Principles of HVAC Duct Sizing

Standards

• Duct sizing should adhere to the standards in “Manual D – Residential Duct Design,” according to the U.S. Department of Energy. The Air Conditioning Contractors of America is a non-profit organization that publishes this manual and updates its content. Duct sizing is not an arbitrary technique. Precise mathematical formulas determine correct sizing by considering variables, such as a home’s heating/cooling requirement, the number of rooms, room sizes and insulation levels. The DOE advises using computer or hand calculations to eliminate any guesswork.

Sizing Problems

• If ducts are sized incorrectly, rooms may be too warm or too cool, and comfort levels vary instead of maintaining consistent temperatures. The air blowing into a room may circulate too slowly, which doesn’t condition the room, and sometimes it may blow too strongly, which over-conditions the room and creates loud blowing noises. Blower fans work overtime, which taxes the blower motor and strains the entire system needlessly. System safety devices may shut down the system if motors become overheated when they try to push air through incorrectly sized ducts.

Supply and Return

• Tim Carter at AskTheBuilder.com compares a furnace to a recirculating pump that pumps air instead of water. When calculating duct sizes, it is important to consider the number and size of return air ducts as well as supply ducts. If each main room does not have a return air duct, new supply air must push existing air aside before you feel comfortable. As air is forced from the furnace through ducts, contractors use “Manual D” calculations to factor in the longest path that air must travel. This path’s associated air loss for traveling the longest distance helps determine correct duct size.

Duct Pressure and Friction Rate

• Carter explains static pressure in an HVAC system as blood pressure in a human body. For a system to deliver the correct amount of air into each room simultaneously, the duct system must support proper static pressure. The pressure exerted by the blower must be divided by the number of supply ducts so that sufficient air is delivered to each room. The friction rate helps maintain correct pressure to ensure maximum flow velocity with minimal air loss.