Our Carrier system about 11yrs old uses a 16.5 x 21.5 x 1 "coil air filter." This is the recommended model in the literature. (
Best Carrier 1" Fan Coil Air & Furnace Filter Replacements | Filter Buy) I've tried the "standard" 16 x 20 x 1 and they pretty much get sucked up into the system. This is the original unit in the home (we're 2nd owners) and is around 11yrs old. Very strange size for a "standard" system installed here in T.V. Can't find them locally anywhere.