HVAC Tips / January 19, 2021
How Duct Design Factors Affect Airflow
The ductwork is an integral part of an HVAC system. The ducts deliver fresh air to targeted areas of the house and direct stale air back to the HVAC. Thus, the quality of duct design is critical to your HVAC’s performance and efficiency. Below are some factors that affect duct design and how they affect the overall airflow within the house.
The cross-sectional shape of the duct channels affects the flow of air. Ducts typically come in three shapes:
The cross-sectional shape affects the following:
- The number of corners: This affects airflow speed.
- The amount of duct material: This determines duct weight.
- The surface area of the duct that comes in contact with the air: This determines air friction (and hence airflow speed).
Round ducts are usually the best in all these aspects. However, the technician may use a different shape, depending on other facts, such as the available space.
The duct size is the cross-sectional area of the duct. The duct size determines the volume of air that can pass through a section of the duct at any time. HVAC technicians typically express duct size (cross-sectional area) in square feet. A bigger duct delivers more air than a smaller one.
Duct fittings refer to the attachments or connections that are part of the duct system. Examples of fittings include:
- Elbows used in corners
- End cups
- Reducers for changing duct size
Fittings that protrude into the duct channel affect airflow. For example, airflow in an elbow is not as smooth as airflow over continuous sections of the duct. Thus, HVAC technicians strive to keep the fittings to the minimum number.
Duct length determines how long the air has to travel before reaching the target room (for supply ducts). The house’s size, location of the HVAC unit, and duct design all affect each duct channel’s overall length. However, long duct sizes reduce the flow rate. Thus, HVAC technicians always strive to use the shortest distance to minimize duct length.
Friction exists between the duct’s interior surface and the air flowing inside the duct. Smooth surfaces present less friction than uneven surfaces. Thus, smooth materials are better for duct construction than rougher materials.
Other factors, other than smoothness, affect material choice. For example, an ideal material should be:
With all these factors considers, common duct materials include galvanized steel, aluminum, and fiberglass.
Duct routing refers to the specific location and direction of each duct channel. Factors that determine duct routing include:
- Location of the ducts
- Number of elbows
- Location of the HVAC unit
For example, heated air is lighter than cool air. Thus, a duct system that rises is best for warm air delivery but not cold air delivery. The routing also determines the number of corners or elbows, which affect airflow friction.
Lastly, the government also has a say in your HVAC system, including duct design. Duct codes determine:
- Duct sizing
- Duct routing
- Duct design
- Duct leakage
The codes affect both safety and efficiency of duct systems. For example, most counties in California require pressure testing during HVAC installation or replacement. Note that the HVAC codes prescribe the minimum requirements, but there is no harm exceeding the minimum standards. Don’t forget that these codes may change with time.
Mauzy Heating, Air & Solar has been active in the HVAC industry for over five decades. We will design and install your HVAC system, including the duct system, for maximum efficiency. Contact us for a quote as the first step towards professional and affordable HVAC services.
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