A few months ago I was having a discussion with someone about how school systems often turn the air conditioning off -- or, nearly off -- during the summer months in order to reduce electricity costs.  They may only cool the buildings from 8am to noon so that the conditions are just tolerable for the limited personnel that are actually present.  The discussion was about the effect that a drastic change in humidity has on the wood furniture because the air is very dry in cold winter months, but in summer months with no air conditioning, the air will get very humid.  Lumber expands in humid conditions and shrinks in dry conditions... but that's another story. 

Then someone asked if it really was a cost savings to turn the air conditioning off every day.  My first reaction was, "Of course it's a cost savings because if it's not running, you're not using electricity which costs money!"  He countered with the point that the building still gets hot, and when the AC does come back on, it has to work that much harder to remove all that heat.  The constant off-and-on of the system and the heavy load on it outweigh any savings due to not running it at all for several hours.  So, which is right?

The Engineer in me can't let a question like this just dangle there, begging to be answered!  I really thought I was right, but I wanted to be able to explain the science behind it.  And if you, too, have that same curiosity, read on.

[WARNING:  Wordy science stuff follows.  Proceed at your own risk.]

Picture your house at 8:00 on a summer morning, just after everyone has left and gone to work for the day.  The sun has just come up and it's still a comfortable 74 degrees inside.  As the sun rises and shines directly on your house, it heats up inside.  (Duh!)  This heating represents energy being absorbed all day until you return home at 5:00.  Today we left the AC unit off, and the house is extremely hot at 84 degrees inside.  The amount of energy it took to warm the house by 10 degrees is expressed in BTUs or kiloJoules.  Let's say that the sun pumped 100 BTUs of heat energy into your house during the day.  When you get home and turn the AC unit on, it has to work to remove those 100 BTUs which will reduce the temperature back to a comfortable 74 degrees.  After converting BTUs (energy) to kWh (power), and then on into dollars, you paid the power company to remove 100 BTUs.

Now, picture your house on the same summer morning, but this time you leave the thermostat at a comfortable 74 degrees.  By 8:30, the morning sun has raised the indoor temperature by 1 degree and the AC kicks on.  To reduce the temperature back to 74 degrees, the AC unit has to remove (in our example) 10 BTUs of heat energy.  OK, after ten minutes, the set temperature is reached and the unit shuts off.  Twenty minutes later, the ever-rising sun has again warmed the house to 75 degrees.  The AC unit again runs to pull that heat back out, again at a cost of 10 BTUs.  Throughout the day this routine repeats and actually occurs more frequently as the hot afternoon mercilessly beats down on your roof.  The AC unit may cycle on and off 50 times during the day to maintain 74 degrees with each cycle costing you 10 BTUs.  So, during the day you paid the power company to remove 500 BTUs, much more than in the first example.

You may be saying, "But, Jason, you just made those numbers up.  You could make it work any way you want it to!"  Yes, I did make those numbers up, but there's one more twist to the story.  In the first example with the AC unit off, the house heats up during the day and eventually stabilizes so that it can't absorb any more heat.  As the house warms, the temperature difference between outside and inside decreases, and the house absorbs heat at a slower rate.  Eventually the house is as hot as it's going to get, and no more heat gets into the house.

In the second example with the AC set at 74 degrees, there is always a large difference between outside and inside temperatures, and the house readily absorbs heat all day.  The house is always drawing in heat, and it is always being removed.  So, no matter what the numbers are, a house that is much cooler than the outside acts as a constant heat magnet always drawing in more heat, and the AC unit has to constantly work to expel that heat. 

The moral of the story is that the less heat there is to remove, the lower the cooling costs will be.  So it is a cost savings to turn the AC off when cooling is not needed.

Whew!  All this typing wore me out.  Is it getting hot in here?