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Ductwork Design: What Makes Successful Airflow and What Commonly Inhibits It

Published on: September 4, 2013

No matter what its energy-efficiency rating, your furnace and air conditioner are only as good as your ductwork allows. A well-planned ductwork design delivers the conditioned air your system produces with minimal waste. With the hot summers in the Southeast, especially in Mississippi, Alabama, and Georgia, efficient ductwork design is essential for controlling your cooling bills.

As important as the ducts are, though, it’s rare to find a duct system without major flaws. The average duct system in the U.S. runs at just 60 percent capacity due to undersizing, according to 1994 Department of Energy estimates. And that’s just one potential problem. Other common problems such as air leaks, sharp turns, and excessively long ducts abound. All of these cut your system’s capacity and energy efficiency.

Whether you’re planning a new home or preparing for renovations, knowing what goes into developing an effective ductwork design will help you ensure that your ducts perform as efficiently as possible. Well-designed ductwork benefits you in several ways.

  • Effectiveness — Correctly sized ducts deliver warm or cool air in a way that makes it easy to maintain a comfortable, even temperature in all your rooms.
  • Efficiency — Good ductwork design is free of issues that impair airflow. This lets your system reach optimal efficiency and reduces wear on your system’s components.
  • Health and safety — Well-designed ducts deliver clean air and maintain balanced air pressure in your home. This minimizes your exposure to molds, fumes from fuel-burning appliances, and other pollutants.

How Effective Systems Are Built

A duct system may look simple, but there’s a lot more to designing one than just hooking up some metal tubes. To create the ideal ductwork design for a home, a heating and cooling contractor considers duct location, material, sizing and layout, among other issues.

Integrated design — Ideally, the heating and cooling system should be considered early in a new home’s schematic design phase. This gives the builder a chance to allow enough space in the right areas for the equipment and ductwork. The air handler can be placed in the center of the home, which allows for shorter, more efficient ducts and well-balanced airflow. The ducts can be located in the interior walls and ceilings where they’re less prone to energy loss.

Heating and cooling load calculation — To help HVAC contractors accurately calculate a home’s heating and cooling needs, the Air Conditioning Contractors of America (ACCA) outlines a detailed process in Manual J. Manual J calculation results help your contractor size your heating and cooling equipment and also provide critical information for sizing the ducts. Ideally, your contractor will perform room-by-room calculations, rather than one calculation for the whole house.

Material selection — Sheet metal (galvanized steel), fiberglass duct board, and flex duct (metal wire covered by plastic) can all be part of an efficient ductwork design. Because different duct materials have different friction rates, though, material is an important design consideration. Friction creates drag and slows down airflow, decreasing energy efficiency. For example, sheet metal ducts produce less friction than flex duct.

Fittings selection — Duct fittings include collars, joints, connection cleats, and transitions. These are used to connect ducts to the air handler, registers, and grilles as well as join lengths of duct. While the material matters, the bigger issue is the angles created by the fittings. Sharp turns slow airflow. For example, a T-shaped tee fitting slows airflow more than a Y-shaped wye fitting. In terms of reduced air speed and efficiency, a 90-degree angle in your ducts is like adding 35 feet of duct.

Duct sizing — Once the contractor knows where the ducts will be installed and which materials will be used, the ducts can be sized. For this, some contractors use ACCA Manual D, which accounts for variables like friction and pressure. Other contractors use duct calculators, although these provide less accurate results.

Straight, short, well-supported ducts — Sharp turns, twists, bends and long runs all create drag in your system. Where turns are necessary, contractors aim to use gentle angles. Even so, the duct size may have to be enlarged to compensate. Ducts should also be supported correctly with suspension rings, brackets or straps to prevent sagging, which also interferes with airflow.

Common Design Flaws

Despite all the benefits of an efficient ductwork design, system flaws are the norm in most homes. These flaws lower your heating and cooling system’s energy efficiency, reduce your comfort, and compromise your indoor air quality. When you’re aware of the common flaws, though, you can work with a contractor to find solutions.

Ducts in unconditioned spaces
— When the HVAC system is designed after the house plans are complete, the contractor is left with little choice of where to install the air handler and ducts. Ducts may have to be installed in unconditioned spaces such as the attic or crawl spaces, which reduces their efficiency. You may not be able to move these ducts, but you can minimize heat transfer by insulating them with duct wrap or fiberglass insulation to at least R-8 level.

Overuse of flex duct — Flex duct is inexpensive, easy to install, and more flexible for use in tight spaces. It also creates more drag than other duct materials and is prone to becoming twisted or kinked, or collapsing in places. Because of these downsides, flex duct should be kept to a minimum and the trunk (main) line should never be made of flex duct.

Long duct runs and sharp turns — When the air handler is located far from any part of the house, long ducts must be installed to deliver air there. Long ducts are likely to require multiple turns and joints. The longer and less straight the duct, the more energy it takes to push air through and the greater the risk of leakage and heat transfer. These ducts also pick up more dust.

Incorrectly sized ducts
— Insufficient heating and cooling, noise and drafts are all symptoms of poorly sized ducts. You may also find hot and cold spots around your home because the air is delivered too slowly to mix well with the room air. Undersized ducts force the fan motor to work harder, potentially wearing out your equipment faster.

Lack of return-air ducts
— Every conditioned room should have a return-air grille to bring conditioned air back to the air handler. Many homes have a return-air grille in only one or two rooms, which reduces the system’s energy efficiency and creates pressure imbalances. Where installing return ducts isn’t possible, transfer grilles and jumper ducts help.

Unsealed ducts — In some cases, duct joints are simply pushed together instead of being properly sealed. Ducts like these can leak up to 20 percent of the air they carry. Leakage is even greater in high-efficiency systems. These systems are designed to run for longer periods at low speeds, which means the air stays in the ducts longer and has more time to leak. Unsealed ducts also pull in air contaminants and draw in humidity that encourages mold growth. Fortunately, this problem is easy to solve by sealing the duct joints with mastic gum or metal-backed tape.

Watch Out for Blockages, Too

Needless to say, for your system to work well, air needs to flow through unobstructed. While regular heating and cooling system maintenance help, not even the best ductwork design is immune to obstructions. Clogged filters, fallen insulation and malfunctioning dampers are all possible causes. Even partial blockages in your ducts reduce airflow to one or more of your rooms. Common blockage sources include:

  • Clogged air filter — An air filter that’s clogged with dust and dirt allows air pressure to build up. That pressure can eventually knock the filter out of place and blow it into the ducts where it blocks airflow.
  • Damaged insulation — Fiberglass ducts contain interior insulation that can break loose and fall into the duct, preventing normal airflow.
  • Damaged ducts — Flex duct is prone to collapsing or becoming pinched. The resulting blockage can either slow or stop airflow, or redirect it.
  • Stuck damper — Your ducts contain fire dampers designed to prevent a house fire from spreading via the ductwork. These dampers can become stuck closed or partly closed, blocking airflow. If you have a zoned system, the zoning dampers can also become stuck.
  • Blocked registers and grilles — If even a few of your registers or return air grilles are blocked by furniture or other items, it can affect airflow throughout your entire home. To maintain balanced airflow, keep your registers and grilles clear.

If you notice a sudden change in output or airflow from your system, contact an HVAC technician. Running your system with an obstruction wastes energy, strains your fan motor, and may even create a fire hazard.

Because so much depends on proper ductwork design, the job should be entrusted only to an experienced, knowledgeable contractor. With the free Find a Contractor service we offer at AC Southeast, you can find a skilled local contractor wherever you live in Mississippi, South Carolina, Georgia, Florida or Alabama.