Duct takeoff sizing - http://www.hvacmechanic.com/ Forums

Original message

Quakeserver149

12.205.48.41

"Duct takeoff sizing" , posted Fri 2 Jun 22:35

The job is not going to get outsourced, therefore, I've been assigned to run a new duct to the office portion of the warehouse where I work. The existing 35' takeoff was run about 50 feet farther away than where it could be and then 50 feet of shoddily installed (ie, crushed long runs) independent flex ducts run parallel to the main supply and service the registers in the office. Airflow at those registers is barely detectable.

I plan to run a new ductboard takeoff nearer to the end of the main supply which will run a straight line to the registers, rather than a long L shape. I'll probably block off 1 or 2 of the existing large registers which vent into unimportant space.

I've got manufacturer data about the unit's CFM and static pressure ranges. My question is what target feet per minute I should design for. I've read anywhere from 400 to 4000, which seems quite a range.

Alternatively, a reasonable primer (not just the very basics) of duct design would be helpful.

In the end, I don't want to create an over or undersized duct network. I presume I will crunch the numbers based on the unit data I have and the sizes of the existing vents and then work back from there to decide what allowances I can make with regard to airflow and then design for that goal.

Tips? Redirects?

Replies:

Weber

207.200.116.67

"Re(1):Duct takeoff sizing" , posted Mon 26 Jun 22:51

What size is the unit, and what cfm does it put out? How many supplies and returns will you have, what size are the areas you are going to feed? These are the questions you need to ask yourself before you start to layout the ductwork. and remember that flex and rect. duct have different cfms.

Steve Weber