The duct system plays a huge role in home energy costs, comfort and performance. Poor ductwork design can lead to hot or cold spots, high utility bills, extra wear and tear on HVAC equipment and bad odors or unbalanced air pressure that cause doors to slam.
Efficient ductwork design creates a network of freeways that allow the conditioned air to travel throughout your house without obstruction. Learn how to properly size and layout ductwork using the principles found in this article.
Ventilation
A duct system distributes heated or cooled air from a furnace, heat pump or AC unit to rooms or areas of the house. It is a vital component in ensuring that conditioned air gets to all parts of the home, but poor duct design can make the system less efficient or even unreliable.
Ductwork can be made from a variety of materials, including galvanised sheet metal or aluminium. Aluminium is lightweight and can be shaped into the desired duct shape by a fabricating shop. Ducts can also be lined with fibrous glass insulation, polyurethane foam, phenolic foam or fibreglass, which provide built-in thermal and sound insulation.
Ideally, a duct system should be located inside the conditioned space of the house to reduce energy loss. Ducts that run outside the conditioned space are subject to a substantial loss of energy, as warm air can leak into an area that should be receiving conditioned air and cool air can leak out of an area that shouldn’t be getting any conditioned air.
A duct system may have many different components to help regulate the flow of conditioned air throughout the house, such as turnouts, which reduce duct resistance by allowing some of the air to be diverted into small branch ducts, dampers, which can open and close the flow of air, and smoke and fire dampers, which are used in locations where a duct runs between separate fire compartments. The connections between the various sections of ductwork are normally sealed with mastic, a special duct sealant that is often reinforced with a layer of metallic tape to increase its strength and prevent it from coming loose over time.
Conduction
In ductwork design, conduction refers to the way heat moves through a system of pipes. The conductive properties of the duct material are important because they influence how much air moves through it.
Good ductwork design includes proper sizing and layout. It also minimizes leakage and internal air friction to increase efficiency and comfort.
To ensure that a home’s ductwork is sized properly, technicians use software and the ACCA’s Manual D (residential) or Manual Q (commercial). The process involves determining room volume and layout, entering specifics like floor type and room direction (for heat gain), and then using a duct sizing calculator to determine how large a duct should be.
Ducts that are too small cause airflow problems. For example, a 3-ton air conditioner and furnace pushing air through a 2-ton duct system would be the equivalent of a traffic jam in a major city.
Another important aspect of ductwork is creating a circulating pattern that removes unconditioned air from rooms and returns it to the HVAC system for heating or cooling. If a room doesn’t have a return air duct, the HVAC system will constantly run to fill that space with warm or cool air—which drives up energy costs and introduces dirt and bad odors into the forced-air system. To avoid this, ducts should be routed to the nearest return vent, and ideally through conditioned spaces.
Heat Transfer
Ducts can be made of sheet metal, fiberglass, flexible plastic or a combination. They are often lined with thermal insulation made from glass wool, polyester or other materials. Insulation is either blown or compressed between the duct wall and the inside surface of the duct.
Air ducts are also fitted with turning vanes at changes of direction to minimize turbulence and resistance to air flow. Other ductwork accessories include take-offs, which are differently shaped fittings that allow a small portion of the main duct’s flow to be diverted into branch ducts; volume control dampers to adjust the amount of airflow; and smoke or fire dampers to seal off ducts in the event of a fire.
The goal of a ductwork system is to create a circulating pattern that takes unconditioned air from throughout the house and returns it to your furnace or air handler, where it can be heated or cooled. Without this circulating air pattern, your heating and cooling system has to work harder than necessary just to pump enough warm or cool air into a room.
To make sure your ductwork system works properly, have it professionally tested. An HVAC professional will use a blower fan to pressurize the ductwork and a computer program to calculate how much air is being lost due to leaks, obstructions or other problems in the ductwork.
Insulation
All ductwork must have adequate insulation to keep the inside of the ducts from heating up too rapidly or cooling down too quickly. This insulation also protects the duct from corrosion. The R-value (thermal resistance) of the insulation determines its effectiveness. The higher the R-value, the more effective it is.
The insulating material in the duct must be properly sized to match the heat transfer characteristics of the ductwork and the system it supports. Insufficiently sized insulating materials will result in air flow problems that may damage the equipment and compromise occupant comfort. Incorrectly sized insulated ducts will reduce the performance of the HVAC system, resulting in high utility bills and poor indoor air quality.
Ideally, each room with a supply vent should also have a return vent to ensure proper air flow through the system. All return and supply ducts should be sized to handle the maximum loads expected during normal operating conditions of the system.
Ductwork should be fabricated of industry-standard sheet metal, fiberglass or approved flexible duct. Wooden building channels and cavities such as wall voids should never be used as a cost-cutting substitute for hard ductwork. Air balancing dampers should be installed in branch ducts as close as possible to the primary trunk duct.
Ductwork design errors can cause hot and cold spots, poor air flow, extra wear on your system, increased maintenance costs, high energy bills, excessive humidity in living spaces, lingering bad odors, distracting noises and even mold growth in some cases. The duct system should be properly designed and sized, installed using the best practices and then tested to ensure it is functioning as intended.