Why Sizing Ducts Is Important Before You Install A New HVAC System


If you’re having a new HVAC system installed in your home, you’ll want to be sure that your contractor includes sizing ducts in the process. Without properly sized ducts, you could compromise your return on investment. Without adequate air flow and well-designed ducts that match your system, you could end up with significantly reduced home comfort and much higher operating costs.

Manual D Sizing Method

The industry standard for sizing ducts is called Manual D, a method developed by the Air Conditioning Contractors of America (ACCA) and used across the HVAC industry. A Manual D method includes:

  • Evaluating air flow needs – Each room in your home requires a different amount of air flow, and a sizing calculation will ensure that the duct’s design and materials can meet that need.
  • Controlling noise factors – When air moves through ducts, it can be noisy. Making adjustments to the ducts can help to reduce noise, often by switching out ducts in lieu of more favorable, noise-reducing duct materials.
  • Retrofitting the duct’s design if necessary – With the right design, materials and air flow, your contractor will ensure that your ductwork system delivers in terms of comfort and energy savings.
  • Sealing the system – Leaks can account for significant energy losses, so sealing ducts will help to enhance efficiency.
  • Insulating where necessary – Ducts can also lose energy when portions of the system are installed in unconditioned areas of your home, like an attic. In these cases, ducts should be insulated to reduce losses from conduction.

Special Considerations

Sizing ducts will also take into account the type of cooling system your home uses. Heat pumps produce a consistent, even air flow, and they often require larger ducts than a forced-air system does. Forced-air central air conditioners operate differently, producing short but strong blasts of cooled air. If you’re upgrading your existing A/C to a more efficient one, you may be able to install a smaller-sized system. In both cases, sizing the ducts will ensure that they can handle the air flow and capacity of the newly installed system.


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