Education
For the last 20-30 years, manufacturers of HVAC equipment have been making an effort to make the systems they manufacture more efficient. This was originally driven by rising energy costs, and has more recently been driven by increased awareness over environmental issues. There are several methods for making HVAC systems more efficient.
Heating energy
Water heating is more efficient for heating buildings and was the standard many years ago. Today forced air systems can double for air conditioning and are more popular. The most efficient central heating method is geothermal heating.
Energy efficiency can be improved even more in central heating systems by introducing zoned heating. This allows a more granular application of heat, similar to non-central heating systems. Zones are controlled by multiple thermostats. In water heating systems the thermostats control zone valves, and in forced air systems they control zone dampers inside the vents which selectively block the flow of air.
Ventilation Energy recovery
Energy recovery systems sometimes utilize heat recovery ventilation or energy recovery ventilation systems that employ heat exchangers or enthalpy wheels to recover sensible or latent heat from exhausted air. This is done by transfer of energy to the incoming outside fresh air.
Air conditioning energy
The performance of vapor compression refrigeration cycles is limited by thermodynamics. These AC and heat pump devices move heat rather than convert it from one form to another, so thermal efficiencies do not appropriately describe the performance of these devices. The Coefficient-of-Performance (COP) measures performance, but this dimensionless measure has not been adopted, but rather the Energy Efficiency Ratio (EER). To more accurately describe the performance of air conditioning equipment over a typical cooling season a modified version of the EER is used, and is the Seasonal Energy Efficiency Ratio (SEER). The SEER article describes it further, and presents some economic comparisons using this useful performance measure.
BTU - British Thermal Unit. The quantity of heat required to raise 1 pound of water by 1 degree Fahrenheit.
SEER - is an abbreviation for Seasonal Energy Efficiency Rating. It is the most commonly used measure of the efficiency of consumer central air conditioning systems. (EER, or Energy Efficiency Rating is the most commonly used measure of efficiency for commericial air conditioning systems.)
CFM - Cubic feet per minute, which is a measure of the volume of air flowing in a system