Heating and Cooling
In much of the fall and spring, and in some days during the winter and summer you can improve the comfort of your indoor air just by opening windows, shading west-facing and south-facing windows during the summer and letting the sunlight in during the winter. Be sure to open windows or doors on opposite sides of your home to allow for cross-ventilation. With a multi-story home, you can take advantage of stack effect ventilation which uses the natural tendency of hot air to rise to pull cool air in from below.
Mechanical Ventilation-Heating and Air Conditioning
When it is too hot or cold outside for natural ventilation, your home's Heating, Ventilation, and Air Conditioning (HVAC) system is responsible for maintaining a comfortable temperature and humidity of your indoor air. It accounts for one of the largest sources of energy consumption in your house.
The SEER Rating of an air conditioning system stands for Seasonal Energy Efficiency Ratio. The higher the SEER Rating, the more energy efficient it is, and the less you will pay to cool your home.
HVAC systems manufactured before 2005 may have a SEER as low as 9. Inefficient window air-conditioning units often have a SEER of around 10. The minimum SEER rating allowed by current code is 13. Good heat pump systems will have SEER Ratings in the high teens and low twenties. Ductless mini-split heat pumps can reach 30. Ground Source Heat Pumps (Geothermal) can theoretically achieve a SEER Rating of 75, but in climate zone 4 (where we live) their actual SEER rating will be closer to the highest performing conventional heat pump.
TVA's Energy Right Heat Pump Program has some useful information and may even be able to help finance your retrofit: TVA Energy Right Heat Pump
Energy Star also has some great information about heating and cooling. Energy Star Heating and Cooling
They also made this great graphic: