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October 2011 Energy Solutions
How low can your thermostat go?
 by James Dulley
Engineering Consultant
Setting your thermostat back in winter can save energy and money
Q: I hear how important it is to lower my thermostat setting during winter. It seems it would just take more energy to reheat the house each morning. What is the best thermostat setting for the most savings?
A: Selecting the proper temperatures throughout the day and night can be a bit confusing. You want to balance comfort with energy — and dollar — savings. It is surprising how comfortable you can be at a lower indoor temperature once you become accustomed to it. Thereafter, you find yourself uncomfortable at higher indoor temperatures that used to seem normal.
It actually does save energy overall if you lower the temperature setting on your central furnace or heat pump thermostat. The actual amount of dollar savings depends primarily upon how low you set the thermostat, how long you have it set back, and, to a lesser degree, your climate.
There are also other advantages to lowering the thermostat setting during winter. If your house temperature is lower, it requires less moisture indoors to keep the indoor air at a given relative humidity level. The fact that your furnace or heat pump runs less at a lower indoor temperature means the equipment will last longer and need fewer repairs.
If you look at setback savings charts, don’t be confused by the fact that the percentage savings are actually higher in milder climates than in colder climates. This is because the total amount of energy used to keep a house comfortably warm in a cold climate is much greater than in a warm climate. This makes the base number larger in cold climates so the percentage savings are less even though the dollar savings are greater.
It is a common myth that it takes as much energy to reheat a house, in the morning for example, as was saved during the temperature setback period overnight. The amount of heat a house loses through its walls, ceilings, and floors is directly proportional to the difference between the indoor and the outdoor temperatures. Air leakage into and out of your house also increases with larger temperature differences.
When the indoor temperature is set lower, the indoor-to-outdoor temperature difference is smaller, so less heat is lost from your house. During the summer, the same is true in reverse. If less heat is lost from your house, your furnace has to use less gas, oil, or electricity to create the heat to replace it. The amount of heat used to reheat the house, therefore, is less than the amount saved over the temperature setback period.
The only time a temperature setback may not be wise is if you have a heat pump with backup electric resistance heat and an old thermostat. When it is time to reheat the house and you set the thermostat higher again, the expensive backup electric resistance heater may come on. For a long eight-hour setback, you will likely still save overall, but not for just a short couple-hour setback.
If you have a heat pump, install a special setback thermostat, designed for heat pumps. These heat pump thermostats have electronic circuitry to keep the backup resistance heating elements off after the setback period. My own heat pump thermostat works this way, and it also allows me to block out the resistance heating when the outdoor temperature is above a certain temperature. I have mine set at 20 degrees.
There is not a “best” thermostat setting for all homes and climates. The lower you set it, the greater the overall savings will be. The amount of savings per degree for each nighttime eight-hour setback period ranges from 1 percent to 3 percent. Because many people are also gone during the daytime, the temperature can be set lower for about 16 hours per day. Unless there are some health problems in your family, 62 degrees is comfortable if you are wearing long sleeves or a sweater.
In moderate climates, let your comfort dictate how low you initially set the furnace or heat pump thermostat. As you get used to the lower temperatures and wear a sweater, you will be able to gradually lower it more. In colder climates, excessive window condensation often limits how low the indoor temperature can be set. In order to set the temperature lower, you will have to reduce the indoor humidity level.
James Dulley is a nationally syndicated engineering consultant based in Cincinnati.
If you have a question about energy use or energy-efficient products, send it to: James Dulley, Electric Consumer, 6906 Royalgreen Drive, Cincinnati, OH 45244; or visit www.dulley.com.
For more information
Touchstone Energy’s “Together We Save” website offers a variety of information on how to conserve energy and use electricity more efficiently. It includes a savings calculator and videos, including some with Indiana’s own Bob Geswein from Harrison REMC, that show you how to save. The site also includes multiple links, graphics and other information. Check it all out at:
www.togetherwesave.com.
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