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Thermostat Theories

The "Ask REMC" column features questions from our customers. Feel free to submit your questions about REMC operations, energy efficiency issue, electric safety, whatever is on your mind that you would like to have answered. This month's question is one that has been asked by several customers and is fielded by Harrison REMC Energy Services Specialist, Bob Geswein.

Q: During the summer, when I leave my home in the morning, I adjust my thermostat setting up. I have understood that this will save electricity because my air conditioner won't run as much during the time no one is home. I have recently heard that it's just as economical to leave your thermostat at the same setting while you're gone. I would like to know which is the best method.

A: This is a question many folks have concerns about. The first strategy mentioned regardging setting your thermostat higher when no one is home gets its electricity savings from the fact that the AC unit doesn't run as much during the day as it would with a lower thermostat setting. However, my personal experience in my home and through working with homeowners while in the heating and cooling industry has made me strongly consider the second strategy of leaving the thermostat setting alone.

Here's why:
If you normally set your thermostat at 78 degrees while at home but raise it to say, 85 degrees during the day, three significant events happen. The air temperature in your home is allowed to rise to 85 degrees . Second and more significantly, all the wall, floor and ceiling surfaces, appliances, furniture and other objects will also rise in temperature to 85 degrees. Third, the relative humidity in the home increases at the same time.
When you return to your home and reset the thermostat to 78 degrees, it will normally be at about 5:00 p.m. As your AC unit runs to cool your home, it will have to run continuously for a long period of time during this, the hottest part of the day, to remove the large amount of heat that built up in your home. A considerable number of us, myself included, are not patient and expect relief quickly. When relief is not prompt, we may feel that the AC unit is not large enough to meet our needs.
The fact is, your HVAC contractor and your AC manufacture do not generally agree with you about your AC unit being too small. If the system was designed and sized correctly when installed, you are likely to experience this lengthy period of discomfort when you return home and reset the thermostat back to your "comfort" setting.

Here are some reasons why:

  • Correctly designed and sized AC units do not have the "extra" capacity to bring down the hot household temperatures quickly. Most systems are sized to maintain your home's comfort level, not change it quickly. When outdoor temperatures exceed 95 degrees, it is even more difficult for the unit to change indoor temperature on demand.
  • During the period of time that our AC is running to meet the 78-degree setting of our thermostat, the air temperature in our homes drops first. The wall, floor and ceiling surfaces, our appliances, furniture and other objects that have mass continue to hold heat longer than the air. This means when we stand over our cabinets preparing the evening meal, the counter top is warm and radiating this heat towards us. Also, if we relax in our recliner to read the paper, our body's full length is now in contact with an 85-degree surface and we'll feel uncomfortable. This means our total comfort won't be restored in our home until all these objects have also been cooled back down to 78 degrees. This can literally take hours instead of minutes.
  • A major component of our comfort in addition to temperature is humidity levels. Our AC units remove humidity from our homes while they are cooling them. During the day, if the thermostat was re-set allowing higher temperatures, our AC unit didn't run as much and humidity levels in our home will now be higher also. As in the case of the objects in our homes, it takes longer to reduce humidity levels than it does to reduce air temperatures. This condition extends the length of time it takes our home to reach the total comfort level we desire.

These three conditions defeat our AC unit's ability to quickly provide the comfort we desire after arriving home to an 85-degree environment. Our AC unit must run for a long period to restore the comfort level we desire. This can offset any electricity savings your AC unit may have achieved by not running during the day.
From your REMC's perspective, your AC unit running continuously during the hottest part of the day, along with all your neighbors, contributes to a "peak demand" for electricity that sometimes is tough to satisfy. Allowing your AC units to maintain a cooler temperature in your home during the day also allows more AC units to cycle on and off during this time, which helps reduce this "peak demand." If the reduction of peak demand is substantial enough, REMC may not need to purchase as many KW's of more expensive electricity on the open market or build more generation capacity just to meet this demand. You'll win with more comfort and less expensive electricity.

Thank you for your question. I hope this information has been helpful to you. I invite any of our customers to send their questions to:


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