You've probably read or heard that a heating unit has to work harder to warm up a cold house than to maintain the temperature in an already cozy space. This is what the U.S. Department of Energy likes to call a "common misconception" [source: Department of Energy].
The truth is that it requires more energy to keep the house at its normal temperature than to heat it back to that temperature after dialing the thermostat down. Heat naturally moves to places where it's cold. So if your heat is up, it is constantly moving from the inside of your house to the outside, even if your house is well-insulated. A home loses energy more slowly once the temperature inside drops below normal levels. The longer the house remains cold, the more energy it saves compared to the energy lost that comes when the heater is humming along at its normal temperature [sources: Department of Energy, Sierra Club].
The same principle holds for home cooling purposes. The higher the air temperature rises above typical levels inside the house, the slower it loses energy. The slower it loses energy, the easier it is to re-cool the home when you get out of bed or return at night [source: Department of Energy].
That doesn't mean you should shut the furnace or air conditioning unit off entirely before you leave your house, especially if you're going to be gone for a while. When a house gets too cold, it puts the pipes at danger of freezing. When it gets too hot, condensed air can do a number on wood floors, cabinets and other surfaces [source: Martin].
If you're looking for a sweet spot, keep the thermostat at about 68 degrees F (20 degrees C) when you're home and drop it down to about 55 degrees (13 degrees C) before you go out or go to bed. The same goes for cooling costs: Keep the house warmer than normal when you're not home and try to leave the thermostat at around 78 degrees F (26 degrees C) otherwise [sources: Department of Energy, Sierra Club].
According to the U.S. Department of Energy, a family that sets back its thermostat by about 10 to 15 degrees for eight hours a day while sleeping or out of the house can save 5 to 15 percent a year on home heating costs.