Heat Pumps Provide Cooling
There are basically two ways to get cool air — 1) move air across a cool surface, or 2) take the heat out of the air.
The first one, moving air across a cool surface is something most people have experienced or can visualize, but the second one, actually removing heat from the air, requires a little more imagination (or quite a bit of math and science).
Let’s say it is 100 degrees outside. Since heat always wants to move to where it is colder, it finds every crack it can to get into your 75 degree house. To force heat to move in the other direction — out of your house and into the wild — we use a heat pump, which is part of a standard air conditioner.
Think of a heat pump as a loop of pipe that goes between the indoors and the outdoors. If fluid or gas (we’ll call it the “refrigerant”) in that pipe is colder than the indoor temperature as it travels indoors, then it will absorb heat from the house… leaving the house a little cooler. Then that same refrigerant, as it gets to the outdoor part of the loop needs to be hotter than the outside temperature so the heat will move to the outdoors.
The two pieces of equipment that are needed to change the temperature of the refrigerant are a compressor and an expansion nozzle. When the refrigerant is compressed, it becomes hot (say 120 degrees). Then it travels through the outdoor pipe section and heat flows away from the pipe into the outdoor air (which is 100 degrees).
Then the refrigerant gets to the expansion nozzle. At this stage it is a gas and expanding a gas drops its temperature. If you have ever used a CO2 cartridge to fill a bike tube, you know that as the tire fills with air, the cartridge gets very cold. So the refrigerant gets to the expansion nozzle and drops its temperature to, say, 50 degrees. It then travels through the pipe in your house (which is at 75 degrees) and now the heat wants to move out of the air in your house to the 50 degree refrigerant.
The effect is that you are taking heat out of the house and moving it to the outdoors.
Here is a simple drawing:
Heat pumps have been used for decades to provide us with refrigeration. The heat pump moves the heat from inside the refrigerator to the room it is standing in. Next time you are next to a working fridge, feel around behind or on top of it to see where the heat is being exhausted.
Heat Pumps Provide Heat
One more very cool thing is that a heat pump can run backwards or forwards, which means if one day you want your house to be cooler than the outdoors, and the next day you want it to be hotter than the outdoors, you can basically run the heat pump in either direction to get the heating or cooling you like.
Not all heat pump equipment is designed to run in both directions, but this is basically how a mini-split AC/heating system works.