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Building Science

The Fundamentals of Rigid Duct Design

Observe these principles of fluid flow to deliver the right amount of air

These rigid duct fittings are pre-insulated and pre-sealed with mastic on one of Matt Risinger's jobsites.
Image Credit: Energy Vanguard

At the end of this month, I’m giving a little presentation at the ASHRAE conference in Las Vegas. Actually, I’m doing one third of the whole presentation, which is titled Flex Ducts, Hard Ducts and No Ducts: Migration Patterns for Duct Hunters (or not) in the Land of Thermal Comfort. My part is on hard ducts.

Chris VanRite is doing flex duct, and Robert Bean will cover the no-ducts part (which doesn’t refer to ductless minisplits but rather to hydronic distribution). We get 15 minutes each, so I’ll elaborate on my part a bit here.

Before duct design

Designing a duct system is important, but there are a few critical steps that come first. Number one is the load calculation using a protocol like ACCA’s Manual J or the ASHRAE Handbook of Fundamentals. You’ve got to know how much heating and cooling you need for each room. Then those BTU/h requirements immediately translate to room-by-room air flow requirements in cubic feet per minute (cfm). It’s done automatically in the software we use (RightSuite Universal by WrightSoft).

Once you know the BTU/h and cfm numbers for the building, you need to select the right equipment. ACCA’s Manual S protocol helps you do that. There’s more to it than just finding a piece of equipment that meets the total heating and cooling loads for the home. You’ve got to make sure you adjust for the indoor and outdoor design conditions of the home. Ideally, you have the manufacturer’s performance data tables to help you get it right.

Then you’re ready to start designing the duct system.

Getting the right air flow

If you take a fan out into your yard on a calm day and turn it on, you’ll…

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