When is air velocity low enough to use a restrictive filter?
I am trying to find out when it would be acceptable to use restrictive air filters, like the Filtrete red/purple/blue label filters sold in the big box stores.
My understanding is that with sufficiently low air velocity (i.e. sufficiently large filter area), the static pressure can be held to acceptable levels.
My upstairs unit has a 2.5 ton air handler and a 20x25 filter grill. The downstairs one is 3.5 ton with two 20x20 filters (heat pump split system). If I simplify by assuming the entire filter area is usable, I get 288 fpm upstairs, and 252 fpm downstairs. At those velocities, what kind of air filters can I use w/o risk of damaging my equipment?