On most air conditioner remote controls, you can find buttons such as Heat / Cool. Users must switch to the cooling mode in summer, and to the heating mode in winter. Curiously, Heat 25°C(or 77°F) is usually too hot, while Cool 25°C is too cold, which may cause confusion to careless users.

Here is my question: is this button necessary at all? Why don't the air conditioner simply try to keep the room at 25°C, regardless of the current temperature?

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  • Wow, I have never seen a remote. Why not just a phone app? Unfortunately, thermostats are not yet able to sense how the temperature / humidity / day-night / sunny-cloudy conditions will feel to a person in the room. Until then, we have to nudge the thermostat endlessly as we go throughout the day, the sun comes out from a cloud heating the roof over the room, etc. Temperature is felt by each person individually and varying over time and circumstances (hungry or tired --> cold, etc.). We should all just move to Hawaii and then it would be great! (Reminds me of the book Stand on Zanzibar.)
    – user67695
    Feb 23, 2017 at 15:15

1 Answer 1


Most air conditioning systems are built to do all three things.

They can be used to set a fixed temperature but this is incredibly inefficient as the heating and cooling systems are effectively working against each other as the heater needs to over heat the area for the sensor to register the correct temperature for a short while before it gets too hot and the cooling needs to kick in, overcooling the area, and so on.

To solve this, a wider range of temperatures is used. Cooling can be set to kick in when the temperature is above say 23° and the heating when the temperature is below 19°. That way the two systems are only reacting to the rooms natural temperature rather than the temperature created by the opposing system. For actual numbers it's probably best to ask a heating engineer.

There may also be cases where there really is no need to heat an area but it must be kept below a given temperature or vice versa.

  • 1
    The range of temperatures between the heat and cool turn-on points is called the "dead zone". It is typically 4 C or 6 F so that, as mentioned, the two systems are not constantly swapping back and forth. This has always been a part of thermostats, and in the old mechanical ones the two sliders were unable to get closer together than that, although you could separate them farther.
    – user67695
    Feb 23, 2017 at 15:12

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