The Right Size Air Conditioner for Your Home

Installing the proper size air conditioner in your home can make all the difference in your comfort and budget.  Install a unit that’s too large, and you’ll be wasting money on higher electric bills with suboptimal performance.  Install a unit that’s too small, and your A/C won’t have enough power to lower the indoor temperature to a comfortable level, roasting you during the Summer’s hottest months.  

Here’s a brief summary of how air conditioners work. Thoughts on right-sizing your air conditioner follow at the bottom of the article.

How Air Conditioners Work

Air conditioners transfer heat from the air inside your home to the air outside.  Of course, when its 95 degrees outside, this seems pretty magical.  How can an air conditioner produce cool, 55-degree air when it’s smoldering outside?  The answer lies in something called the evaporation cycle, whick takes advantage of a convenient rule many of us learned in high school chemistry:  if you compress a gas, all other things equal, its temperature goes up.  If you decompress a gas, all other things being equal, its temperature goes down.  Your air conditioner is filled with a special gas called freon that is constantly being compressed and decompressed.  Freon’s temperature fluctuates significantly under compression, making it ideal for use in an air conditioner. 

While most of us refer to our home’s outdoor unit as the air conditioner, there are actually two major parts of an air conditioning system:  the outdoor compressor/condenser unit, and the indoor air circulation / evaporation unit.  These two units are connected by copper piping that contains the freon.  Inside both units, the copper piping is coiled / finned to create a large surface area for air to pass over for heat exchange. 

In a nutshell, here’s how the evaporation cycle works (remember, this is a cycle, so the starting point is arbitrary).

1. Low pressure, relatively cool freon gas is pushed to the outside unit.
2. The outside unit compresses the freon gas which heats up the freon significantly (remember high school chemistry!).
3. The freeon then passes through a series of coils (or fins) in the outside unit called a condenser.  A fan blows air over the coils/fins.  This cools the freon gas to the ambient outdoor temperature.  In the cooling process, the freon is converted to a liquid.  Remember, the liquid is still compressed.
4. The compressed freon liquid is pushed to the interior unit where it is allowed to decompress and evaporate.  As it evaporates back into a gas, it cools down significantly, well below the ambient outdoor temperature, and even below the indoor temperature.
5. Air is blown over the copper coils/fins containing the decompressed freon, thus cooling the air.  This air is ultimatly blown through the duct work in your home to cool your family.  Note that when air passes over the coils, there is also a dehumidifying effect.  Water in the air condenses on the coils and then falls to the bottom of the handler where it is pumped out of your house or drained to the sump pump.  This dehumidication makes the air in your home feel even cooler on your skin.
6. The decompressed freeon’s temperature decreases as the air is blown over it.  It is then pumped back out to the exterior unit where the cycle is repeated at Step #1.

That’s pretty much it.  Of course, this is a high level view.  There’s a few more parts involved, but this will get you through at the next cocktail party.  So what about right-sizing your air conditioner?

When an Air Conditioner is Too Small

This one is simple and straightforward: if you install an air conditioning unit that is too small for your house, the unit will not be able to keep up with the cooling demands of your home.

When an Air Conditioner is Too Large

This one is a bit more complicated: the thought of cooling down the whole house in a shorter period of time is certainly alluring. The problem with purchasing a large air conditioner is that it actually cools the air so quickly that the unit turns off before the air is sufficiently dehumidified (see Step 7 above).  As a result, you must set the thermostat lower to achieve the same comfort level, which wastes energy and leaves you with suboptimal performance.

What do you think?  Did you install the right sized air conditioner in your home?

Photo by DaynaR.