If you want to get cleaner air to breathe, you may maintain your furnace air filter. Having done it correctly, you can also maintain the working condition too. But before changing your air filter, you should measure and order the correct size and thickness. It is recommended to change your filter at least once a month with a new filter.
 However, if you don't change your filter every month, then you should at least inspect and assess the cleanliness. There are reasons for changing it instantly. They are: dirt, if it is covered in hairs or other particles, or if you smoke in the house. The quality of the filter influences on the interval for changing it. Cheap filters are needed to be changed often.
The general guide to buying a new filter is, if you can see though it does not buy it. Usually, it means that the dirt will not be trapped as the air cannot be filtered properly. But if it is too thick it will affect the flow of air which could modify the performance of your furnace. Be sure, you will be able to prevent some health illness such as asthma if you buy the best filter.
It is recommended to look for a higher efficiency pleated air filter as these will trap more allergens in the air. Different types of pollutants as well as the pet dander form the air which causes a lot of allergies are trapped with these filters.
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