How low can your thermostat go?

Balancing comfort and savings
By James Dulley
How low can your thermostat go?

Setting your thermostat back in the winter can cut your power bill . The key is finding the temperature at which you and your family are comfortable.

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 actually does save energy overall if you lower the temperature setting on your central furnace or heat pump thermostat. It is surprising how comfortable you can be at a lower indoor temperature once you become accustomed to it. The actual amount of dollar savings depend 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 is likely to 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. 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.

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.

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 sweaters and thicker shirts, you will be able to gradually lower it more.

About the Author

James Dulley is an engineer and syndicated columnist for the National Rural Electric Cooperative Association. www.dulley.com

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