Actual dimensions are 20.75 x 20.75 x 4.38. Accumulair brand: 21x21x4.5 Ruud PD540018 style whole house air filters, exactly engineered to be an alternative to the original brand Ruud PD540018 filters. Completely box-formed - no forming or combs needed. MERV classified to ASHRAE 52.2 test specs ensuring particulate filtration to the level of the original brand Ruud PD540018 filter. Note: The example filter image shown is not true to size and is there to show configuration purposes only. Brand names and Ruud PD540018 P/N used for filter substitution identification purposes.
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