Without efficient, effective ductwork design, the cooled and heated air produced by your air conditioner and furnace goes nowhere. In the typical home, between 400 and 700 cubic feet per minute of conditioned air circulates through the ducts. It’s an established fact that in homes with ductwork extending throughout all living spaces, up to 30 percent of that air never reaches the rooms it’s intended for. Wasted through ductwork leaks, pressure imbalances and thermal losses, this is household comfort you get the bill for every month – but never get the benefit of.
Done right, ductwork design is calculated utilizing professional software, constructed of quality materials, and then maintained with periodic leak detection tests, sealing procedures and, should the need arise, professional duct cleaning.
Ductwork design wasn’t always the precise science it is today, particularly in residential applications. Back when energy prices were lower, mass-produced homes often received one-size-fits-all ductwork, designed and fabricated for lowest cost to the builder rather than highest energy efficiency and comfort value for the homeowner. Since ductwork is usually permanently installed unless or until a major duct failure occurs or the home is dismantled for renovation, owners of these homes often have been stuck with the consequences of cost-cutting in ductwork design for decades. Rising operating costs, inconsistent temperature control and even poor indoor air quality are common results. In many cases the real reason behind these shortfalls isn’t even recognized, but simply dismissed as the status quo for an energy-inefficient home.
The Manual Approach
Adhering to state-of-the-art principles of ductwork design means going by the book. Except, it’s not a book anymore; it’s industry-standard duct design software published by the Air Conditioning Contractors of America. Manual D is the principle reference for determining the optimum specifications of ductwork installation. It incorporates heat and cooling load data generated by Manual J load calculation software that determines the exact BTUs of cool or hot air required to maintain each room at a comfortable temperature.
After performing the load calculation, a qualified HVAC contractor will reference Manual D to make critical decisions about the diameter of the ducts, the layout of the ductwork, and the use of attachments like elbows, tees, diffusers and grilles, all of which directly affect airflow, the lifeblood of any forced-air heating and cooling system.
Good ductwork design is not an afterthought, nor is it separate from other aspects of the construction process. It’s part of the overall home design, from the drawing board up. Taking duct efficiency and performance issues into consideration begins early, before construction of the home or the start of major renovation, and ideally results from close collaboration between the architect, the builder and the HVAC contractor. Here are some of the guiding concepts that characterize professional ductwork:
- Short and straight. By situating the air handler as centrally as possible to the system, the length of supply and return ducts can be minimized. The shorter the effective length of the ductwork, the less resistance to airflow and fewer potential spots for leakage. Ducts that incorporate an excessive number of right-angle turns, frequent bends and unnecessarily long spans are generally not energy efficient.
- Conditioned response. Where possible, ductwork should be routed only through conditioned zones of the home. Installing A/C ducts in an unconditioned attic that soars to 120 degrees or more during summer is a recipe for thermal loss and poor efficiency. Same goes for heating ducts running through acutely cold zones like the crawl space under the home. In addition, because leakage in return ducts sucks air into the system where ducts are routed through unconditioned areas, air drawn into the system through leaks may be tainted with mold spores, dust, insulation fibers and other contaminants.
- One supply, one return. Ideally, every room where a supply vent is installed should have its own dedicated return duct to convey air back to the air handler. This arrangement facilitates precise system air balancing and allows air volume into rooms to be accurately tweaked to the exact load requirements of the room. Where costs prohibit dedicated return ducts, a central return may be installed in a common area such as a hallway. However, accommodations must be included in the design to ensure a clear air path back to the central return from all rooms. These may include air pass-through grilles in room doors and/or jumper ducts installed in ceilings to link airflow between multiple rooms, then convey it to the central return duct.
- Avoid the void. One of the most energy inefficient practices of duct design in the bad old days was utilizing wall voids, spaces between ceiling joists and other structural channels as substitutes for proper hard ductwork. When these building voids are enclosed and used to move air, leakage is high, both into and out of the airflow. As the house ages and wooden materials shrink, air loss increases and operating costs continue to rise. Air quality is also compromised. All ductwork should be fabricated of sheet metal, fiberglass or approved flexible duct material, and installed according to current accepted practices for efficiency and healthy air quality.
- Seal the deal. All duct spans should be sealed at joints, elbows, tees and other intersections. Today’s standards call for sealing all junctures with mastic and/or foil tape. Duct tape is not an acceptable recourse. In addition, all junctures should be mechanically fastened with screws – not merely press-fit together – and lengthy spans should be properly supported with brackets to prevent sagging over time.
- Strike a balance. Airflow into rooms should be balanced using air volume measurement devices to ensure that incoming airflow through supply vents is equal to the amount drawn out through return vents. This establishes a state of neutral air pressure in each room, the optimum condition for efficient cooling and heating. When the air balance tips toward the positive and the room is over-pressurized, conditioned air is forced out through structural cracks and gaps. When the room is under-pressurized due to insufficient supply air, hot or cold humid outside air may be sucked indoors, as well as unhealthy, unfiltered air from the attic or crawl space.
After design and installation, the entire system should be pressure-tested. A duct blaster test utilizes a fan to pressurize the duct system, plus sensors to measure internal pressure and airflow through the fan. Linked to a computer, data from these sensors allows the system to calculate the extent of air leakage from the ductwork. Most local building codes now impose limits on maximum leakage, both when ductwork is initially installed and later, when HVAC equipment is upgraded or the duct layout is extended for home renovation.
With the system pressurized, the HVAC technician can inject artificial smoke into the ductwork to identify the location of leaks for sealing. Where numerous small pinhole leaks are discovered in existing installed ductwork, aerosol sealant may be injected into the ducts to seal the leaks from the inside out.
Duct Cleaning—Or Not
Professional duct cleaning may or may not be advisable later in the service life of the system. It all depends upon the circumstances and specific existing conditions of the ductwork. Here are some scenarios that may warrant further consideration of duct cleaning:
- If active mold growth is discovered inside ductwork – and confirmed by laboratory tests of mold samples, not just visual observation – cleaning and disinfecting ducts and attached HVAC equipment is advised to stop the circulation of toxic mold spores throughout the home.
- If ducts have ever been exposed to water infiltration that spawns mold growth, from a leaky roof, for example, an inspection of the interior spans of ductwork by a professional with the necessary equipment to do a thorough job is called for.
- If the presence of rodents or other vermin is discovered inside ductwork.
- If dust is observed emanating from registers when the blower fan cycles on. Over time, ductwork may simply accumulate excess common dust deposited by years of circulating air. In this case, duct cleaning may be advisable, particularly if occupants of the home are susceptible to allergic reactions to airborne dust.
For more information on proper ductwork design to promote efficiency and comfort, please contact the professionals at Logan Heating, Air Conditioning & Electric.
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