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Any heat exchange system as the air conditioner (AC) depends mainly on the common refrigeration cycle: evaporator, compressor, condenser and the expansion valve. Then, what does the AC do in order to provide various temperatures for the cooled air indoors?

Specifically, if I lower the temperature of the cooling air from 20°C (68°F) to 18°C (64°F), what does the AC do to achieve this lower temperature?

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  • Sounds like you've got a pretty good understanding of how AC works. Care to edit your question to be more specific about exactly what part you're confused about?
    – FreeMan
    Commented Jan 27, 2023 at 12:48
  • Are you referring to a temperature control on the front of the AC unit, where one can dial in 20 C (68 F) or 22 C (72 F) and the AC gets the air to that temperature? Commented Jan 27, 2023 at 12:49
  • @Triplefault Yes that is exactly what I mean Commented Jan 27, 2023 at 12:52
  • @FreeMan Ok, thanks for your clarification Commented Jan 27, 2023 at 12:53
  • I have edited your question to ask what I think you're asking. If I've missed, you are free to edit it further yourself to clarify it. However, I'll write up an answer based on that.
    – FreeMan
    Commented Jan 27, 2023 at 12:58

1 Answer 1

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Air conditioning (and heating) is controlled by a thermostat where you select the room air temperature you'd like.

The AC system, as a whole, will only put out cold air at (roughly) one temperature - very cold (hold your hand over the vent when the AC is blowing - the air is quite chilly in comparison to room temp). It relies on mixing this very cold air with the air already in the room to achieve the temperature requested by the thermostat. It does not put out 68°F air if you set your thermostat to 68°F. If it did, and the room air was currently at 70°F, your house would never get down to 68° because there are things in the house (like you) that are producing heat in addition to the house leaking cooled air to the atmosphere and absorbing heat from the sun. It has to produce much colder air to cool the house in a (reasonably) short period of time, overcoming all the heat sources in the house in addition to the cool air losses.

The thermostat ('stat) has a thermometer built in to it as well. The 'stat compares the actual room air temperature (from the thermometer) to the desired set point. If the actual temp is above (usually by 2°F, ~1°C), then it calls for the AC system to turn on.

The AC system will pump out very cold air. Usually around 25-30°F colder than the outside air temperature. It blows this air into the house, where it is mixed with the air already in the room, slowly cooling the overall temp of the room.

Once the overall room temp has dropped below (again, usually by 2°F) the temp requested by the 'stat, the 'stat tells the AC machinery to turn off. The temp in the room immediately (but, generally, slowly) starts to rise again until the air temp is above the set point and the whole thing starts over.

The 2°F range above and below the set point allow for hysteresis and prevent the AC equipment from "short cycling". Short cycling will cause the machinery to freeze, preventing it from working short term (until it's defrosted) and will shorten the lifespan of the entire system. Additionally, without this 2° "dead zone", the AC system would be blowing nearly constantly, cooling the house below the temperature desired, slowly making it colder and colder, causing you to adjust the 'stat setting up, turning off the AC, allowing the house to get warmer, causing you to turn the 'stat down, turning on the AC... It just does all that for you instead of making you do it yourself.

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    Thanks for the reminder about short cycling. I'm in the process of implementing my own temperature controls/smart remotes. The system's existing logic should be sufficient to prevent short cycling but it's something to be aware of, especially when evaluating the success of my project
    – keshlam
    Commented Jan 27, 2023 at 15:12
  • There's also the newer two-stage and variable speed compressors that can operate at different "power levels", but these are rare compared to the single speed compressor you are assuming in this answer.
    – JPhi1618
    Commented Jan 27, 2023 at 15:45
  • Do those actually put out different air temperatures, @JPhi1618, or do they just blow cold air at different speeds? I dunno, so I'm asking...
    – FreeMan
    Commented Jan 27, 2023 at 15:48
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    There's actually two options - you can have a multi-speed blower that can blow at full speed when cooling and then drop down to a lower speed to circulate air when the compressor isn't running. Then there are multi-speed compressors that actually cool more or less, and they run almost continuously, but spend most of their time at a low power level. The control schemes are pretty manufacturer specific, so you are normally locked into their proprietary thermostats and repair parts are a lot more expensive since they are less common and proprietary. That's likely the reason they are so rare.
    – JPhi1618
    Commented Jan 27, 2023 at 15:56
  • Not that rare - most (probably not all - mine are, and they are dirt-common) mini-split units are variable speed drive. They vary both compressor speed and fan speed to meet load.
    – Ecnerwal
    Commented Jan 27, 2023 at 21:23

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