Mechanical air conditioning can be provided by... 1) Central ducted AC, 2) Central ductless AC (mini-splits), 3) Window or Room air conditioners, 4) Portable air conditioners, 5) Evaporative Coolers and 6) Heat pumps.
New central air conditioners are a lot more energy efficient than older units. Their SEER (energy coefficient) can now be 20 or more, enabling them to cut cooling bills in half.
When buying, pay attention to new features like multi-speed blower motors, and consider carefully the “zoning” issue. Why should you cool rooms that you are not using?
Zoning is very important and room air conditioners and - most of all - ductless mini-splits can provide it without the issues and the costs of traditional central units. Central air conditioners do not make sense in very energy efficient homes, properly designed, insulated and sealed, with a good landscaping.
Central Ducted AC
Room air conditioners cool one room at a time, providing significant energy savings.
But for those who need more than just cooling a room, ductless mini-split units can be a great alternative to traditional central AC from an energy-efficiency standpoint.
Ductless AC is a tested technology, largely used in Europe and Asia. They can be the best mechanical air conditioning choice for energy-efficient homes.
Several manufacturers are offering portable air conditioners.
Contrary to window air conditioners, they sit completely inside the room; but most of them are very energy-inefficient: unvented units do not remove any heat from the room; they just move heat from one part of the room to another part.
See for details: Portable AC
Evaporative coolers can be a great alternative to air conditioners. They consume a small fraction of the electricity of conventional AC. They are a good choice, from an energy-efficiency perspective. The problem is that they are not effective in humid weather conditions, and do not make sense in moderate and cold climates.
Heat pumps are – at their core – air conditioners able to provide both heating and cooling. The disadvantage is that they aren't effective at heating, at very low temperatures.
See, for details: Heat pump vs. Air Conditioners
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