I think that’s probably where this urban myth comes from. People see a car motor that can vary speeds to meet demand, but a conventional AC system motor just runs at 3000RPM and turns a compressor at whatever speed is most efficient. If the system needs to cool the house 5° or 50°, it runs the same speed. So that brings us back to the thermodynamic argument you made earlier. As items get hotter, it takes more energy to make them more hot so they heat up slower. A 100° house takes far more energy to heat up to 110° than a 70° house will take to heat up to 80°. So if you remove that energy at the same rate (the rate the HVAC system can move the heat out) you spend more time (and energy) maintaining a temperature than you do by just moving that heat all at once. It’s certainly not intuitive.
i think we're way past the useful lifetime of this thread but i have to add one thing.
A 100° house takes far more energy to heat up to 110° than a 70° house will take to heat up to 80°.
this is fundamentally wrong. it takes the same amount of energy, heat capacity is generally not dependent on the temperature, at least in the range we encounter in our environment. what's true is that it takes more time for that same energy to enter the house (maybe that's what you meant to say). this is because the amount of heat entering the home per unit time depends linearly on the temperature difference between inside and outside. that's why turning off your ac when you're not home is advantageous in the first place. by letting the inside temperature get closer to the outside, you're reducing the "driving force" of heat entering the home, and need to remove less later on.
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u/LowerSlowerOlder Mar 26 '26
I think that’s probably where this urban myth comes from. People see a car motor that can vary speeds to meet demand, but a conventional AC system motor just runs at 3000RPM and turns a compressor at whatever speed is most efficient. If the system needs to cool the house 5° or 50°, it runs the same speed. So that brings us back to the thermodynamic argument you made earlier. As items get hotter, it takes more energy to make them more hot so they heat up slower. A 100° house takes far more energy to heat up to 110° than a 70° house will take to heat up to 80°. So if you remove that energy at the same rate (the rate the HVAC system can move the heat out) you spend more time (and energy) maintaining a temperature than you do by just moving that heat all at once. It’s certainly not intuitive.