What is the optimum thermostat setback temperature?

expert EXPERT April 26, 2007

If you consider energy consumption as your only criteria, then you would program the thermostat to completely shut off the heat between 8 AM and 5 PM assuming that no one is home. The temperature in your house would drop substantially, however your heating system works most efficiently when it is running continuously, and therefore you would use the least amount of energy letting your house cool down and then bringing it up to 68 degrees Fahrenheit at 5 PM. What prevents you from doing this is the humidity levels within your home. Assuming your home has a 40 percent relative humidity level at 68F, the humidity level would soar to 100 percent and produce condensation on every surface in your home, if the temperature dropped to approximately 44F. It would literally be raining in your house.

At a temperature well above 44F, the relative humidity will be high enough to form condensation on the coldest surfaces within your house -- the windows. The temperature at which condensation will form in your windows, is partly a function of outdoor air temperature, indoor relative humidity at 68F, and your windows (single glazed, double glazed, argon filled, etc.). As long as you are not getting condensation on your windows, feel free to keep lowering the temperature during the day, when no one is home.