The size of the furnace or vent filter must be correct or the filter won't be able to do its job properly. If the filter is too small, it will come loose and some of the air (and debris) will not pass through the air filter. The oven filters should fit snugly together, but there is no need to force them into position. If you have to force the filter into its slot, it's probably too big. Inserting an incorrectly sized filter may cause it to deform, damage it, or reduce its ability to function correctly.
The filters are slightly smaller than their slot for easy replacement. Some HVAC units may need a filter with unique or unusual dimensions. In these cases, it is necessary to order a custom filter. Round to the nearest inch to create the nominal filter size, which is the measurement you'll use when buying a filter. When you buy an air filter online, the most important thing to know is that air filters come in two sizes.
You must understand the nominal size (which is a round size often used to label the filter) and the actual size. For example, a size of 20x30x1 may appear on the filter label, but if you measure the filter you will see that the actual size is smaller than the nominal air filter size of 20x30x1. When you buy a filter online, the actual size appears in the product bullet or in the product description. Most air conditioning and HVAC units use standard-sized filters, but some require the purchase and installation of a custom-sized air filter. If your filter doesn't have the actual dimensions on the outside of the filter, you can measure it to get the correct dimensions. Filters don't need to have a “hermetic” seal, but they should be able to be installed and uninstalled easily and without problems.
The actual size, as you may have guessed, is the actual dimensions of the filter in length, width, and thickness. If you have an old filter from your oven on which the dimensions are printed, then determining the size is not a problem. This size is usually on the filter just below the nominal size and is usually labeled as the actual size. However, keep in mind that those dimensions are not necessarily equal to the actual dimensions of the opening of your oven where the filter is installed. Even so, understanding what all those numbers mean can be tricky if you're not sure what they mean. Poorly sized or incorrectly installed filter frames can cause air leaks, significantly reducing filter efficiency.
So to make sure you have the right size filter, especially if you're replacing it with a different brand, it's important to confirm its actual size.
