How Does an Air Conditioner Work? Simple Explanation for Kids
An air conditioner does not make cold. It moves heat. It picks up heat from the air inside your house and carries it outside, so the air left inside feels cool.
That is the big secret. Cold is not a thing you can add. Cold is just what we call it when there is less heat. So an air conditioner is really a heat mover. It works like a bucket brigade for heat: scoop it up inside, carry it out, dump it outside, come back for more.
On this page, each part has three levels. Read the Rookie part first. If you want more, open the Pro box. If you love the real science, open the Expert box.
The five steps, in order
- Air comes in. Warm air from your rooms gets pulled into the big air grille called the return, usually on a wall or ceiling in a hallway.
- The filter catches dust. The air goes through a filter that grabs dust, fur and lint.
- The cold coil grabs the heat. The air blows across a very cold metal coil. Heat jumps from the air into the coil.
- Cool air goes back to your rooms. A big fan called the blower pushes the cooler air through ducts and out the vents in your ceilings.
- The heat goes outside. A special liquid called refrigerant carries the heat through copper pipes to the outside unit. A fan blows the heat away into the outdoor air.
The heat loop: how heat rides outside
Rookie: Inside the air conditioner there is a special liquid called refrigerant. It runs in a loop through copper pipes, from inside your house to the outside unit and back again. Inside, it soaks up heat like a sponge soaks up water. Outside, it squeezes the heat out. Then it goes back in for more.
Pro level: why does the refrigerant get cold?
Refrigerant can boil at a very low temperature, much colder than ice. When something boils, it soaks up lots of heat. So when refrigerant boils inside the indoor coil, it pulls heat out of the air blowing past.
Then a pump called the compressor, in the outside unit, squeezes the refrigerant. Squeezing makes it hot, even hotter than a summer day. Hot things give heat to cooler things, so the hot refrigerant gives its heat to the outdoor air.
Expert level: the four big parts of the loop
Scientists call this the refrigeration cycle. It has four main parts:
- Evaporator coil (inside): low-pressure refrigerant boils and soaks up heat from your house air.
- Compressor (outside): squeezes the refrigerant gas so it gets hot and high pressure.
- Condenser coil (outside): the hot gas gives off heat to the outdoor air and turns back into a liquid.
- Metering device: a tiny opening. The liquid squirts through it, the pressure drops, and it gets very cold again, ready for the next trip.
Heat always flows from warmer things to cooler things on its own. The compressor is what lets the machine push heat “uphill”, from a cool house out into hot summer air. That push takes energy, which is why an air conditioner uses electricity.
The cold coil
Rookie: A coil is a bunch of metal tubes with thin metal fins all around, a bit like a radiator in a car. The refrigerant flows through the tubes and makes the whole coil very cold. When your warm room air blows across it, heat jumps from the air into the cold metal. The air comes out the other side cooler.
Pro level: why does the coil have fins?
The thin fins give the coil lots more metal for the air to touch. More touching means more heat can move. It is the same reason you cool down faster in a swimming pool than standing next to one. If the fins get bent flat or covered in dust, less air can touch the metal, so less heat moves.
Expert level: how cold is the coil?
When it is working, the indoor coil is often about as cold as the inside of a fridge, but not cold enough to freeze water. The air that comes out of your vents is usually a good bit cooler than the air that went in. Technicians measure both with thermometers to see if the system is moving heat the way it should.
Where the water comes from
Rookie: Air has water in it that you cannot see. It is called water vapor. When that air touches the cold coil, some of the water vapor turns back into tiny drops of liquid water. This is called condensation. It is the same thing that makes a cold glass “sweat” on a hot day.
The drops drip into a pan under the coil. Then they flow out through a pipe called the drain line. Often the pipe ends outside, near the house. That is why you may see water dripping outside on a hot day.
Pro level: so an air conditioner dries the air too?
Yes. Every drop that drips out is water that used to be floating in your house air. Drier air feels cooler on your skin, because your sweat can dry faster. In a muggy place like Austin, the drying part is a big deal. On a sticky summer day, an air conditioner can drip a lot of water.
Expert level: the dew point
The temperature where water vapor starts turning into drops is called the dew point. The indoor coil is kept colder than the dew point of your house air, so water keeps condensing on it. The more water in the air, the higher the dew point and the more water drips out.
Want to see condensation yourself? Try Cloud in a Glass in the Kitchen Table Lab.
The filter
Rookie: Before the air gets to the coil, it goes through a filter. A filter is a flat frame with a soft, bendy screen inside. It catches dust, pet fur and lint so they do not pile up on the coil and the fan. Think of it as the doormat for the machine.
A filter fills up over time. A full filter is like trying to sip a milkshake through a squished straw. Less air gets through, so the machine has to work harder.
Pro level: which way does the air go?
Filters have an arrow printed on the side. The arrow points the way the air flows, toward the machine. Changing the filter is a grown-up job, but you can be the one who reminds them. Many families write the change date on the filter’s edge.
Expert level: what the numbers on a filter mean
Many filters have a rating number. A higher number means the screen can catch smaller bits of dust. A screen that catches more also lets air through a little less easily. The best filter for a house is one the system was built to work with, which is something a technician can help a grown-up choose.
The outside unit
Rookie: The big metal box outside your house is where the heat goes. It holds the compressor, an outside coil and a big fan on top. The fan pulls outdoor air through the coil and blows the heat up and away. On a summer day, the air blowing out the top is warm. Ask a grown-up to tell you how it feels. That warm air is heat that used to be inside your house.
Pro level: why it needs room around it
The outside unit needs lots of air flowing through it to get rid of heat. Bushes, toys or piles of leaves around it are like putting a blanket over it. Grown-ups keep plants and stuff a few feet away. Your job: if you see something leaning on it or blocking it, tell a grown-up.
Expert level: why is it louder in summer?
On the hottest days, the outside air is already hot, so getting rid of heat is harder. The compressor and fan run longer to keep up. That is why you hear the outside unit humming for a long time in July and August in Austin.
Try it: be a heat detective
- With a grown-up, stand under a ceiling vent while the AC is running. Can you feel cool air coming down?
- Now find the big return grille, usually on a hallway wall or ceiling, where air gets pulled in. If you can reach it from the floor, hold your hand near it. Does that air feel cool or warm?
- If you have a kitchen thermometer, a grown-up can hold it up to each one for a minute. Which number is lower?
Wait, but why…?
- Why does the AC drip outside? And other house mysteries
- How does the heater work? (Hint: a heat pump runs this loop backwards.)
- Take a tour inside the machine
- The Why Bank: 60+ short answers
- Hero Words: every hard word, explained
Next stop on the Air Trail: Inside the Machine. Or see the whole trail on the Air Trail page. Writing a school report? Visit the Homework Helper.