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From the simplest device to the most complex system, design affects function. When it comes to HVAC equipment and the systems that heat and cool your home, the concept of design affects not only the comfort level in your living spaces, but how much you pay to maintain that comfort. The ductwork system that extends from your central furnace, air conditioner or heat pump is a vital component of your HVAC system, and its design has a significant impact on the efficiency and performance of your heating and cooling equipment. The following brief introduction to ductwork design can help you understand how ducts affect heating and cooling systems, how they work with your HVAC equipment to maintain indoor comfort, and how they can improve heating and cooling efficiency while reducing your monthly costs.

The Basics of Ductwork

The basic function of ductwork is to provide a distribution network for the heated or cooled air that comes from your furnace, heat pump or A/C. Ductwork is the system of large pipes or tubes extending from your main HVAC equipment to points throughout your home. The duct system is essential to making sure heated and cooled air gets into your indoor spaces. Without ducts, this conditioned air would have no way to travel throughout your home.

Your HVAC equipment creates, at the simplest level, a large air circulation system that continually refreshes the supply of heated or cooled air that maintains your home’s indoor temperatures. The HVAC unit produces heated or cooled air that is blown into the supply ductwork by powerful fans. The conditioned air travels the length of the ductwork and exits from vents attached at several different termination points. The conditioned air enters rooms and areas in your home, where it heats or cools your indoor spaces as appropriate. Expended air that has lost its heating or cooling energy is brought back into the system by return ducts. The expended air travels the length of the return ducts and back to the HVAC unit, where it is filtered and heated or cooled again. The cycle then repeats, with freshly conditioned air being sent back out through the supply ducts.

Why Be Concerned About Duct Design?

The nature of the forced-air HVAC air circulation system means that all of the conditioned air that affects your indoor environment travels through the ductwork. This means that any problems with the ductwork will affect how efficiently that conditioned air travels, and how much of it actually reaches the inside of your home to do the job of heating and cooling. Duct design, and the condition of the ductwork itself, has a substantial effect on this air distribution.

Poorly designed ductwork can decrease the efficiency of air distribution by as much as 75 percent. What’s worse, badly designed and installed ductwork, damaged ductwork, or improperly sealed ductwork can account for the loss of small percentage to 100 percent of the conditioned air traveling through the ducts. With that much air being lost:

  • Your HVAC equipment will have to struggle to keep your home at the temperature you want. This will increase wear and tear on components and parts and make it more likely that a malfunction or breakdown will occur. Your HVAC equipment’s lifespan will be shortened, and you will have to make repairs that could have been prevented.
  • You will lose an ample portion of the heated or cooled air that you’ve already paid for. This air loss will result in a substantial loss of energy and money.
  • You will have to pay for additional heated or cooled air to make up for the conditioned air lost to air leaks or inefficient duct design. This will make your monthly energy bills much higher than they need to be.

Poorly designed or damaged ducts also increase the chance that particulates, contaminants and even dangerous exhaust gases could be pulled into the air distribution network and sent into your indoor environment.

Troublesome ducts can suck dust, pollen, mold, fibers and other particulates into the system and out into your home’s air, reducing indoor air quality. Increased amounts of contaminants in your indoor air can trigger asthma or allergy attacks and threaten respiratory health.

Excessive amounts of particulates could cause your forced-air system’s filters to get dirty quicker, resulting in the need for more frequent filter changes. Clogged filters will reduce airflow within the HVAC system, which is a major cause of equipment damage and malfunctions.

Backdrafting can allow dangerous gases such as carbon monoxide to be pulled into the ductwork and discharged inside your home. Carbon monoxide, which is produced as a byproduct of burning fuel, is extremely dangerous and potentially lethal.

Effective Duct Design

Your HVAC professional should be well aware of accepted industry standards and best practices for duct design and installation. Some of the most important elements of duct design include:

  • Applying a standard ductwork configuration: Ductwork is usually designed and installed in patterns that promote efficiency and performance. These patterns will affect how well conditioned air is distributed. Sometimes space restrictions or other factors will influence the type of duct design pattern that can be used. Whenever possible, however, stick with proven duct configurations that have been shown to be effective. The two most common designs are radial, which has the HVAC unit in the center and ducts radiating outward from that point, and trunk-and-branch, which uses one large “trunk” duct with several smaller ducts branching out from it.
  • Sizing the ductwork correctly: The physical size of the ductwork will affect how well it functions. Ducts that are too small won’t be able to carry enough air to keep your home heated or cooled. Air velocity could also increase, which will make the ducts noisy. Too-large ducts can waste air and energy, which increases costs. Ask your HVAC professional to size your ductwork using accepted industry standards, such as those published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).
  • Ensuring enough return ducts: Airflow within the system will be affected by how many return ducts are available to bring air back for reconditioning. If the system can’t get enough air back to the heating or cooling equipment, it won’t be able to produce enough air for conditioning. Make sure there’s a return duct in each room that receives heated or cooled air from a supply duct, or fixtures such as louvered doors that allow air movement between rooms. If needed, install return air grilles in large areas such as hallways and bigger rooms, or under stairwells. Jumper ducts can be used to connect duct sections between rooms.
  • Putting supply duct vents in areas away from exhaust fans or ventilation systems: If supply ducts are too close to exhaust equipment, conditioned air can be sent outdoors before it has a chance to benefit your indoor environment. This results in a large and unnecessary waste of conditioned air. Make sure supply vents, where the heated and cooled air exits into your home, are placed well away from exhaust or ventilation fans.
  • Making sure the ductwork is properly installed, sealed and insulated: All sections of ductwork should fit together tightly, creating a solid connection. Sections can be mechanically sealed with sheet metal screws or other appropriate fastener to make connections stronger and less likely to come loose over time. All connections should be sealed with duct mastic, a specialized rubbery sealant intended for use on ductwork, or with strong metal tape. Ironically, standard duct tape isn’t recommended for this task; the adhesive on duct tape tends to dry out and let the tape fall off. Ducts should be insulated with rigid fiber board insulation or wrapped with standard blanket-style insulation. This will prevent energy loss through the relatively thin material of the ducts themselves.
  • Avoiding putting ductwork in unconditioned areas: If possible, ductwork should be installed in or very close to areas of your home that already receive conditioning. Putting ductwork in uninsulated attics or basements, crawl spaces or garages will increase the chance of energy loss as conditioned air travels through the ducts. When ducts do run through these areas, make sure the ducts are properly insulated.
  • Using a “hard-ducted” system: Make sure the ductwork system uses actual duct material from end to end. Some older configurations may use a crawl space, spaces between walls studs, under-floor areas, or other building cavities for air distribution. This will result in substantial air and energy loss and a decrease in HVAC system performance.
  • Evaluating the condition of existing older ductwork: If you already have ductwork in place, make sure it still works properly. Determine its age and condition. Make sure it’s providing enough airflow into rooms in your home. Have the duct system tested for efficiency, leaks and insulation, and whether the duct design should be revised.

Griffith Energy Services, Inc. applies its more than 100 years of professional experience to every job it does for HVAC customers in Baltimore and the surrounding communities. Contact us today for more information on the importance of duct design, and for expert help designing and installing a ductwork system that will keep your home comfortable in all seasons.


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