The most energy efficient hvac system available is Geothermal hvac systems that use ground temperature for a heat source or exchange medium. The ground temperature just a few feet below the surface remains relatively constant during climate extremes. Like a cave, the temperature is warmer than the air above it in the winter and cooler than the air in the summer.
Traditionally hvac systems have used outside air as an exchange medium which isn’t nearly as efficient as ground source heat.
Geothermal systems (also known as ground source heat pumps or GeoExchange) typical energy savings are 25% to 70% depending on type of unit and climate. They use 25% to 50% less electricity than conventional heating and cooling systems. They also improve humidity control by maintaining about 50% relative indoor humidity.
According to the U.S. EPA, Geothermal systems have the lowest life cycle cost of any hvac system. These systems can be more costly to initially install, however, the maintenance and repair is much lower over the lifetime of the unit.
The energy savings of a Geothermal system alone will allow you to recoup the initial installation costs fairly quickly.
These units can be heating and cooling systems (which is the most common), central heating only, or hot water only.