What Are Air Ducts in a House?
The ducts in a home deliver air to heat or cool the entire house, and air duct designs are different for every home. There are many factors that can affect air duct design, including the layout of the house and its BTU requirements, which will be explained below.
How House Layout Affects Air Duct Design
The way a house is designed and built affects the layout of the ductwork. The air handler should always be centralized in the house, and all the branches of ducts should be about equal in length for optimal efficiency. Where the ducts are placed depends on what type of space is available.
For example, the air handler is often placed in the middle of the basement, with the ducts branching out from there, and the vents blowing up into the rooms above. But not all houses have a basement. In this case, the air handler can be placed on the ground floor, and the vents can run either through the concrete pad underneath the floor or through the attic with the vents in the ceiling.
Air duct design is usually planned out early in the home building process, but older homes can sometimes have ductwork added if there is space, such as in the attic.
Choosing the Right Duct Materials
Residential ductwork is most commonly made of galvanized steel, but that is not the only choice on the market. These are the materials ductwork can be made of:
Coated in zinc to prevent rusting. Available in a wide range of sizes. Can be insulated with duct wrap. Also available in double-wall. Solid and durable.
Lightweight and easy to install. Customizable. Available in a wide range of sizes, including rectangular and round.
Coiled with wire, best for tight spaces; used most often for ventilation. Lightweight and easy to manipulate, but not as easy to clean as metal. Can withstand high temperatures and can be flame retardant.
Made of hardy polyvinyl chloride, often chosen for exposed use because of its attractive color and low profile.
Polyurethane or phenolic foam board covered with aluminum foil. Lightweight and energy-efficient, but not hardy.
Fiberglass boards are scored and folded into a rectangular duct shape. Absorbs interior sound for a very quiet system.
Factoring BTUs for Air Duct Design
Before planning the air duct design, HVAC companies must first factor in a home’s thermal load. They will consider the size of the home, how well it is insulated, and the climate zone to calculate the heating and cooling load. This load is represented as BTUs per hour, or how much heat energy must be added during cool weather or removed during warm weather.
BTU stands for British Thermal Unit, which is the unit of measurement for heat energy. For the most efficient heating and cooling, one must have a unit that can heat or cool the appropriate amount of BTUs for the space in their home. Air conditioners are rated by how much heat energy they can remove per hour, and heaters are rated on how much heat energy they can produce and distribute per hour.
Although the BTU load largely affects the size of a heating and cooling unit, it can also affect the air duct design. For example, a small house that requires a small amount of BTUs will need shorter duct branches. The BTU will also affect how many vents are in each room, and the diameter of the ducts.
A certified HVAC technician will be able to determine a home’s BTU heating and cooling load, as well as recommend duct material based on one’s specific needs.