Understanding Your Thermostat

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?—Don G.

Selecting the proper temperatures throughout the day and night can be 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.

It 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 on how low you set the thermostat, how long you have it set back, and to a lesser degree, your climate.

There are 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.

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 overnight. The amount of heat a house loses through its walls, ceilings, and floors is directly proportional to the difference between the indoor and 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 uses 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 two-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 to 3 percent. Since many people are gone during the day, 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.