How Does Central Heating and Cooling Work?

How does central cooling work?

Air conditioners function by taking heat and humidity from your home and releasing it outside, to ultimately drop the temperature in your home. In order to complete this process, all parts of the central cooling system must work together.

Central cooling system options

In most cases, central air conditioning refers to a split-system air conditioner or a heat pump, both have an outdoor and indoor unit. The indoor and outdoor units work together to distribute cool air through a system of ducts in your home. Together, they are composed of five main parts: a thermostat, an outdoor unit (holds a fan, condenser, and condenser coil), an indoor unit (holds a fan and evaporator coil), copper tubes that connect the outdoor and indoor units, and ductwork throughout the home. While a split-system AC only cools, a heat pump can reverse the flow of refrigerant to heat the home, so the process works in reverse.

A packaged air conditioner is the final type of central cooling. A packaged unit works the same way as a split system AC or heat pump, except the whole unit sits outside the house; there is no equipment inside the home. The packaged unit takes air from your house through the return ductwork, cools it, and then supplies it back to the house through a second set of ductworks, called the supply ductwork.

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How does heat move?

Heat moves in three ways: Radiation, conduction, and convection.

Radiation happens when heat moves as energy waves, called infrared waves, directly from its source to something else. This is how the heat from the Sun gets to Earth.  In fact, all hot things radiate heat to cooler things. When the heat waves hits the cooler thing, they make the molecules of the cooler object speed up.  When the molecules of that object speed up, the object becomes hotter. 

Infrared waves are part of a spectrum of energy waves known as the electromagnetic spectrum. The electromagnetic spectrum includes all kinds of energy that can travel in waves, including light, heat, x-rays, radio waves, ultraviolet waves and microwaves.

All of these kinds of waves contain a lot of energy. Also, they can all travel through deep space. That's why we can see sunlight from stars billions of light years away. The light from them radiates to us.

Conduction is another way that heat moves. Heat is a form of energy, and when it comes into contact with matter (Anything that you can touch physically) it makes the atoms and molecules move. Once atoms or molecules are moving, they collide with other atoms or molecules, making them move too. These, then bump into other molecules and make them move, too. In this way, the heat is transferred through matter.

Conduction is what makes the handle of a pot hot when only the bottom of the pot is touching the stove. The heat from the burner starts the molecules in contact with the burner start to move. Those molecules bump against others in the pot, which bump others, until all the molecules in the pot, including in the handle, are moving. When someone touches the pot handle, they feel the heat. The heat has moved from the burner to the cook's hand through conduction.

Conduction is an important way that heat travels in space, but only within a spacecraft. Since there is very little matter in deep space, heat cannot leave a spacecraft by conduction.

Convection is a very important way that heat moves on Earth, but is not very important in space. Convection happens when a substance that can flow, like water or air is heated in the presence of gravity. When air or water is in the presence of gravity, the gravity pulls all of it down. The bottom of the air or water becomes denser because it is pulled down and also pushed down by the weight of the molecules on top of it.

When there is heat at the bottom of this air or water, the air or water molecules in contact with the heat start to move, and the molecules spread apart. The heated air or water becomes less dense. It rises up until it gets to air or water with the same density as it has, and when it gets there, it pushes the air or water that was there out of the way. At the same time, new air or water fills the space that was vacated when the heated molecules rose up. The air or water that gets pushed out of the way falls down. This sets up a circular motion. Air or water is heated at the bottom, travels to the top, cools, gets denser, falls, is heated again and the whole cycle starts again. Convection does not occur in space because there is no gravity.

Ovens work by convection. The heating coils at the bottom of the oven heat the air which climbs to the top, cools slightly, and falls down again.