How to Calculate Return Duct Size for an HVAC

Furnaces create hot air using burners. Once the air is heated it is blown through the home using a blower. The air is carried through a duct system into the house. Eventually this circulating air loses its heat and needs to be heated again. Return ducts draw the air back into the furnace. These return ducts need to be the correct size to prevent air and energy loss. Calculating duct size for your heating, ventilation and air-conditioning system -- HVAC -- is made easier using a duct calculator, available for about $15 from select retailers. This devices are commonly known as ductulators.

Things You'll Need

• Measuring tape
• Ductulator

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Instructions

1. • 1

     Determine whether you want to use circular or rectangular ducts for your system. Different shapes need to be sized differently.

   • 2

     Locate the size of the HVAC unit on the condenser outside of the home.

   • 3

     Determine the cubic feet per minute (CFM) of air flow needed for your home. This number should be between 600 and 900. Keep in mind that the higher the CFM the noisier your unit will be.

   • 4

     Measure how long the return-air duct needs to be to fit to your return duct.

   • 5

     Add the square feet of air ducts needed to the duct calculator. Use 0.1" of friction per 100 square feet of duct. The ductulator gives you a measurement for both square and round ducts.