We recommend at minimum a 1" pleated filter. Do not use 3M Filtrete or Angelhair filters. These filter types are much too restrictive and will cause damage to your system (from our experiences with them). Washable filters are okay if they are correctly maintained. However, most people fail to maintain them properly, and this causes problems with the system. To properly clean your washable filter, you should first vacuum it entirely from the incoming airside. Once it is completely clean, flush with water in the opposite direction of airflow. If you flush the filter from the wrong direction or fail to vacuum it, you will just further embed dirt and debris into the filter. The cleaning process must be done monthly without exception. 5" high-efficiency filter are a good upgrade to the standard 1". They are about 20 times more effective at filtering the air inside your home. The new Accuclean/Clean Effects filtration system from American Standard/Trane and the April Aire 5000 electronic filtration system from April Aire are 100 times better at filtering the indoor air than the standard 1" filter. These systems are great upgrades for those with allergies and sensitivity to other indoor pollutants. For further information on indoor air quality products, see our indoor air quality section of this website or call us directly at (281) 355-1500.