I am new here, I did search for an answer to this question but did not find one.
I have a 5 ton HVAC system with a 25x20x5 filter (marv13) I have a magnahelic (sp?) between the return and supply it reads .07 (inches of water) with out a filter, .31 with a new filter and .34 with a 2 month old filter. An 8 foot long, 5 inch ridged duct closest to the supply of the HVAC unit has 420 ft/ min speed with out a filter and 380 ft/min with a new AND the 2 month old filter. (We run the HVAC fan 24/7 so the filter gets dirty way faster than if it was in intermittent use.)
My question:what is the best way to determine if the filter needs to be changed? Pressure differential increase or air speed decrease?