Explain Like I'm Five

AC as refrigerant, not coolant. Although that might be semantics.

AC is a type of heat pump. You are moving heat from inside to outside. The physics behind a heat pump, is fairly simple. There are three principles to work with.

1. Take a gas, compress it, and it gets super hot.
2. Take a compressed gas, let it rapidly expand, and it gets super cold
3. Different temperature gases move from hot to cold

Let's focus on AC since most folks are used to that concept. In an AC, you have a closed loop of refrigerant. Outside, there is a compressor that compresses the gas, which makes it very hot. The gas is pushed through a radiator with a fan pulling air through it. Since hot moves to cold, the heat trapped in the gas moves to the outside air, and the gas is slightly cooled. (As long as the gas is hotter than outside).

Inside, there is an expansion valve that lets the gas rapdly expand, making it super cold. It is pumped through a radiator that has inside air blowing over it with a fan. Since hot moves to cold, the heat in the inside air moves to the cold gas, cooling the inside air. (As long as the gas is cooler than the inside temperature). It is then pumped outside to start the loop again.

So, inside gets cooler while the heat is moved outside. The physics also establishes the limits of the heat exchange. You will only grab heat from inside if the expansion makes the gas colder than the air inside. Typically it expands to around 0 degrees. Likewise, it will dump the heat outside if the gas is hotter than outside air. The compressor typically makes it 130 to 140 degrees (temps vary depending on many factors).

To use a heat pump for heating rather than cooling, reverse the process. Pick up the heat from outside and dump it inside. This will work as long as outside is warmer than the expanded gas (0 degrees or so). Although you can get some that go to around -20.