It’s nice to find relief from a hot and humid summer’s day inside a cool and comfortable home, or to shake off the chill and keep warm and cozy during a long Chicago-area cold spell. In order for your home to stay comfortable from season to season, the air conditioner and furnace must have an efficient means to deliver cooled and heated air to your living spaces. This air-delivery system is your home’s ductwork.
If you’re upgrading your HVAC equipment, retrofitting, adding a room to your home or undertaking new construction, take a tour of a well-designed duct system and learn how it plays an integral role in home comfort, energy savings and the performance of all your home-comfort systems.
Components of a Ductwork System
Tucked away out of sight, perhaps in the attic, basement or other inconspicuous area of your home, is a network of tubes which provide a channel for the cooling and heating systems to deliver cooled and heated air to the living spaces. These tubes are most often constructed of metal, rigid ductboard or plastic-lined flex ducts, but the duct tubing is only one component of a ductwork system. The other components, strategically and professionally designed and installed, are equally important for the efficiency of the ductwork system. These are the components:
- The plenum is the “root” of a ductwork system which joins the duct trunk to the furnace/air-conditioning blower.
- Supply ducts extend from the duct trunk or plenum to the air-supply outlets.
- Return ducts provide an air-return channel from return grilles to the furnace/air conditioner.
- Air-supply outlets are typically located on floors, high on walls or ceilings, and serve as a connection point for supply ducts which allows supply air to reach the living spaces.
- Air-return grilles are typically larger and fewer in number than air-supply outlets. However, for optimal air supply and air return, it’s advantageous to install an air-return grille for each air-supply outlet.
- Jumper ducts and transfer grilles provide a channel for unrestricted airflow from one room or area to the next. These components are commonly used when obstructions to free airflow exist between supply outlets and return grilles, such as doors or sometimes room additions.
- Collars, screws, mastic, metal tape and mesh are the components used to seal duct connections. Mastic, metal tape and mesh are also used to seal larger holes and rips in ducts.
- Metal ducts are round or rectangular bare ducts. Galvanized steel is the metal of choice, as the zinc coating is resistant to mold and is easier to clean.
- Rigid ductboard is typically manufactured from resin-bonded glass fibers (compacted fiberglass). High-quality ductboard is extremely energy-efficient, resists moisture, delivers excellent acoustic properties, provides better indoor-air quality (IAQ) and more.Flex ducts are accordion-style ducts consisting of a plastic membrane wrapped in insulation.
- Flex ducts are very convenient and efficient for tight spaces and turns, but still require expert knowledge for proper sealing and installation despite their simple maneuverability and handling.
Objectives of Good Duct Design
Good duct design involves configuring a duct layout acquiescent to the design of the home to maximize home comfort and energy-efficient heating and cooling, and to promote good IAQ. These are the objectives of good duct design:
- Minimize heat-energy losses through duct walls by taking the shortest route possible from blower to supply outlets while utilizing spaces within the insulation barrier, which could include raised floors, drop ceilings and sealed chases. Ducts located outside the insulation barrier (e.g. attic, basement or crawl space) should be adequately insulated to minimize heat gains and losses.
- Manual D from the Air Conditioning Contractors of America (ACCA) should be used to properly size the ductwork to ensure the pressure drop across the blower is within manufacturer specifications.
- Maintain neutral air pressure inside the home with balanced air supply, air return and free airflow through the living spaces.
- Duct seams for metal ducts should be impermeably sealed using sheet-metal screws, mastic sealant and metal tape. Expansion collars and metal tape are used for sealing flex ducts. It’s wise to install an expansion collar at the plenum and blower connection to reduce or eliminate loud “boom” noises created by the expansion and contraction of metal ducts due to temperature changes.
- Metal ducts should be installed with one inch or more of clear space between the duct walls and any home structure, such as timber, pipes and walls. Due to temperature changes inside ducts, metal ducts expand and contract. Any metal ducts that come into contact with other structures may emit annoying noises during this expansion and contraction.
The number of return grilles should equal the number of supply outlets. If this is not practical, measures should be taken to ensure free airflow through the living spaces from the supply outlets to the return grilles. This may include the installation of transfer grilles in doors and walls and/or jumper ducts connecting one room or area with another.
The two layouts most suitable for ducts installed within the conditioned spaces of a home are radial ducts and trunk-and-branch ducts. Each configuration has the potential to efficiently meet all the requirements for good duct design. The radial configuration consists of duct runs that extend from the plenum, much like the spokes on a wheel extend from the hub. This configuration delivers a few benefits over trunk-and-branch:
- Radial duct systems, in general, distribute air more evenly to the supply outlets than trunk-and-branch configurations because there are not multiple runouts which affect pressure. Equal amounts of heated or cooled air enter each branch of a radial system at the plenum.
- A zoning system is more efficient with a radial duct system when the automatic duct doors are properly installed near the plenum.
- A radial duct system contains fewer duct seams, which means fewer locations for air leaks to develop.
The trunk-and-branch configuration consists of a large duct “trunk” which connects to the plenum and extends for the length of the home. “Branches” (runouts) extend from the trunk to the supply outlets. For optimal airflow from trunks to branches, utilize “Y-branches” for runouts rather than right-angle “T-branches.”
Ductwork Inspection and Testing
Professional ductwork inspection and testing should be performed after any modifications, repairs and new installation to confirm the integrity of the ducts for air leaks and air pressure.
A visual inspection of the duct system seeks to locate obvious air leaks, loose connections and uninsulated ducts in non-conditioned spaces. While a visual inspection is generally a straight-forward procedure, sometimes ducts are located in inaccessible areas, such as sealed up inside walls. If a duct system has been tested and sealed, but issues with uneven air supply still exist at the outlets, your HVAC professional may need to use video in these spaces to inspect ducts for disconnections and leaks. It's best to avoid such circumstances in the first place with good duct design.
Professional testing involves the use of a blower door to pinpoint small leaks, and to help test air pressure through the ductwork. With new duct installation, it's important to test ductwork before any insulation is applied to the surfaces, or else potential areas for leaks will be obstructed and obscured from view.
Repairing Inefficient Ductwork
Repairing an inefficient duct system may involve sealing air leaks or loose joints, adding insulation, untangling flex ducts or any number of other issues that adversely affect efficient airflow.
Unusual noises are prime indicators of duct issues. Rattling noises indicate loose duct connections at seams or at supply outlets. Loose duct connections (and air leaks) are best remedied with mastic sealant and metal tape. Loose connections at supply outlets are best solved using mastic, metal duct collars and metal tape to make a lasting air-tight seal.
Banging noises may be a sign that ducts are coming into contact with building structure -- expansion and contraction. You may try installing acoustic dampers to the surface of ducts to help quiet the noise. Otherwise, the ducts would need to be repositioned by a professional.
While professional duct repair yields the best results, you may also perform a visual inspection of your ducts and make subsequent repairs if needed. However, ducts are usually located in tight spaces or in the attic where nail protrusions and other hazards exist. You may also run the risk of falling through the ceiling, depending on the design of your attic. Excessive heat and cold are issues with which to contend for much of the year, too.
That being said, basic do-it-yourself duct repair is possible if you're handy with tools. For best results, ask your HVAC professional about duct inspection and repair during your annual cooling and heating systems preventive maintenance.
If you would like more information about the repair, inspection or installation of your ductwork system, contact a professional HVAC contractor who understands the integral role ducts perform in an HVAC system. Call Comfort24-7 today, and we'll be happy to help. We proudly serve homeowners throughout the greater Chicago area, as well as NW Indiana and SW Michigan.