If a home is having issues with the air distribution and flow, improperly designed or installed ductwork may be the cause.
Poor design, improper installation, and lack of proper maintenance of a residential HVAC duct system can contribute to occupant discomfort, excess energy consumption, shortened equipment life span and poor indoor air quality. On the other hand, a well-designed and -installed duct system that is properly sized and configured to ensure efficient airflow will minimize energy losses and stress on the equipment, as well as help to maintain occupant comfort.
The Basics
Residential ducted air systems are HVAC systems that use motor-driven fans to distribute air for occupant comfort within designated interior rooms and spaces. These are often referred to as “forced-air” systems. Most residential ducted HVAC systems include common components, such as a heating and cooling unit, ductwork, supply registers, return-air grilles and automatic controls. Accessory equipment, like humidifiers, ultraviolet lights and filtration systems, may be used to enhance the performance of the heating and cooling systems and can provide specific space-conditioning benefits.
These air-distribution systems vary in the type and amount of ductwork used. Most forced-air systems require ducts that use custom shop-fabricated fittings and manufactured fittings that when assembled form the duct distribution system. Ductwork is often routed with many constraints that may include limited space for the duct distribution system, routing problems and obstructions, minimizing overall first cost and certain code restrictions. Careful duct system layout and sizing is required to properly distribute the conditioned air throughout the home while providing consistent comfort to the occupants.
The sizing, routing, type of duct transitions and the amount of flexible duct used can have a significant impact on the energy efficiency of any ducted air system. Selecting a duct size or system design that is marginal for the required airflows will increase duct static pressure, which in turn will decrease the air volume delivered to the space. Reduced air volume negatively impacts the HVAC equipment efficiency. Poor duct design can result in uneven space conditioning with hot and cold spots. Abrupt changes in airflow direction, large numbers of elbows and fittings, excessive lengths and improper installation of flexible ducts will also decrease the overall HVAC system airflow volumes, which reduce the HVAC system efficiency. To help promote smooth airflow and reduce energy consumption, the layout of duct runs should be optimized to minimize excessive fitting use, restrictions and obstructions whenever possible.
The Science
Proper load calculations are essential for determining the heating and cooling requirements of a residential space. Load calculations take into account various factors, such as square footage, insulation levels, window types, orientation and local climate conditions. Once the loads are calculated for all spaces, the total load is used to size and select the HVAC equipment, as well as properly size the duct distribution system. This ensures each room receives the correct amount of conditioned air. Each supply register and return-grille size will be based on these airflow requirements. The largest airflow (heating or cooling airflow) must be used to size each register, grille and branch duct, which will, in turn, be used to size the main duct supplying the branch ducts. Duct sizes are selected based on airflow velocity, friction loss, and static pressure while factoring in the available ductwork routes and installed system components.
The best practice is to locate the air handler or furnace as close to the center of the zone to be served as possible. This provides the best starting point for a superior duct system. A well-designed, properly sized and sealed duct system having gradual transitions, smooth airflow and minimum energy losses will save far more over time than the added initial cost to install a quality duct system. Ducts installed entirely within the insulated enclosure of the home, when properly designed and installed, have the lowest energy loss. When ducts and plenums carrying conditioned air are located outside the insulated enclosure of the home or conditioned space, the heat gain or loss must be accounted for in the design air quantity and total sensible load. System load must be calculated when routing ducts through attics, outdoors or other unconditioned spaces. Alternate routing of ductwork to avoid these conditions may be more desirable than increasing the system load. Furthermore, the use of additional insulation on the ductwork will prevent condensation and greatly reduce the impact of duct heat gains or losses.
Figure 1-1 from the SMACNA Residential Comfort System Installation Standards, below, shows an extended plenum system, which is a common duct design used in residential construction. The ductwork consists of a main trunk that serves as an elongated plenum with branch duct properly located and a single thermostat operating a single HVAC system. Larger residences or 2-story homes may have two or more individual HVAC systems, each with its own thermostat to directly manage each zone’s heating and cooling requirements.
Additional Considerations
In addition to temperature and humidity control, HVAC systems play a crucial role in maintaining indoor air quality. Forced air systems require the air being circulated to be filtered. Filtering of the air will reduce dirt and dust in the conditioned space, keep ducts and air coils within the air-handling equipment clean, and is a key component to maintaining the overall health of the system. High-efficiency filters (Merv 13, as an example) are recommended to capture particles effectively without impeding airflow. Ease of access to change or service filters is important; experience has shown filters that are difficult to access or remove and install do not get changed as required.
New homes are now required by code to have very tight enclosure construction. Improved energy codes and standards have increased the requirements for thermal insulation and sealing of ductwork. Excessive duct leakage can have a negative impact on the HVAC system performance. All ducts outside of the thermal enclosure must be sealed. The ductwork should always be carefully constructed, assembled and installed to minimize duct air leakage. Sealing of ducts is always recommended even when the duct is located entirely within the conditioned space. Existing duct systems should be checked for air leakage and sealed and insulated as required by the most recent code or local ordinances.
To reduce the transfer of noise from the HVAC equipment into the duct system, it is desirable to internally line at least the first few feet of the supply and return air plenum closest to the HVAC equipment. Duct lining must be installed in accordance with SMACNA’s HVAC Duct Construction Standards—Metal and Flexible. Sound-absorbing liners may be of any thickness but where the duct liner is intended to provide code-required insulation it must have sufficient insulation (R-value) to meet that requirement.
The Benefits
Residential heating and air-conditioning systems require careful duct system layout and sizing to efficiently and effectively distribute the conditioned air. A well-designed HVAC duct system will maintain space temperature, remove airborne contaminants, control humidity, operate quietly and minimize energy consumption.
If you have any questions regarding HVAC duct systems for homes, find a SMACNA member contractor in your local area.
IMAGES: SMACNA
