Some people try to wash or vacuum their furnace filters to extend the life. Washing your furnace filter with water is generally a bad idea. Many filtering materials do not hold up well when wet, and washing your furnace filter significantly increases the risk for mold or microbiological growth on the filter itself (which will release those mold spores into the air that is circulating throughout your home).
Vacuuming your furnace filter is a healthier alternative to washing your furnace filter, but is still not recommended. Although you may slightly prolong the life of your furnace filter, you will probably make a mess doing it, and a vacuumed filter will release a lot of dust and particulate into the ductwork when you first re-install it. Additionally, most furnace filters are made from a delicate filtering material and are prone to ripping or tearing, which will render the filter useless.
There are certain filters that are intended to be washed, called washable air filters. Make sure your filter is truly a washable air filter before you attempt to wash it. If you’re not sure what type of filter you have we recommend that you err on the side of caution and replace your existing filter.