So, you're buying a new heating or air conditioning system, and you want to figure out what size to use.

In your quest for the correct size, don't let a contractor fool you into thinking a "quickie" method is actually accurate. They may be useful for very rough sizing, but not for the final size.

Here are some of the "quickie" methods some contractors may use to size a system.

Never use any of these to determine the final size.

• The contractor walks in the house, looks at the existing unit and recommends that the replacement unit be the same size, or larger. This obviously does not take into account any improvements made to the house or mistakes made in sizing the original unit.
• The contractor asks you how many square feet of living space there are in your house, then tells you what size unit you need. This is called "sizing by square footage" and is the most commonly used inaccurate method of sizing.
  A typical value used for air conditioners is one ton (12,000 Btu/hour) per 500 square feet (46 m2). This does not take into account differences among house orientation, insulation levels, design, construction, energy efficiency or intended use of the system.
  You may get different answers from different contractors who use this technique. In that case, they may have a different "rule of thumb," or one of them may be using the "lowest cost" method. This involves adjusting the square footage rule so that whatever the contractor has in their warehouse becomes the right size for you. Since the "in-stock" unit costs the contractor (but not necessarily you) less to install, this becomes the "lowest cost" method.
• Another rough method for sizing heating systems involves a prepared chart.
  On the chart, you fill in the floor area of all the heated rooms, and the levels of insulation in the floors, walls, and ceilings. Then you find the category that best describes the house. Then, you multiply both the higher and lower numbers for heat loss in Btu per hour per square foot by the floor area of the home to give you a rough range for the heating load.
  Although a chart like this looks official, not all houses fit the profile given. There is also no accounting for outstanding factors.

To save some time the above methods are often used for a first "guess" or rough estimate. If so, then it should be plainly stated to you that this is the case. The final bid should be based on the results of the use of a procedure such as Manual J.

Source:Consumer Energy Information: Energy Efficiency and Renewable Energy Network, U.S. Department of Energy