Watch the video on Duct System Design.
Most outlaws agree—the three steps to make a crime go well are to plan it, pull it and run like... heck. When it comes time to put the HVAC duct system in a home, builders might take this tip from the villains. Plan it (design), pull it (install), and help the air run like heck (seal the ducts).
Shortly after the home's HVAC system has been properly sized and the required airflow calculated by a mechanical engineer, the HVAC trade contractor will begin to design the home's duct system. Though often overlooked, poorly designed or unsealed ducts systems can reduce the HVAC system's efficiency by up to 40%, costing the homeowner money and creating a drafty, uncomfortable home.
By understanding the basics of duct design, builders can not only help communicate better with trade contractors, they can help deliver a higher quality product to their customers. The best practice when it comes to duct work is to make sure the system is designed, installed, and sealed properly.
Design of the duct system usually happens at the same time the house plans are drawn up. The American Conditioning Contractors of America (ACCA) Manual D is an invaluable guide for designing a successful ductwork system, and can help the mechanical contractor with the task.
Here are some of the steps your HVAC trade contractor should follow when designing the duct system:
- Calculate the home's heating loads and cooling loads.
- Select the proper size equipment for the home using the ACCA's Manual S: Residential Equipment Selection guide. This has to be done before duct sizes are calculated because the specific blower data will determine the duct sizing criteria.
- A scale drawing of the home's air distribution system is sketched to show equipment placement, register sizes and locations, air flows associated with each register, location and lengths of ducts.
- Determine the size of the ducts based on calculations and information about blower data, room size, and air flow following the procedures in the ACCA's Manual D. It's important that the HVAC contractor properly sizes both the main trunk and branches of the duct systems. This will help make sure that each room will receive the appropriate amount of conditioned air.
After several design sketches have been created and evaluated, the best design should be selected to heat and cool the house based on integration with architectural layout and structural systems. With a finished plan in hand, the HVAC trade contractor can then begin to install the system.
When installing the duct system, be sure to communicate that the HVAC trade contractor pays special attention to the joints in the system. A paste called mastic should be used to help seal all connections to make sure all the air is delivered to the living spaces. If the joints and connections seep conditioned air, the overall system will operate at a lower efficiency and needlessly waste energy.
Builders today are probably tired of being told to keep their 'ducts in a row', but the real value of properly designing, installing, and sealing HVAC system ducts is something the homeowner can appreciate for years to come. Anything less would be criminal.
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