Many times when researching equipment for duct cleaning vacuums you will find manufacturers posting the CFM suction ratings on their vacuums as “Free Air” or “bench Test”.
The Free air test is the rating on the impeller wheel of the vacuum, simply put it is the maximum amount of airflow the impeller can produce with out any restriction. It seems as though many manufacturers prefer to use this rating over the actual vacuums suction ratings in an attempt to hide the amount of restriction created by the vacuums filter banks.
In any vacuum system a filter will naturally restrict the air flow whether the filter bank in on the input side of the impeller or on the output side any type of blockage will restrict the vacuums ability to get air in and out. When looking at a duct cleaning vacuum system don’t be misled by free air test numbers, or model numbers that like to make you believe a vacuum has more suction then it really does. The next time you see a vacuum with a moel number like ETC5000 or a cfm rating that states 5000CFM then “free air ” make sure you ask for the real cfms ratings.