According to the US Department of Energy, when it’s time for a new heating system, choosing one of the correct size is critical to getting the best efficiency, comfort, and lowest maintenance and operating costs over the life of the new system. Some national surveys have determined that the most common sizing mistake is in over-sizing. You might think that MORE heating or cooling capacity is a good thing, but it may not be!

Over-sizing not only makes the new system more expensive to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Over-sized heating equipment also often creates uncomfortable and large temperature swings in the house.

It is the installer’s job to perform the correct sizing calculation for your home. However, many installers only check the existing system and sell you one just like it, or even worse, one that’s larger. This is a not a correct sizing method and not in your best interests! Other methods include simple “rules of thumb” based on the size of your home or using a chart that accounts for a variety of factors. While these methods might provide a first estimate, they should not be used to size your system.

You should insist on a Manual J, “Residential Load Calculation,” before signing a contract. This is the nationally recognized standard for calculating the proper size of heating and cooling equipment for your home, and it takes into consideration all of the factors that influence the proper sizing of your comfort system including:

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

Many factors affect a home’s heating or cooling requirement, or “load.” A good installer will measure walls, ceilings, floor space, and windows to determine the room volumes, and will assess the R-value of the home’s insulation, windows, and building materials. They will also include an inspection of the duct system, including size, condition, and insulation.

Don’t settle for a “guestimate” when it comes to the comfort and efficiency of your home. Insist on a thorough evaluation using Manual J. Make sure your home is “just right”, and remember, bigger may not be better!

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