Load Calculations: How Manuals J and D Should Figure Into Your Replacement or Upgrade

If you’ve done your homework and decided on a highly efficient air conditioner that meets your family’s needs, you might think your job’s done. But you would be wrong. A key determinant in your family’s comfort for years to come will be your contractor’s expertise in sizing the system–based on an accurate cooling load calculation–and making sure your ductwork delivers all the cool air that you need.

Correctly sizing a system is critical because a system that’s too large will cycle on and off frequently and fail to sufficiently dehumidify your home. A system that’s too small will work too hard, drive up your energy bills and develop problems early in its expected lifespan.

The Air Conditioning Contractors of America has developed a system for making the necessary load calculations called Manuals J and D. The inputs for these software tools include:


  • Cubic footage of your home.
  • Layout of home.
  • Insulation levels in the walls and attic.
  • Windows, their size, placement and energy efficiency.
  • Rates of air infiltration.
  • Landscaping factors.
  • Heat-producing appliances, lighting types and electronics.
  • Household size and member ages.
  • Preferred indoor temperatures.

Manual J determines the tonnage of the HVAC system that you need. Once your HVAC professional has determined the size, he moves on to Manual D, which prescribes the design of the ductwork. Depending on how appropriately your home’s original HVAC equipment was sized and installed, you may not need to make any changes to the duct layout. But your contractor should inspect your ducts to evaluate their integrity.

Of course, you want your cooling load to be as low as possible, and you can help to ensure that by making efficiency improvements before your new system goes in. For example, you could improve the insulation level in the attic and seal any air leaks. If you’re more enterprising, you might consider getting energy-efficient windows or landscaping your yard to lower the heat buildup in your home. Make enough improvements, and you might be able to get a smaller cooling system.

For more information about load calculations and air conditioner installation, contact Beyer Boys. We’ve proudly provided HVAC services to homeowners in the San Antonio area since 1990.

Our goal is to help educate our customers in the greater San Antonio, Texas area about energy and home comfort issues (specific to HVAC systems).

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