Why do we save money/energy if we set our furnace thermostat lower (say from 70 to 65 degrees?

Question

Assume that we're talking the same temperature (say 20 degrees) outside. Same house, thermostat set 5 degrees colder. Aren't we still talking about heating the same volume of air 2 degrees? (Assuming that the furnace kicks on when the temperature drops 2 degrees.) Whether we're talking heating from 68 to 70 or 63 to 65, it's still the same amount of heat/energy required. Where's the energy (and hence money) savings?

Answer 1

no man, we're talking about heating the home from 20 degrees.

It takes less energy to warm the home 45 degrees than 50 degrees.

Your analogy is kind of like "I had to push start my friends van 2 blocks, isn't that just as hard as pushing it 4"? You're pushing the same van, but you're pushing it further.

Answer 2

The rate of heat loss from the house to the outside is determined by leakage, radiation, the temperature difference between the inside and outside, and the insulation value of the building materials.

For instance, about 17% of heat loss is through single pane windows. They are poor insulators.

The rate of heat loss here follows a formula k x dt, where k is a constant related to the material and dt is the difference between the inside and outside temperatures. So if k remains constant and the difference in temperatures goes up, the rate of loss is greater.

You can see, then, that the greater the temperature difference, the higher the rate of loss. The difference between 20 and 70 degrees is 50 degrees. The difference between 20 and 65 degrees is only 45, or 10% less. If losses through construction materials were the only heat was loss, you could expect a 10% savings on your heating bill.

Answer 3

Actually, the heater puts out air that is hotter than the temperature that you actually want the house to be. So if you have your thermostat set 70 degrees, it's heating the air that is being pumped out to 80 degrees (or something along those lines). If you only have the thermostat set to 65, it will heat it to 75, which is less. Because it's heating the air that is being pumped into your house to a lesser degree, it saves you energy.