refer to your HVAC system and specific filter for maintenance guidelines, you should generally change filters at least every season, if not once a month. If the filter is not the correct size, either too small or too large, air will flow around it instead of through it and not all of the air will be filtered out. Filters don't need to have a “hermetic seal”, but they should fit easily into your unit without being able to move freely.