5” furnace filter and why buy a 5” furnace filter One of the most used furnace filter is the 5” furnace filter. A 5”furnace filters with a MERV rating greater than 10, works very effectively. MERV rating stands for Minimum Efficiency Reporting Value, which ranges from 1-20.MERV rating measures the efficiency of the furnace filter. Higher the MERV rating better is the efficiency of the furnace filter. By furnace filter, we mean the thickness of ... 4/1/2019 ~ Phoenix, Arizona ~ 0.0 mi.