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Manual J and Manual D: The correct way to size a system


Correct system sizing requires the consideration of many factors other than simply reading the nameplate of the existing unit. Key factors for correctly sizing a heating and cooling system include the following:

  • The local climate
  • Size, shape, and orientation of the house
  • Insulation levels
  • Window area, location, and type
  • Air infiltration rates
  • The types and efficiencies of lights and major home appliances (which give off heat),

Homeowners should insist that contractors use a correct sizing calculation before signing a contract. This service may be offered to homeowners by major heating equipment manufacturers, conscientious heating and air conditioning contractors, or other professionals. Manual J, “Residential Load Calculation,” published by the Air Conditioning Contractors of America (ACCA), is the recommended method for use in the United States. There are also many user-friendly computer software packages or worksheets that can simplify the calculation procedure. You should make sure that the procedure used by the contractor follows Manual J. If ducts are part of the installation, they should be sized using the ACCA’s Manual D, “Residential Duct Design.” The ACCA also offers a comprehensive guide for choosing home heating and cooling systems, called Manual S, “Residential Equipment Selection.”