SEER is useful for comparing one model to another much in the same way that a car's calculate MPG is useful. It's not an accurate prediction of exactly how efficient the system is, but it can tell you which one is more efficient. Also, since SEER is based on a "cooling season", what region you live in will determine how long or short your cooling season is.
If you are using existing ducting, it will have to be inspected. Proper ducting loses around 2% to 5% of your energy. Old, leaking ducts can lose 50% or more. A contractor will need to have the ducts inspected and replace any parts ahead of time. If you are changing the size of your HVAC system because of significant changes to your home, you might need to replace the ductwork regardless.
Some systems include an "economizer mode", which is sometimes called a "free-cooling mode". When economizing, the control system will open (fully or partially) the outside air damper and close (fully or partially) the return air damper. This will cause fresh, outside air to be supplied to the system. When the outside air is cooler than the demanded cool air, this will allow the demand to be met without using the mechanical supply of cooling (typically chilled water or a direct expansion "DX" unit), thus saving energy. The control system can compare the temperature of the outside air vs. return air, or it can compare the enthalpy of the air, as is frequently done in climates where humidity is more of an issue. In both cases, the outside air must be less energetic than the return air for the system to enter the economizer mode.
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