For those of us who live in hot summer environments, the air conditioner is one of the biggest energy hogs in our home (and the reason I can live in the hot, humid South in the summer).  I live in Atlanta, and for the past month and a half it has been over 90° F every day, which requires me to run my AC a lot.  In the cooler months, my electricity bill may run about $35 (I heat and cook with natural gas).  In the summer months our electricity bill will run about $120, and almost all of this increase is due to our air conditioners, even though I try and program our thermostats as efficiently as possible.

We've written a fairly popular post on "How much Electricity does the Air Conditioner Use" but I'm not sure how much time we've devoted to explaining how air conditioning works.  It's not necessary to know how something works in order to save energy, but it sure does help.  It will help you understand why preventative maintenance on your air conditioner will help save you money both in operating and early replacement costs!

Allison Bailes over at Energy Vanguard has written a great post titled The Magic of Cold, How your Air Conditioner Works - Part 1.   He goes over the basics in the first post.

What makes an air conditioner work is a thermodynamic cycle called the refrigeration cycle. It's a series of changes in temperature, pressure, and state (liquid/vapor) that the refrigerant undergoes as it removes heat from the house. The refrigerant is a special fluid that changes between liquid and vapor at convenient temperatures for pulling heat out of air that's at about 75° F and dumping it into air that's above 90° F. It's what travels through those copper pipes, one insulated and one uninsulated, that connect the indoor part of your air conditioner to the outdoor part.

I'm going to focus this discussion on the most common type of central air conditioning system - the air-source, split system. It's called air source because it dumps the heat from inside the house into the outside air, as opposed to a ground-source or water-source system that dump the heat into, well, the ground or some water. It's called a split system because there's a unit that sits outside making all that noise all summer long and another component that's inside the house somewhere, maybe in the attic or crawl space. Other types of air conditioners still follow the same refrigeration cycle, but the locations of some of the pieces differ.

In the second part of the of Energy Vanguard's series on how an air conditioner works, Allison gives an intermediate explanation of what is happening in the system that keeps us from sweating in our homes in the summer.  He mentions the 4 basic parts of an air conditioner (also listed in Allison's picture at the top of this article):

1. Evaporator Coil
2. Compressor
3. Condenser Coil
4. Expansion Valve

But as Allison explains, the real magic of the air conditioner system happens at the expansion valve:

Once the refrigerant gets back to the indoor unit, it passes through the expansion valve, and the magic of the refrigeration cycle happens here. The high pressure, relatively warm liquid runs into a constriction that doesn't allow the refrigerant to pass through easily. As a result, when the liquid does get through to the other side, it finds itself in a much lower pressure. When the pressure drops like this, so does the temperature - a lot! This is what makes air conditioning possible. Without being able to get the refrigerant down to temperatures below the air in your home, an air conditioner wouldn't be able to work. Why? Because heat flows from warmer to cooler, the old second law of thermodynamics again.

While you don't need to know how it works, knowing how your air conditioning system works will play a role in helping you understand how to use it more efficiently.