system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5. Is it better to have your thermostat turned down when you are out of the house and have it come back on when you get home or to keep it at a constant temp?

I got in a fight with my girlfriend about this and I actually have never gotten specific info on which is better, so fill me in guys!
gpt
It is better to turn down your heat when you are not home. tl;dr: The amount of energy it takes to bring your house from cold to warm is less than the amount of energy required to keep it warm all day.

To answer your question, you need to define *better*. I'll take better to mean *wastes the least amount of energy*. If each technique keeps your house the same temperature when you are home (the energy inside is constant), what we need to compare is **how much energy gets leaked outside**.

When your house is hotter inside than outside, the energy (heat) will move from areas of higher concentration to lower concentration. The warmer your house is than the outside, the faster this will happen. It's the same reason why you can boil water faster on a stovetop set to high than a stovetop set to medium-high. The hotter something is (like the burner), the faster heat will flow from the source to the surrounding environment (like the water). You can look up Newton's Law of Cooling for equations that describe this effect.

So, when you keep the house warm all day, you leak heat outside at a constant rate. The only way to leak heat faster is if you heat the house above room temperature. If you turn down your heat when you're gone, at first the heat leaks at the same rate as a heated house, but as the house cools the rate of heat leaking will slow down simply because there is less of a temperature difference between outside and inside. When you need to reheat the cold house, it doesn't matter that it might feel like you are using a lot of energy, as you bring the temperature of the house up you are always leaking heat at a slower rate than if you kept it that temperature the entire time.