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Thermostat Settings

Thermostat Settings and Programmable Thermostats

        written on 6/30/04

        This article is largely in response to questions I have received about thermostat settings and programmable t-stats.  Since air conditioning accounts for the largest portion of your electric bill in summer months, adjusting the thermostat can make a huge difference.  DOE, utility companies, and other energy people recommend programming thermostats to 78 in the summer.  I usually keep mine 79-80, although not everyone is comfortable at that temperature.  Turning ceiling fans on can make it feel several degrees cooler.

        Here is a partial explanation of why the savings occur.  The air conditioner works to maintain a set temperature.  The "load" or amount of heat which must be removed is largely related to the difference between the outside, and inside temperatures.  If it is 95 outside and the t-stat setting is 78, then the difference is 17 deg.  If the t-stat is set to 71 instead, the difference is now 24 deg.  In this case, the AC now has to work 41% harder to keep the indoor temperature at 71 as compared to 78.  Since the AC equipment is not 100% efficient, this can result in more than 41% increase in the cooling portion of the utility bill.

        I have read several articles which attempt to quantifying the extra cost for lowering the thermostat.  One said it adds 8% to the cooling portion of your bill for every degree lower than 78.  Another reads each degree below 78 will increase your bill by as much as 3%.  Actual numbers will vary from house to house with the overall efficiency of your home and conditioning equipment.

        Also, if you are out of the house on a regular basis for work or play, a programmable t-stat can automatically adjust the temperature while nobody is home to save even more.  It can be programmed to cool the house back down before you get home, so you will not have to walk into a warm house.  Depending on the size of your house and how cool you keep it 80-85 is about the highest that you would want to set the temp while you are out.

        After installing a programmable thermostat in my house, I recorded about 2 hours a day less A/C run time.  The payback time for my thermostat was only one month.  There are many types and brands of programmable thermostats, so you may need to check with your local HVAC dealer or Service Company to match the correct one for your equipment.

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