HVAC Duct Design Basics: What You Should Know

Energy-efficient, comfortable HVAC design for your Phoenix home or business really begins with HVAC Duct Design basics. The ducts that professional HVAC technicians will run through the ceilings and walls of your building can be thought of as the veins and arteries of your ventilation and climate system. If the ducts are incorrectly sized or laid out poorly, even the most expensive furnace and central air conditioner will under-perform.

Duct work Design

For most HVAC contractors, the mathematics and calculations of good duct sizing and layout are done by applying the ACCA’s Manuals D (residential) and Q (commercial). Instead of estimating how large a duct each space needs, technicians measure room volume and layout, enter specifics like floor type, room direction (for heat gain) and other factors. The software and manual provides a blueprint for successful Ductwork Design.


Conduction is one of three challenges to effective HVAC system design. While this overview is strictly HVAC for beginners, everyone can understand the idea of heat conduction; it is how heat moves through solids.
If you have flexible ducts running through an unheated attic, for example, the furnace in your home or business is pumping cleaned, heated air through the unheated attic. Conduction robs the system of that expensively heated air, as the heat conducts through the ductwork. In summer, hot attic air conducts heat into the same duct that is trying to deliver cool air.
Solutions to fight conduction include using insulated ductwork, ensuring tight seams (using mastic instead of infamous “duct tape”) and sometimes using rigid, insulated ductwork.

Air Leakage

Air leakage is like tossing dollar bills into the air and waving goodbye to them as they blow away in the breeze. Small holes, open seams and ill-fitting ducts cause air to move from the pressurized ductwork out into your walls, ceiling or attic.
Whether hot or cool air is escaping, this air leakage represents an expensive waste of money.
Truly professional HVAC technicians work very hard to install ductwork free of any leaks. They also pressure-test the system to check for leaks and then close them, if any are found.

Zone Pressurization

A third issue with sloppy ductwork design and installation is zone pressurization. Since many modern duct designs do not include return air registers in every room, the system balances by keeping doors open.
When a door is closed, the supply duct pressurizes that room while delivering a normal airflow to other rooms. The pressurized room leaks air out around windows, robbing your home of energy efficiency.

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