Like a lotta things, the answer is "it depends"
First question is: Is it a big fat plenum or a high velocity discharge? think of the air as a fire hose, is the water being put into a 55 gal drum or a bucket?
Originally Posted by ka24det
the high velocity discharge needs gentle size transitions OR the high velocity air gets turbulent and the velocity doesn't convert to static pressure. It's called system effect losses
The other thing to remember is that the choice of sheet metal or ductboard ( people think the extra resistance of board is a big deal) is not much different in air delivery because most of your duct losses are the fittings!
In fact the extra resistance of ductboard is more than offset with it's really low leakage Ductboard is expected to be 1/8 the leakage of sealed residential duct
That's right Boys and Girls, leak class 6 vs. leak class 48 NOT A Typo
So the real advice is to think about the discharge velocity for your immediate problem of doing a good tie in and if the customer is honest and tells you they have air delivery problems now, your new box even with a 1 hp ECM motor is NOT GOING TO FIX IT Now you need to really understand what is hanging off the box and figure out the fix
A high velocity discharge
would be less than one sq fi discharge area more than 1000 FPM
The best thing is to make a real transition that would gently open from the discharge size to the duct size you need with no more than 15 degree angles 1 inch to 6 OR just know that you are going to take a pressure drop from the stated pressure available for the airflow you need Probably going to lose .1 or .15 at the unit and that's if the return duct is short and sweet
Beyond that not seeing what you have, it is tough to suggest anymore