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AC show room - how to determine what size ac unit to buy

The size of your AC will determine how efficient it runs and how well it cools your home. If it isn’t the right size, you’ll experience big problems that can lead to costly repairs and sweltering summer nights in Florida.

But how do you determine what size AC unit to buy? Can you just find the cubic feet and climate zones, plug them into an equation and discover the perfect tonnage? 


You need to run a proper load calculation on the property that is unique to your home (the block load). Then, that amount is compared to the cooling capacity (BTUs) of the AC. 

If that’s confusing, you wouldn’t be the only one. Calculating the right size unit involves science and experience. Mathematics is only a portion of the work! 

And just because a contractor installs new cooling systems, doesn’t mean they know how to do a load calculation or choose the right size unit for your home. What you really need is a cooling system professional who understands thermodynamics and air conditioning. style=”font-weight: 400;”> The experts at Action Air Conditioning are trained to determine what size AC unit you need. 

This isn’t a gimmick created by the heating and cooling industry. The Department of Energy and the ACCA (Air Conditioning Contractors of America) both recommend that every home has a Manual J load calculation completed before installing an HVAC system. 

What is a Load Calculation?

Experts can help you to determine what size AC unit to buy by deducing how heat and cold travel into and out of your home through a series of tests, measurements, and observations. A standardized way to decode this mystery is called the Manual J load calculation because it was created by a man named Manual J.

Consider how much heat collects in your home from natural and artificial sources, and how much of your AC literally flies out the door every time you open it. A load calculation decides how much heat your house loses and receives over the course of an hour.

What Factors Influence AC Unit Size

There are many crucial details that dictate how much heat enters your home and how many BTUs your AC needs to be to combat that heat effectively. Every tiny factor, from the energy efficiency of your home to the angle of the sun, will influence how much heat creeps in and how much power you need to keep it cool and dry indoors.


Square footage is important, but it can’t be depended on alone. There are other factors to consider, such as:

  • Insulation
  • Direction
  • Construction
  • Doors
  • Windows
  • Shade/Sun
  • Ceiling height
  • Appliances
  • and many more…

There could be two identical houses located in the same neighborhood that require different AC unit sizes. What direction their windows face will change the heat load they receive from the sun. 

What Size AC Unit Do You Own?

When you are looking to buy a new AC unit and have the air conditioner professionally installed, it’s always a good idea to consider if your previous AC was the right size. 

But exactly how do you do that? 

By observing its performance. 

If you think your previous AC was functioning well with a long life, it may be wise to stick with the same size. Why mess with perfection when your home was cool, dry, and your unit met or exceeded its life expectancy? Those are the exact goals we are looking for when choosing the right unit size!

The best AC size for your house brings your house’s temperature down to the set degree, while balancing humidity and efficiency. There are three major indicators that an AC is the correct size.

Balanced Temperature

If your house was a sweltering 84 degrees and you set your thermostat to 74 degrees. Ideally, it should take 3 hours to cool off your entire home. At this time, the temperature should be consistent throughout; none of the rooms will be overly chilled or hot.

When it doesn’t happen this way, we know there is an issue with the HVAC system. If the ventilation system and AC unit are in good shape, there is a problem with the unit’s size.

Balanced Humidity

An efficient AC dehumidifies the air as it cools. As the air handler draws in hot, stale air, the evaporator coils draw out moisture. When the indoor humidity level is between 40% and 60%, you feel cooler, more comfortable, and healthier. 

Balanced Run Time

When your AC is the perfect size, it runs for about 15 minutes on an 85-degree day. Every hour it should cycle about two or three times, turning on 15 minutes and shutting off for another 10 to 15 minutes. Only on very hot days will it run continually. 

Receive a Qualified Assessment to Determine Your Optimal AC Unit Size 

Call Action Air Conditioning today and schedule a visit from one of our trained and certified heating and cooling specialists. We will assess your home and help you to decide what the best AC unit size is for you. 

Load calculations are more than just a set of math equations. Where you live, what appliances run in your home, and even how high or low you like to set your thermostat are all factors to consider when choosing a new air conditioner unit. 

Is there an air conditioner company near me that can help me size my AC correctly?

Yes! If you live in Hillsborough, Pinellas, or Pasco County, contact our experts or visit the Action Air Conditioning showroom in Tampa, Florida.