i understand the diameter of the duct feed is important but how do you match the register size?Do you use the biggest size available for the diam. duct your using,i.e. 7" duct feed or return with a 6x12 register.How do you know when to go to a 4x10 register?
Re: register sizes
many factors will determine the register size. if its a return bigger is always better. supply can be differnt sizes depending on the desired effect. a smaller register will give you more velocity but a larger one will give you less air noise.