How does an air conditioner work?

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.

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One thing I'd like to know is - is there an alternative to a dehumidifier in the basement, which would exhaust heat out the window, and also dehumidify? I presume that just a portable AC with an exhaust hose would work but I'm not certain... If a dehumidifier uses 750W, that's a 750W space heater in your basement... in the summer.... which then must be sucked out again by ANOTHER appliance, your AC. :( Seems like there must be a better way, and I'm not sure if portable AC would be that better way...?
Live in Atlanta too and sadly have to observe that construction is pretty shoddy and state-of-the-earth. Understanding AC and running it efficiently is only part of the solution. Start from the Roof of your house. ATTICs typically based on their roofing material (which is usually black) attracts and stores heat and raises the temperature to 140 degree on a hot summer day. Then your brilliantly stored attic AC which is now positioned at the Equator is supposed to deliver cooling to the UPPER area (bedrooms) of the house. There is no magic to it, the AC unit plus its dehumidifying unit will maintain a 10 degree differential with outside temperature provided you bath the unit in ice (which is not the case). Lower your attic temperature with a good quality solar exhaust fan (and not a circulation fan) at the highest point of the roof and the longest diagonal point between the front and rear of the house. This might sound very complicated but it is not. Buy a good quality Solar fan and get a good installer to do the job. This might set you back a few 100 dollars, but the rewards will be instantaneous and your AC unit will not be working so hard to keep the house cool. In addition, windows should be well shaded in Summer or winter as they are the source of greatest heat loss or heat gain. HOPE this helps. Then the AC will need minimum maintenance and will work for 20 years and at minimum load and cycling.

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