I'm hoping this is a relatively simple question to answer, but I just couldn't find it on Google.

With regard to air conditioning (or any cooling system that relies on compressing/evaporating), how do you actually control the 'cooling' temperature? After compressing/pressurizing the refrigerant, I understand that it becomes cool due to releasing heat, however, how do you control the level of coolness?

Is temperature directly related the amount of pressure being applied to the refrigerant? I.e, more pressure leads to lower temperature, less pressure leads to higher temperature?

Thank you!

  • 2
    $\begingroup$ You generally do not try to control the temperature of the refrigerant directly. Instead, you cycle the system on and off, varying the duty cycle to control the temperature of whatever it's cooling. $\endgroup$
    – Dave Tweed
    Commented Aug 16, 2016 at 18:15
  • $\begingroup$ @DaveTweed Is that achieved via some kind of switching valve? Where would that valve typically be located? $\endgroup$
    – Izzo
    Commented Aug 16, 2016 at 19:27
  • 1
    $\begingroup$ No, it's usually an electrical switch that controls the power to the compressor. If the temperature is too high, run the compressor; it it's low enough, turn off the compressor. Generally, the temperature-sensitive switch (a.k.a. "thermostat") has a certain amount of hysteresis so that the on-off cycles do not occur too frequently. $\endgroup$
    – Dave Tweed
    Commented Aug 16, 2016 at 20:48

1 Answer 1


The temperature of evaporated refrigerant gas is constant as a property of refrigerant material. And usually it is considerably below the thermostat setting in the controlled space. The design of the system relies on the circulation of cooled air and its gradual warming up to deliver the cool air at desired temperature so that it is still cool enough to be effective and not too cool to be uncomfortable. Thermostats cut off the power to the compressor pump which is the starting point of refrigeration four phase cycle.
Thermostats have built in delays as to switch the system back on or off with enough laps to allow the system get back to equilibrium and prevent freezing of valves or undue heat at the start up of the pump.

Along this path of delivery of the cool air there are incidental materials such as partitions, ducting, furniture, any material with high thermal index, that act as a moderator of temperature by absorbing heat from ambient air and delivering it back when the air coming from the unit it too cold. So the system's sudden switch on and off cycles are tempered into gradients of temperature that are hard to notice to occupants!


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