If the non metal ductwork is filthy you should just have it replaced AND sealed. Also most duct programs aren't very properly built. Try to look for appropriate sizing and don't go affordable with one or two intakes (return grills) You need them through-out your house. Right here in http://capistrano-beach71369.free-blogz.com/19563075/5-easy-facts-about-air-duct-cleaning-orange-county-described