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Today I want to talk a little about over-sized air conditioners. We often hear the term “the bigger, the better.” While this is true for many things, it is not the case when it comes to purchasing an air conditioning system. Even here in central Texas where it is not uncommon to see some days near 110 degrees and many days over 100 degrees in the summer, it is important that your air conditioner system not be too big. There are three main reasons that a properly-sized system is important.

Reason #1 – Your Comfort

The primary purpose of an air conditioner is to make it comfortable for those in the home. When an air conditioning unit is running, it is doing two things – both of which are related to your comfort. First, and most obvious, is that the air conditioner cools the air in the home. As it pulls air over the cold evaporator coils, the air is normally cooled about 15 to 20 degrees.  Cooling the air is only half the battle when it comes to helping to make your home more comfortable.

The second thing your A/C system does to help with the comfort in your home is to remove moisture from the air in you home. In this area of Texas, a properly-operating system can remove five or more gallons of water from the air in your home every day. This is a lot of water, and it is a critical function of an air conditioning system.

Why does an over-sized air conditioner system not remove moisture adequately? As the air is pulled over the evaporator coil, the air temperature is lowered below the dew point of the air. The dew point of air is the temperature at which the moisture in the air will begin to change from a vapor to a liquid in a process called condensation. This is the same thing that happens if you have a glass of ice water sitting out in warm weather. Moisture in the air will condense on the outside of the glass and begin to pTypical evaporator coiluddle at the bottom of the glass. Likewise, moisture droplets form on the air conditioner’s evaporator coil and trickle down into the pan below the coil. Eventually, a lot of water will accumulate in this pan. When enough water accumulates, it begins to flow into the condensate drain line which will take the water outside the home. By removing this moisture from the air in the home, the humidity level in the home is lowered, making it much more comfortable for everyone in the home.

If the system does not run for long enough, then not as much water will condense out of the air, and a lot of the water that does condense will just accumulate in the pan and will never exit the home. The water that is left in the pan will simply re-evaporate and end up back in the home.

Reason #2 – Increased Life of the AC unit

Almost all mechanical equipment works hardest when it is first starting up or when it is slowing down or stopping. Let’s look at a familiar example. When you are driving your car down the highway at 75 mph, your car is not working very hard. Momentum is providing much of the work to move the car, everything is lubricated because the oil is being pumped everywhere it is needed, etc. Everything is running at optimal speed for the conditions. Your vehicle is operating in steady state. However, when you first start your car, the bearings, crackshaft, etc are not lubricated. All of the moving parts in your car are in a state of flux as they try to get up to speed. All of this is hard on your vehicle.

The same is true for your air conditioning system. It receives the most wear and tear when it first starts up and when it is coming to a stop. Therefore, it stands to reason that the fewer start-stop cycles that it goes through, the less wear and tear it will undergo, and the longer it will last.

A properly sized unit should run for 10 – 15 minutes at a time on a mild day. You can expect it to run two or three times every hour. On an extremely hot day, your AC system will run for significantly longer – perhaps for hours without shutting off. This is normal, and is an indication that it is properly sized.

Reason #3 – Lower Initial Cost

It stands to reason that a smaller system will be less costly to purchase than will a larger system, so for this reason it will save you money not to install an over-sized system.

What Size AC System do I Need?

This depends on a lot of factors. There is a rule of thumb that says in our climate in Central Texas that you need about 1 ton of cooling for every 500 square feet of area in the home. Keep in mind that this is just a rule of thumb. The best way to know for sure is to have a “Manual J” calculation completed on your home. This is something that HVAC contractors and energy auditors can do for you. A Manual J calculation takes many factors into account when determining the size of air conditioner that you need. Some of these factors include the square footage of your home, the amount of insulation in the walls and attic, the type of windows and the direction that they are facing, how well the home is shaded, etc. With a Manual J calculation completed, you should know just what size AC system your home requires.

If you are in the market for a new AC system, do yourself and your family a favor, and make sure to purchase the right size system for your home. It will be less costly to purchase, it will make your family much more comfortable, and it should last longer than will an oversized system.

If you have questions or comments about the size of your A/C system, please feel free to leave a comment or reach out to Mike at

Mike Morgan

Professional Home Inspector, TREC #6231


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