What Size Central Air Conditioner Do I Need?

Replacing or installing your home’s central air conditioner? Whether you’re trying to gauge costs or just want to make sure everything’s set up correctly, you might have questions about what AC system is best for your home—which includes getting the size right.

AC sizing requires a professional HVAC service technician’s help. Contrary to popular belief, sizing your AC correctly is not as simple as, “Your home is X square feet, therefore you need an X-sized air conditioner.”

To help familiarize you with the AC sizing process, we’ll go over the following:

Air conditioner sizes are measured in British Thermal Units (BTUs). Your air conditioner’s BTUs measure how much heat your AC can remove from the inside of your home to dump outside in an hour.

Residential air conditioners generally range anywhere from 12,000 - 60,000 BTUs. The more BTUs, the more heat your system can remove from your home (i.e. cool your home more effectively).

If your AC is oversized (i.e., has more BTUs than it needs), it will be able to quickly cool your home. While that seems like a benefit, the oversized AC will quickly shut off once it finishes meeting your cooling demands. This constant short-cycling can lead to higher monthly energy bills, inconsistent cooling throughout your home and a shortened lifespan for your system.

If your AC is undersized (i.e., has fewer BTUs than it needs), it will not be able to meet your cooling demands and will strain to try to get your home to the set temperature, leading to similar consequences as an oversized AC.

Your AC shouldn’t be too big or small for your home. Like the Goldilocks principle, it has to be just right.

To find the right-size AC for your home, a professional HVAC service technician needs to do some calculations...

HVAC service technicians use (or should use) the Manual J Heat Load Calculation when determining how many BTUs your air conditioner should have. This calculation analyzes how much heat your AC needs to transfer from the inside of your home to the outside to keep your home cool.

So, what all goes into this calculation? Your HVAC service technician needs to gather key information about your home, including:

• Square footage
• Climate of where you live
• Layout (multiple levels, basement, etc.)
• Number, size, and age of windows and glass doors
• Type of flooring
• How shaded your roof, walls and home are
• Ceiling height (e.g., do you have vaulted windows)
• The quality of your home’s insulation
• If your air ducts are sealed and insulated
• Number of residents
• ...And much, much more!

As you can see, contractors who use the Manual J Heat Load Calculation look at a lot to produce the best AC size for your home.

With that in mind, be cautious around a contractor who gives you a quick rule of thumb based on square footage to determine your system’s size. They’re not doing their due diligence, and you could wind up with an incorrectly sized AC!

If you currently have a central air conditioner and just need a replacement, you might be wondering if you could just swap it with an AC of the same size. Although you could take this route, there are a few problems with doing so that could cost you time and money later on.

Here are two reasons why you shouldn’t blindly go with your current AC size:

1. Your current AC may be the wrong size
2. Your house might have changed

Your current air conditioner might not be the right size for your home (which could have shortened its lifespan and be the reason you need a replacement). Many unprofessional contractors size a system using a “rule of thumb” (a guesstimate based on your home’s square footage) instead of taking the time to do the right calculations.

If your previous contractor did perform the right calculations for your AC, you should still consider updating the calculation. Why? If your home has changed, your cooling needs have also changed. You might not need the same-sized AC anymore. Everything from adding new rooms to planting more trees around your home can change up your AC size needs.