What is a load calculation?

What is a Load Calculation or “load calc”?                             

For air conditioning and heating systems it is a very comprehensive calculation to determine the proper size heating/cooling system for your home.  Regardless of if you are building a new home, or replacing an existing air conditioning system, starting with a professional load calculation is the first step to  be done to insure maximum efficiency, longevity, and comfort from your air conditioning system.

Most load calculations are based on a book called “manual J” .  In this modern era this is generally done using a load calculation software program.  When I started in the HVAC (heating, ventilation, air conditioning) business almost everyone was using rules of thumb/guessing on proper size air conditioners.  these rules of thumb were closer to right back in the old days.  Most houses were roughly square, had similar insulation and construction, and had similar number and types of windows.  Houses today vary drastically!

Some important things a load calculation considers:

  • square footage
  • ceiling height
  • window type and amount
  • insulation type and amount
  • wall construction
  • direction structure faces
  • # of occupants
  • lighting and appliances
  • air tightness of structure
  • region (determines temperature load calc is using)
  • temp around air conditioned space
  • location of equipment and ducts
  • attic ventilation
  • and much more

As you can see, a lot goes into a heat load, or load calculation.  The proper size HVAC equipment is crucial.  Sadly, many contractors still guess and many places dont require a load calc to pull a permit.

Problems of over-sizing:

  • shortened equipment life
  • higher utility bills
  • high humidity
  • mold growth
  • hot/cold spots
  • excessive noise
  • uncomfortable house

That’s just to name a few problems.

Energy Star, every air conditioning manufacturer, and every major air conditioning organization agree that Proper sizing is the first step to a proper heating and air conditioning system installation.  An improper installation of HVAC equipment can cost you so much more in the long run than you think you are saving by going with the lowest bid.

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