ductwork designBetween your furnace or air conditioner and creating a comfortable environment in your living spaces you can find the principles of proper ductwork design. Airflow is your system’s life force, and ducts are the conduit that makes your home a livable, energy-efficient environment. Unfortunately, the reverse is also true. Where ducts are not accorded appropriate attention and priority – whether in the ductwork design and layout or the installation of the ducts – a variety of problems relating to high operating costs, low comfort and even unhealthy indoor air quality may plague the homeowner, often going unresolved for years or even decades.

Best Air Duct Guidelines

Proper ductwork design has multiple objectives:

  • To distribute conditioned air in appropriate volume to all rooms in the home and at a consistent temperature to meet thermostat settings.
  • To balance supply air entering each room with return air drawn out of the room in order to produce neutral air pressure, the optimum condition for best comfort, performance and energy efficiency.
  • To allow your furnace and air conditioner to meet manufacturer’s efficiency ratings while placing minimal load and wear and tear on the system.
  • To prevent infiltration of unconditioned air into the system and avoid contamination of indoor air quality.
  • To keep operating costs low and reduce the environmental impact of your home.

Duckwork Should Be Calculated

None of the above happens by accident, nor by guesswork or applying ballpark estimates instead of proven scientific design methods individualized to each specific home. While ductwork design often received less consideration in past eras of cheaper energy, today it takes proper precedence in new homes and is often included in renovation plans for existing homes, where ducts can be upgraded as part of the project. Getting from the ductwork design goals above to a working system that performs under the day-to-day, all-season demands of the average household means adopting certain established tenets of good ductwork design that are standard in the industry.

  • Calculate cooling and heating loads first. Using Manual J, the industry-standard load calculation software, the exact amount of BTUs of heating and cooling required to keep each room comfortable can be accurately determined. From this data, an HVAC professional can utilize the Manual D duct sizing program to figure the supply air requirements of every room and the diameter of the ductwork necessary to deliver that volume.
  • Take the shortest and straightest route possible from the air handler to each room. Reducing the physical length of ductwork by using proper design principles increases energy efficiency and performance. Unnecessarily long spans, as well as the excessive use of curves, bends and elbows in ductwork layout, create internal air friction that increases the effective length of ductwork far above its actual physical length.
  • Use conditioned routes if possible. Design the layout to route as much of the ductwork as possible through heated or cooled zones of the home. Where ducts must pass through unconditioned zones like the attic or crawl space, insulate against thermal loss or gain to a level of R-8 for supply ducts and R-6 for return ducts.
  • Include a dedicated return duct for every room with a supply register. Where the ductwork design mandates a single central return, instead provisions such as air pass-through grills or jumper ducts connecting rooms without returns should be made to ensure a clear air path back to the central return.
  • No substitutes for the real thing. Most local codes today prohibit using structural cavities in the house such as the spaces between ceiling or floor joists or interior wall voids as a replacement for proper ductwork. Use only “hard” ducts of sheet metal, fiberboard or approved flexible ductwork.
  • Make air balancing easy. Designs should specify manual dampers in each branch duct to facilitate air balancing to individual rooms. These should be located at the take-off point where each branch duct diverges from the main trunk duct.
  • Mechanically secure all spans of ductwork with sheet metal screws or other fasteners, then seal each joint with mastic. Using a standard duct blower method, pressure-test the finished system to check air flow and verify leakage. Most local codes today enforce maximum allowed leakage specs for new ductwork or when installing upgrades to existing systems. Permissible leakage varies per location, but typical figures average around 6 percent of the total airflow volume for new construction or 15 percent when existing ductwork is being replaced or a new A/C or furnace installed.

For more information on the efficiency and performance benefits of professional ductwork design, consult the professionals at Air Specialty.

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