Cost to Maintain Established Temperature

Old 06-14-11, 11:45 AM
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Cost to Maintain Established Temperature

My wife and I have an ongoing argument that, don't tell her, I'm beginning to see her logic behind.

I have always kept the AC set at 74 as I've heard it costs less to keep it a little warmer. My wife likes it around 72.

She argues that once it is at a certain temperature it will cost the same to maintain that temp whether it is 72 or 74. Sitting here thinking about it I see her thinking.

So, in a well insulated house, ours is, would it not cost the same to maintain the temp at 72 as it would to maintain it at 74?
Old 06-14-11, 02:49 PM
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Let me put it this way, the cost difference between maintaining 72 vs. 74 is minimal. If you were to ask about 68 vs. 78 then it would likely cost noticeably less to maintain the higher temperature.

Here is the science. Heat ALWAYS travels from high to low, meaning that higher temperature areas always lose temperature to lower temperature areas. Quite simply, there is no such thing as cold, only an absence of heat. The greater the temperature difference the faster the heat transfer, or to put it into your scenario the greater the difference is between the temperature inside your house and outside the faster the temperature will change AND the greater the amount of energy required to maintain any particular temperature inside.

For example, if the outside temperature is 75 and the inside temperature is 73 then with no mechanical cooling the inside temperature will slowly rise until it reaches the outside temperature. If the outside temperature is 90 then the inside temperature will rise at a much faster rate because the DIFFERENCE between outside and inside is greater. This also means that if you desire 73 degrees (I'll split the difference between your desired temperature and your wife's) and it is 78 degrees outside it will take less energy to maintain the inside temperature than it will if the outside temperature rises to 90.

The difference between 72 and 90 is large but the difference between 74 and 90 is only very slightly less with the difference between 73 and 90 (a compromise) being only slightly more energy used than maintaining 74 and slightly less than maintaining 72. Since the outside temperature is constantly changing it is almost impossible to make any kind of absolute energy vs. temperature statement that would would always be true.

There are several other issues also that are important. Each human body inside the house generates approximately 600 BTUs per hour to raising the inside temperature and more if they are doing anything more than just sitting around reading the paper. Cooking, of course will add to the heat inside and raise temperatures. Use of any lighting or electronic equipment also raises temperatures. Relative humidity plays a big role in how people "feel" at different temperatures and how well your home is insulated AND air-sealed can make a major difference in how much energy is used to maintain a certain inside temperature.

So, in short, I suggest that you compromise on 73 and both admit that the other has some valid points to their argument. The difference in energy consumption will be minimal.
Old 06-19-11, 09:02 AM
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You need to send your wife back to her high school physics class.

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