If your home’s more than 20 years old, chances are efficient ductwork didn’t occupy a high priority in its design and build process. When energy prices were low, other considerations such as cost-cutting and quick installation likely took precedence. Ductwork design in the old days was frequently relegated to rule-of-thumb engineering based on a one-size-fits-all philosophy. All that changed when energy costs skyrocketed and poor ductwork design was identified as one of the principle suspects in high utility bills, inferior household comfort and even unhealthy indoor air quality.
Today, avoiding duct design shortcomings—or correcting them where they exist—is recognized as a critical part of maximizing home energy efficiency and optimizing performance. The scientific approach to duct design in residential forced-air heating and cooling is based on the concept of the entire building as a system in which all HVAC-related components interact. In that scenario, airflow is the blood and ductwork is the circulatory network. Shortfalls in one area inevitably affect efficiency or performance in another. Ducts in today’s homes are designed into the structure early in the process. Consideration is given to how proper duct layout can be integrated into the plans and, where necessary, structural design changes may be made to accommodate the requirements for efficient, effective ductwork.
Whether it’s a planned home still on the drawing board or an existing house about to undergo renovation, these are some of the principles of good ductwork design for 21st century standards of energy conservation and enhanced comfort performance.
Ductwork Design Principles
- Design by the book. Actually, it’s not a paper book anymore—it’s software. Manual J and Manual D are calculators published by the Air Conditioning Contractors of America and essential to proper duct design. Manual J computes the heating and cooling load—the BTU’s and cubic-feet-per-minute volume of hot and cold air, respectively—that must be delivered to each room to maintain a comfortable temperature. Manual D integrates that data into calculations that determine the optimum main trunk and branch duct diameters as well as the most efficient layout and design. These critical factors cannot be left to “guesstimates”
- Place the air handler in the layout as close as possible to the center of the home. Also, minimize the number of elbows and bends incorporated in the design. This decreases factors such as static pressure and internal air friction that affect airflow.
- Route as much ductwork as possible within the conditioned envelope of the home. Ducts that run through the attic, crawlspace, unfinished basement or any other zone subject to acute temperatures suffer thermal losses or gains that impact energy efficiency. Where unconditioned routes are unavoidable, supply ductwork in these zones should be insulated to a level of R8 and return ducts to R6.
- Don’t place ducts in exterior walls. Ductwork inside an exterior wall takes up vital space that should be occupied by insulation, instead, to retard heat transfer to or from the outdoors. Also, ducts routed through exterior walls are more likely to create condensation inside wall voids, an invitation to rot and mold.
- Ideally, include a dedicated return duct in every room with a supply vent. This creates the best scenario for balanced airflow. However, in houses where a single central return exists in a common area like a hallway, ensure that a clear air pathway exists from the conditioned rooms to the central return. Installing air pass-through grilles in doorways or jumper ducts in ceilings that link multiple rooms to the vicinity of the central return are options to maintain a free flow of return air.
- Install only approved “hard” ductwork. In the past, building cavities such as wall voids or joist bays in the ceiling were simply covered with wood and utilized as substitute return ducts to cut costs. All ducts should be of sheet metal, fiberglass or approved flex duct. In most areas, converting structural voids to use as ductwork now violates local building codes.
- Seal ducts at installation and mechanically fasten duct spans. Use mastic, not duct tape, to seal spans of ductwork as they are assembled. Then secure the joints between spans with sheet metal screws.
- Pressure test with a duct blower to verify proper sealing before ducts are enclosed in walls or ceilings. In most locales, the standard for duct sealing requires limiting tested leakage to the range of 5 to 10 percent of total system airflow.
For more on the guidelines for proper ductwork design and installation, contact the professionals at Goose Creek.
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