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Do programmable thermostats really save money?

In theory they are a great idea, in reality they have actually been shown to cost you more.

It is easier for your furnace to maintain a consistent temperature than it is to bring the temperature up 10 degrees or more. Everything in your home absorbs heat; the furniture you are sitting on, the floors and walls… If you set the temperature much lower than your normal setting, everything cools off. If the furnace is off for 8 hours while you are at work, all the heat stored in your walls and furniture is spent and now your furnace has to work to reheat them. You can sit in a 68º room and be comfortable, however, if everything in the room is colder than 68º then it absorbs the heat from the room and the room “feels” colder.Ice Furniture

It is more efficient and comfortable to maintain a relatively consistent temperature.

In May 2009, the EPA suspended Energy Star certification for programmable thermostats [PDF], writing:

EPA has been unable to confirm any improvement in terms of the savings delivered by programmable thermostats and has no credible basis for continuing to extend the current Energy Star specification.

Best practices to save money on heating/cooling
1. Program the thermostat to lower heat by 8 percent when away from home or asleep. (At 72º that’s less than 6º)
2. Keep blinds, shades and draperies open during the day to allow sunshine in to warm up the room.
3. Properly seal and insulate the attic, exterior walls, floors, basements and crawl spaces.
4. Install a weather stop to prevent cold air from coming in and warm air from escaping.
5. Use exhaust fans in the bathroom and kitchen as little as possible.
6. Change air filters at least every three months to prevent dust and dirt buildup.
7. Close vents and doors to unused rooms or bedrooms when not in use.
Programmable thermostats if used properly can offer some savings, however, it has been shown that few people actually use them correctly.