Eli5: What really happens when you change the temperature in an AC (Cooling) ?
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Most home climate control works on a few pretty simple mechanisms.
First is the thermostat. It takes the temperature. And then you also tell it if you want it to be hotter, or colder, and what temperature you want it to go to. If you want it to heat your home to 75 degrees, it’ll take the temperature and if the temp in the room is under 75 degrees it will tell the furnace to turn on. The furnace will heat the room. And once the room is 75 degrees the thermostat tells the furnace to turn off. Same thing for the AC except backwards, it’ll tell the AC to turn on if it’s hotter in the room than desired.
The other two/three parts are the AC, the furnace, and the vents.
Vents are pretty simple, they’re just pathways for the heated/cooled air to be distributed through the whole building as equally as possible. Typically there’s fans somewhere in the vent to pull/push the air and keep it moving. Those fans will likely be tied into the thermostat, so the fans only turn on when the furnace or AC is on.
Furnace. The furnace is pretty simple. You burn stuff, it gets hot. Most homes (at least in America) run on natural gas. The furnace makes a ton of heat, and uses a copper (or some other thermally conductive material) manifold to transfer the heat without transferring the cooked air. As the air doesn’t have much oxygen after burning, it needs to be vented outside. That manifold near the furnace heats air in the vents and presto, hot air in the vents.
Larger buildings will use a boiler in junction with the furnace. Where the furnace just heats the boiler. The boiler is full of water, rather than air, and the hot steam is what is sent to wherever the heat is needed. Steam is better at holding heat, so if you have to send heat a long way (like all the way up a highrise) it’s going to do better than air. Air might cool off by the time it gets where it’s going. Steam will still be hot. But of course you don’t want to just dump steam in places so you use a radiator. A type of heat exchanger. By having a lot of surface area it has more space to trade heat with the ambient air in the room it’s heating.
The AC is the complicated one. An air conditioner has 7 important parts.
First is your refrigerant. Freon is typically used, but it needs to be something that will remain liquid at really high and really low temperatures. It also needs to be able to exchange heat pretty readily. And it needs to be stable, meaning it won’t react or combust under high temps and pressures. There’s probably more stuff that works, but Freon is probably the cheapest.
The rest of the AC works on the Carnot Cycle. Which is the principle that when you compress something or apply pressure that material heats up. That material will eventually dissipate the heat. You can then release the pressure to cool the substance. It will eventually return to temperature equilibrium. Then repeat.
So what happens is you have a closed system of tubes filled with Freon. There are four features in the system. The compressor, a heat exchanger, a throttle, and then another heat exchanger.
So the compressor acts as a pump (to keep the fluid moving the same direction the whole time) and also as a compressor. The compressor pressurizes the Freon, This causes the Freon to get hot. The pump then sends the hot Freon into a heat exchanger, where releases all it’s heat with the help of a fan and cools to room temperature. This makes things pretty hot. It’s why the back of your fridge is hot. It’s why the AC unit outside is really hot. Once the Freon is the same temp as outside it’s pumped back inside and into a throttle. A throttle is a device that increases speed but decreases pressure in liquids. So when the Freon goes through the throttle and loses pressure it gets colder. The now cold Freon is sent through a heat exchanger where a fan blows over the heat exchanger which cools the air and brings the Freon back to room temp. The cold air then goes into your vents to cool your home. The Freon then goes back into the compressor and the cycle repeats.
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