Manual J Load Calculation

  When evaluating the proper equipment size for your home, we must consider 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 U.S. Department of Energy recommends homeowners insist that contractors use a correct sizing calculation like Manual J Load before upgrading their equipment.

Steps a Good Contractor Should Take to Size Your System (source: U.S. Dept. of Energy)

Many factors affect a home's heating or cooling requirement, or "load". A good estimator will measure walls, ceilings, floor space, and windows to determine the room volume, and will assess the R-value of the home's insulation, windows, and building materials. A close estimate of the building's air leakage is also necessary. A blower door test is the best way to measure air leakage.

A good estimate will also include an inspection of the size, condition of seals on joints and insulation, and location of the distribution ducts in forced air systems. The placement of supply and return registers should be appropriate for the system type and size.  The orientation of the house also affects heat gain and heat loss through windows. Overhangs can reduce solar gain through windows. 

To summarize, when designing your new heating and air conditioning system, the contractor you choose should do the following:
      Use a computer program or written calculation procedure to properly size the new system
  Provide a written contract listing the main points of your installation and include the results of the heating and cooling load calculation
      Give you a written warranty on equipment and workmanship

Jon Wayne will always perform a Manual J Load calculation before recommending a new energy efficient heating and cooling system for your home.   


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