Energy Efficiency

Does turning the thermostat down at night really save energy?

An age old argument against turning the thermostat down overnight or while you are away is that it just causes your furnace or AC to have to work harder to catch back up, and therefore negates any savings. But is this true?

So Does it Actually Save Money?

While it is true that this strategy will make your heating and cooling system run a for a while to catch back up to your desired temperature, in total it will still use less energy than if you had maintained that temperature all along.  And the longer the period for which you are able to keep the settings turned down, the more that you will save.

The reason for this is because the bigger the difference in temperatures on the inside of your home compared to the outside, the faster energy is leaked between them. In thermodynamics, the speed at which heat is conducted between two areas is affected by the insulation between them, as well as the temperature difference between the two.

A good analogy to help understand this, is to imagine your home is a bucket of water with a small hole in the bottom. Your furnace or air conditioner is trying to maintain a certain water level, but some of the water is continually leaking out. And the higher the water level, the bigger the hole in the bottom of the bucket becomes. To keep the bucket full all day and night long will take much more water than if you allowed it to drain down closer to the outside level, and then later decide to fill it back up. The amount of water needed in this example directly correlates to amount of energy your HVAC system uses in real life.

Another common misconception people have is that if they set their thermostat really high or low, than the system will cool off or heat up faster. For the majority of furnace and air conditioning systems out there, this is not true.  Think of your thermostat as a simple on/off switch. The thermostat simply tells the system to turn on, and then back off when the desired temperature is reached. The system will not heat your chilly home any faster when you set the thermostat to 85 instead of 72!

So how can you save money and energy?

There are a lot strategies, but some offer more bang for your efforts than others.

  1. Set thermostats to reasonable temperatures, and adjust them down at night or when your you will be away for several hours. Programmable thermostats can help with this, but they are only as good as the program you set them to follow.
  2. Use ceiling fans and portable fans to lower the heat index in hot weather. Although, the temperature does not actually lower, the movement of air creates a wind chill affect and makes us more comfortable.
  3. Seal your home up as well as possible. Drafty windows and doors can have a huge affect on your home temperatures. Focus on these before improving insulation.
  4. Improve your homes thermal insulation. This not only includes the exterior walls, but also the attic.
  5. Newer, high efficiency windows can definitely help too, although they are lower on the list due to the high expense, and it may takes decades to recoup the costs in energy savings. But if you are already replacing your windows for other reasons, then upgrading the efficiency is definitely worth the investment over the life of the new windows. You can also greatly improve the efficiency by installing solar window film to your existing windows.
  6. Improving the ventilation in your attic with additional vents or solar attic fans can reduce the heat gain from your ceilings in hot summer months.
  7. Don’t forgot to look into saving energy on your other appliances as well, such as reducing the energy used by your clothes dryer. Many of these tricks can actually reduce the energy used by your HVAC system at the same time, if they are not heating up your home as much, or if they are transferring your comfortable inside air back outside.

2 thoughts on “Does turning the thermostat down at night really save energy?”

  1. Hi Mark,

    as for your last paragraph “Think of your thermostat as a simple on/off switch.” This is not quite accurate, at least for furnaces, anymore.
    Nowadays there are modulating/variable speed systems that do run on different levels depending on the difference between current and set temperature. If the house is at 70 and the thermostat is set to 72, it will run on a lower level (heating up slower) than if it is set it to 90, at which instance it will run at the highest level (fan speed as well as BTU input) and heat up faster.
    Some even have an outside thermocouple to take into account outside air temperature, both, for determining delta t as well as of incoming combustion air.


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