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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

So a transition from the supply of the a/h to the proper calculated size is right??

I do agree there is nothing wrong with ductboard as long as it is assembled properly, and genduct its a res 2.5 ton heil air handler with an x-13 ecm motor. So not exactly sure if its high velocity or not? But judging from your analogy i would say high velocity.

Like a lotta things, the answer is "it depends"

Originally Posted by ka24det

Yeah well im just the tech thats replacing an a/h and some ductwork. Never got any dimensions

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?
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

Originally Posted by ka24det

Just curious, what is the proper way to size the main trunkline using ductboard?
After using a duculator to find proper size should you start with the size of the throat and transition up? Or just start with the calculated size wich is larger than the throat?

Good rule of thumb is .06 ft/100 on a ductulator. Tranistion up if the outlet from the unit is smaller. Cant really be to big within reason. Nothing wrong with properly sized/installed ductboard in a residence.

I guess ive never seen it properly installed.

Originally Posted by second opinion

If duct board is properly installed the is nothing wrong with it and has its advantages.

+1 Emphasis on properly.

Originally Posted by SgtBone

You should just be able to transition from the size of the supply from the furnace to the size of the trunk. I would stay away from duct board. Your cust will appreciate it later on.

If duct board is properly installed the is nothing wrong with it and has its advantages.

You should just be able to transition from the size of the supply from the furnace to the size of the trunk. I would stay away from duct board. Your cust will appreciate it later on.

Yeah well im just the tech thats replacing an a/h and some ductwork. Never got any dimensions

First step, get a Manual D.

STUD

Duct Sizing

Just curious, what is the proper way to size the main trunkline using ductboard?
After using a duculator to find proper size should you start with the size of the throat and transition up? Or just start with the calculated size wich is larger than the throat?