The foam filter should only to be installed when you are using the vacuum to clean up spills or when water is present where you are vacuuming. The foam filter is designed to prevent water splash-back within the vacuum tank from entering the motor head during operation. This foam filter is for general purpose applications when vacuuming wet debris such as water spills or debris that contains excessive moisture or liquids etc. NOTE: A VACUUM FILTER ONLY WORKS WHEN IT IS CLEAN! IT IS RECOMMENDED THAT THE FILTER BE CLEANED REGULARLY (PRIOR TO EACH USE & PERIODICALLY DURING USE) SO THE FILTER CAN FUNCTION CORRECTLY. CAUTION: DO NOT USE FOAM FILTER IN DRY VACUUMING APPLICATIONS.