Heating & Cooling Energy Savings Explained

During Winter, why does lowering your thermostat during certain times of the day save you money?

Eg: My daughter is at the University of Waterloo and is sharing a home with 4 other students. They're seeing first hand that heat ain't cheap! She called me for some insight on a discussion she was having with her roommates on how they could save money on their heating bills in their house.  She was trying to convince them that when they left the house, they needed to lower the thermostat from 20°C to 18°C. Her roommates’ contention was that by lowering the thermostat when they are away, the heating system will use more energy to get back up to 20°C when they turn it back on. Therefore, they just want to leave the system at 20°C all the time

A common misconception associated with thermostats is that a furnace works harder than normal to warm the space back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. This misconception has been dispelled by years of research and numerous studies. The fuel required to reheat a building to a comfortable temperature is roughly equal to the fuel saved as the building drops to the lower temperature. You save fuel between the time that the temperature stabilizes at the lower level and the next time heat is needed. So, the longer your house remains at the lower temperature, the more energy you save.

Still don’t believe it?  Think of it this way:  Instead of being gone just a few hours, you decide you are tired of the cold and go to Hawaii for the rest of the winter.  Do you honestly think you would be saving more energy by leaving your thermostat at 20ºC as opposed to lowering it for the the amount of time you are sipping Mai Tai’s and getting sun burned on the beach? To re-iterate the above statement: “… the longer your house remains at the lower temperature, the more energy you save.”  It doesn’t matter if this time period is a few hours, days, or years!

These facts don’t change depending on if you live in a moderate, hot, or extremely cold climate.  As long as your location follows the Laws of Thermodynamics (which unless you are in some other Universe, it does) this applies to you!

In the Summer, why does raising your thermostat during certain times of the day save you money?

Eg: If I have the thermostat set at 23ºC, when the temp oputside gets to 24ºC, my a/c turns on.  If I set it at 24ºC, when the temp gets to 25ºC, 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?

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

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 the 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 above example question poses why there was a difference in setting your thermostat to 23°C or 24°C, because no matter what it's 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.  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?

Happy Energy Saving!

Mario