Airwright: HVAC Duct Sizing

Size HVAC ducts the way the ASHRAE friction chart does, and see the size to order, its velocity band and what it costs in static, in one look.

SIZE IT
• Airflow plus friction rate, velocity, or both (the larger size wins): the exact round size and the next stock size up, never down.
• Rectangular and flat oval sizes that carry the same air at the same friction, squarest first, with a warning past 3:1 and 4:1.
• A velocity band for the duct's job: residential trunks, branches and returns, or commercial mains, branches and runouts by noise criterion and by where the duct runs.
• Fourteen duct materials with their ASHRAE roughness, flex duct pushed 4, 15 or 30 percent shorter (measured correction factors), air temperature and elevation.
• Will it fit? Joist and I-joist bays drawn to scale, ceiling spaces or your own opening. When one duct will not fit, it splits the air into two runs.
• A live friction chart: drag the point and see every stock size and velocity.
• Two friction rates at once, for supply and return.

CHECK AND CONVERT
• An existing duct: velocity, friction and velocity pressure at any airflow, or the airflow it carries at a velocity or a friction rate.
• Capacity tables for round and flex duct at one or two friction rates, held to the velocity limit.
• Round, rectangular and flat oval equivalents; solve a side or an aspect ratio.

DESIGN THE SYSTEM
• Design friction rate from the blower's external static, coil, filter, registers and the longest supply and return paths.
• Whole systems: lay out supply and return as a tree, size every section, find the critical path, see what each damper has to take up and the static the blower must deliver. Share it as a PDF.
• 44 fittings with ASHRAE loss coefficients; takeoffs read against the velocity ratio. Equivalent lengths worked out for your duct.
• Pressure drop of a run, and the longest run a pressure budget allows.
• Static pressure test: total external static against the rating, and the airflow read off the blower table.

FIELD TOOLS
Airflow from a sensible load or tons, airflow per room with branch sizes, air changes, registers and grilles, fan laws, pitot traverse, psychrometrics with a chart, mixed air, condensation on ducts and the insulation needed, leakage by seal class, sheet metal and weight by gauge with the code minimum for homes, two-elbow offsets and a unit converter.

SHOWS ITS WORK
Every size has a "How it's worked out" page: air density, velocity, Reynolds number, the Colebrook friction factor and Darcy-Weisbach friction, with your own figures. The engine is checked against ASHRAE Handbook tables and worked examples, residential design-method examples and Lawrence Berkeley National Laboratory flex duct tests.

FOR THE JOB
• Save sizes to jobs under the name you write on the duct; share a one-page duct schedule as a PDF or a spreadsheet.
• US and metric units, light and dark, larger text.
• English, Spanish and French.
• Works offline. No account, no ads, no tracking, one price.

Results are engineering estimates for design and field checks; the project specification, the equipment data and local codes govern. Not affiliated with ASHRAE, ACCA or SMACNA.