An improperly sized air filter can cause a lot of problems. When it's too small, it allows dust, mold, and other contaminants to enter the vents and be expelled into the air, leading to air quality issues. The filters only work if air has to pass through them, and if there is a gap of 1, a disproportionate amount of air will flow through the hole, making the situation even worse. It's possible to cut an air filter if you know what you're doing and if the filters don't weaken too much from the cut.
However, if the filter is too small, there will be a gap around the air cleaner that allows dirty air to circulate around the filter. This is almost like not having any filters at all. Additionally, if the unit does not have enough power, the filter can block air flow and reduce system efficiency. Air filters are designed to block dust, mold, pollen, and finer dust such as charcoal or lead.
This is especially important if you live in an area with poor air quality. While some air filters fit perfectly without requiring any modifications, this may not always be the case, especially if you have ducts that deviate from standard designs. It's important to change or clean your air filter frequently in order to ensure that all of the parts inside your air conditioner are working properly. If the filter is not the right size, either too small or too large, air will flow around the filter rather than through it and not all of the air will be filtered.
In the long run, it's much cheaper to replace the filter regularly instead of having to replace the entire air conditioner. When shopping for an air filter, it's important to measure the interior dimensions of the air filter inlet in order to make sure that you buy one that fits perfectly. If you're unsure which type is best for your individual climate system, consider getting a HEPA filter which can remove almost anything from the air including smoke, odors and microscopic allergens. Beyond HEPA filters, other types are rated based on their minimum efficiency report value or MERV rating.