Turn the Heat Down, or Leave It on When Gone?

Sierra Club's Mr. Green was asked a question that I have been asking since I've had my own place:

I have oil heat. I am gone about 12 hours a day. Should I turn down the heat when I leave, and by how much? Some people tell me that it takes more to reheat the house than to leave it at a constant temperature all the time. --Jeff, Pennsylvania

Your advisors are wrong. It takes more energy to maintain a constant temperature in a house than to turn the temperature down when you go out or go to bed and then reheat. It is a basic thermodynamic law that heat moves to where it is colder. So if your heat is always turned up, there is a constant transfer of that higher heat from the inside to the outside, even in a well insulated building. Hence, maintaining a higher temperature when you’re not inside is, in effect, heating the great outdoors.
Think of yourself as a furnace. Thanks the fact that you’re a warm-blooded creature, your body constantly burns fuel to maintain a temperature of 98.6 degrees. When you jump into a cold bed, your body heat can warm you up fairly quickly, getting you to the same comfort level you’d feel if you’d been lolling there burning up energy all day long. It doesn’t take long for the heat you’re producing to match the amount that is escaping.

So turn the thermostat down to 55 degrees when you’re not home or asleep, and you can save anywhere from 5 to 20 percent on your heating bills, depending on insulation, windows, etc. Also keep the temperature at a 68 degrees max, which is comfortable for most people, and if it’s not, do as Jimmy Carter wisely recommended long ago, and put on a sweater.

If you’re the type of person who forgets to turn the thermostat down, you should consider a programmable thermostat that does this automatically at times you choose.

Thanks, Mr. Green, for finally answering my questions definitively.