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How does an air conditioner work?

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Quick answer

An air conditioner uses a refrigerant that absorbs heat indoors by evaporating, then releases that heat outdoors by condensing. A compressor drives the loop, and an expansion valve resets the pressure. Because it moves heat rather than making cold, the outdoor unit always exhausts warm air.

An air conditioner moves heat from inside a room to the outside air using a refrigerant that evaporates and condenses in a continuous loop. It does not create cold; it removes heat, which is why the outdoor unit blows out warm air.

Step by step

  1. The compressor squeezes the refrigerant gas, raising its pressure and temperature.
  2. The hot, high-pressure gas flows to the outdoor condenser coil and releases heat to the outside air.
  3. As it cools, the refrigerant condenses into a high-pressure liquid.
  4. The expansion valve lets the pressure drop suddenly, making the liquid cold.
  5. The cold liquid enters the indoor evaporator coil and absorbs heat from room air, evaporating back into a gas.
  6. The gas returns to the compressor, and the cycle repeats.

Key things to understand

  1. A refrigerant flows through a closed loop of tubing, changing between liquid and gas.
  2. Inside, the refrigerant evaporates in the evaporator coil, absorbing heat from room air.
  3. The compressor raises the refrigerant's pressure and temperature, making it hotter than outside air.
  4. In the outdoor condenser coil, the refrigerant condenses and releases heat to the outside.
  5. An expansion valve drops the pressure, cooling the liquid before it returns inside.

Labelled diagram

  1. Compressor

    Pumps refrigerant gas to high pressure, raising its temperature.

  2. Condenser coil

    Outdoor coil where hot gas condenses and releases heat outside.

  3. Expansion valve

    Drops the pressure, turning the liquid cold before it goes inside.

  4. Evaporator coil

    Indoor coil where cold refrigerant absorbs heat and evaporates.

  5. Back to compressor

    The gas returns to start the cycle again.

The refrigeration loop in an air conditioner

Real-world examples

  • A window air conditioner has both coils in one box, with the indoor side blowing cold and the outdoor side blowing hot.
  • A split system separates the indoor evaporator unit on a wall from the outdoor condenser unit connected by refrigerant pipes.
  • A car's air conditioner uses the engine to drive its compressor, cooling the cabin while releasing heat through a condenser in front of the radiator.
  • A refrigerator works on the same loop, but its evaporator coil is inside the food compartment and its condenser coil is on the back.

Common mistake

✗ Myth: An air conditioner creates cold air.

✓ Fact: It cannot create cold; it moves heat from indoors to outdoors. The cooling you feel is simply the absence of heat that has been carried away by the refrigerant.

People also ask

What is refrigerant and why is it used?
A refrigerant is a fluid that easily changes between liquid and gas at useful temperatures. It absorbs large amounts of heat when it evaporates and releases that heat when it condenses. Common refrigerants include hydrofluorocarbons, chosen because they boil at very low temperatures.
Why does an air conditioner freeze up?
Ice forms on the indoor coil when the coil gets too cold and moisture in the air freezes on it. This can happen if airflow is blocked by a dirty filter or if the refrigerant charge is low. Restricted airflow means the coil cannot absorb enough heat to stay above freezing.
How is an air conditioner different from a fan?
A fan only moves air around, which helps sweat evaporate from your skin and makes you feel cooler, but it does not lower the room temperature. An air conditioner actually removes heat from the air and exhausts it outside, so the measured temperature drops.
Why does an air conditioner drip water?
The evaporator coil is colder than the room air, so water vapor in that air condenses on the coil just like on a cold drink glass. The collected water drains away through a pipe or pan, which is why a working unit produces a steady trickle.
Does an air conditioner bring in fresh air from outside?
Most window and split units recirculate indoor air and do not bring in outside air. They cool and dehumidify the same air repeatedly. Some larger systems have a fresh-air intake, but a standard room unit only conditions what is already inside.
Why does the outdoor unit blow hot air?
The outdoor coil is where the heat removed from your room, plus the heat generated by the compressor, is released. The fan blows air across that hot coil, so the air leaving the outdoor unit feels warmer than the surrounding air.
What does the thermostat actually control?
The thermostat senses room temperature and switches the compressor and fans on or off to hold a set point. When the room reaches the chosen temperature, it stops cooling; when the room warms again, it restarts. It does not control how cold the refrigerant gets.

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