Why does programming your thermostat save energy?

I recently had a reader ask this question regarding controlling the temperature in her home with her programmable thermostat and air conditioner and how that affects her energy consumption:

I live in Florida so my question has to do with cooling my home.  I notice that if I have the temp set at 76, when the temp gets to 77, my a/c turns on.  If I set it at 79, when the temp gets to 80, my a/c turns on.  To me that's cooling one degree.  So, once my house gets cooled to the desired temp, the system is going to turn on just as frequently.
Therefore,  I cannot understand the thinking behind turning the thermostat to a higher temp when I'm not there.
What am I missing?

And I can understand her confusion.  Both heat flow and electrical consumption are invisible to most of us, so sometimes it's hard to grasp the interplay. As part of my response I linked to an article I wrote last winter about why turning your thermostat down while you are away is a good idea.

When thinking of the best way to answer her question, I tried coming up with an analogy, and then when I originally started writing this post I came up with what I thought was a simpler analogy, but I'm starting completey over and doing whatmy teachers used to say: K.I.S.S. (Keep It Simple Stupid).

In the summer, when the sun rises it heats up the air, which then heats up your home through conduction. The heat that enters your home eventually escapes back into the air.  It does this either:

  1. When the sun goes down (the sun stops adding heat, so the heat escapes back into the atmosphere) OR
  2. Your AC unit removes the heat from your home and dumps it into the atmosphere

That heat is usually measured as BTUs in the U.S.  Don't worry about what a BTU is, just know it is a measure of an amount of heat.  A BTU is a quantity of heat, and a BTU/hr is a rate of heat flow.

Instead of thinking of the heat that enters your home as invisible heat, think of it as a quantity of heat.  When you turn on your air conditioner, you are removing a quantity of heat, and he lower you set your temperature to, the most of that quantity of heat has to be removed (by your AC unit).  The more heat your AC unit has to remove, the harder it has to work, and when your AC unit works harder, it consumes more energy, which costs you more money!

The reader was wondering why there was a difference in setting your thermostat to 76° or 79°, because no matter what she set it at the air conditioner would just turn on when it got one degree hotter, so if it's still going to turn on what does it matter?

The rate of heat flow into your home is controlled by Fourier's Law of Conduction.  I won't go into the law, but it basically says that as the change in temperature (delta T = T1 - T2) increases, so will the heat flow.  In other words, heat is going to enter your home quicker if there is a big difference between the outside air temperature and the inside temperature in your home that is controlled by your air conditoner. If you turn your air conditioner down so your home is cooler, you are increasing the delta T (change in temperature between outside and inside) and the heat will flow into your home quicker, thus forcing your air conditoner to work harder to keep it at the set temperature.

If you raise the temperature of your home in the summer and bring it closer to the outside temperature, the heat flow rate into your home will slow down, menaing your air conditoner wont have to work as hard to keep it cool.  The harder you make your air conditioner work, the more electricity you will use to run it!  Does that make sense?

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Programmable thermostats don't work well with radiant in-floor heat systems, right? (we already have an outdoor reset control-which measures outdoor temps/heating needs) We live in cold, northern Minnesota and during ice-cold winter months, we use a radiant in-floor heating system. (During the "shouder seasons" of the year, our house is heated with solar passive heating through windows oriented to the E/S to capture sunlight- we rarely need the forced-air furnace, when the real cold sets in, we turn on the radiant system and the concrete floor heats up.) We do not use air conditioning. Thanks for your blog, it is very helpful in understanding complicated energy issues.

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