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Dear Pablo: I have heard that using a programmable thermostat to lower the temperature while you are out saves energy but isn't the same amount of energy used to bring the temperature back up when you come home?
The short answer is no. Because the rate of heat transfer through an insulated (or uninsulated for that matter) floor, ceiling, or wall is dependent on the temperature difference between the inside and outside air, less energy is lost to the outside when the inside air is cooler.
The Theoretical House With No Temperature SetbackLet's explore this with a theoretical example. Let's assume a 2,000 ft2 house (40 ft x 50 ft) with 10 ft ceilings and a flat roof. This gives us 2,000 ft2 of floor, 2,000 ft2 of ceiling, and 1,800 ft2 of walls, a total of 5,800 ft2. The outside temperature is 50°F and the desired indoor temperature is 70°F. Let's also assume average insulation coverage measuring an R-Value of 10. Using all of this information we can estimate that the energy loss is 11,600 BTU per hour, meaning that 11,600 BTU per hour need to be introduced into the home by the heater.
The Theoretical House With A Temperature SetbackIf we program our thermostat to lower the indoor temperature by 10°F to 60°F the difference between the indoor and outdoor temperatures will be less, resulting in less heat loss. In fact the heat loss per hour drops in half to 5,800 BTU per hour. While the exact numbers are dependent on variables such as the characteristics of your heater, but the energy used to raise the temperature back to 70°F is roughly equivalent to the energy saved while the temperature was allowed to drop by 10°F.
The Bottom LineWhile the numbers vary based on the size of your home, the thickness of your insulation, and the temperature difference between inside and out, it is clear that lowering your thermostat while you are away (or sleeping) is a good idea.
What About Turning The Thermostat Down By 1°?Of the many energy saving tips out there, one of the most frequent is to turn down your thermostat by just one degree. Using our theoretical house we can calculate that the hourly reduction in heat loss is 580 BTU, or 13,920 BTU per day. At a design efficiency of about 80% a heater would save 17,400 BTU worth of fuel. If your heater runs on natural gas you would save over 5 therms per month, or around $3. You can begin to see how turning your temperature down by several degrees, for at least part of the day, can save you a bundle, and reduce your impact on the climate as well.Pablo Päster is a weekly columnist for TreeHugger.com, an experienced greenhouse gas engineer and the Senior Environmental Program Manager at Hara Software. Send your questions to Pablo(at)TreeHugger.com or submit the via this form and connect to his RSS feed.
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