MERV 13 20X24X12 VariFlow Box Style #2802843. Variflow filters are designed for high and medium efficiency air filtration in all types of commercial, industrial and institutional HVAC installations. Variflow media is made from ulta fine fiber microglass paper. Each efficiency has its own formulation for fiber size and density. Variflow filters are widely used in areas with high humidity or in systems where sensible moisture is likely to reach the filter.