What Size AC Unit Does Your House Need?

Written on: June 23,

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While there are several things to consider when deciding which AC unit to buy, perhaps the most important question you may be asking yourself: What size AC unit do I need?

Not only do you want to avoid wasting money on the wrong air conditioner, but you also don’t want to throw your money out the window each month, as the unnecessarily large AC unit burns up your energy bill. 

Likewise, you don’t want to buy a unit that’s too small for your house, which means it will be running all the time to try and cool you down, eventually leading you to buy a second unit. 

Thankfully, you can avoid guessing games and calculate the right size AC unit your house needs. 

Calculating The Right Size AC Unit

There are several things you will need to consider, all of which will affect the size of the AC unit you will need for your house:

• What is the square footage of your house?
• What geographic region do you live in, and what is the local climate like?
• How much insulation do you have and how old is it?
• How many windows do you have in your house?
• Does your house get a lot of shade or sun? 
• How old is your ductwork and how old is it?
• How many people live in your house?

To come up with a ballpark, the three most important numbers you need to know:

1. The square footage of your house
2. The required tonnage of the unit
3. The height of your ceilings

What Is AC Unit Tonnage?

Tonnage actually has nothing to do with weight. A ton refers to the AC unit’s capacity to cool the air, in other words, how much heat the unit can remove in one hour. 

That capacity is measured in BTUs or British Thermal Units. Why BTU? A long time ago, they determined it takes 286,000 BTU to completely melt one ton of ice in a 24-hour period. HVAC experts eventually divided that number by 24 hours and came up with 12,000 BTU/1-ton AC capacity. 

Using that measurement, a two-ton AC unit will remove 24,000 BTUs, while a three-ton AC unit will remove 36,000 BTUs. Therefore, the more tonnage the AC unit is rated for, the more air it can cool down.

DIY Calculations

To  get a rough estimate of the tonnage you will need from your AC unit, use the following calculation:

Square footage of your home x 25 (estimated energy to cool one square foot is 25 BTU) / 12,000 –1 =  AC Unit Tonnage

For example, the calculations for a 1,200-square-foot home:

(1,200 x 25) / 12,000 –1 = 1.5 Tons

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You would need a 1.5 ton AC unit for your house. 

If you live in a drier or hotter region, don’t subtract 1. 

For example, the calculations for a 1,200-square-foot home located in New Mexico:

(1,200 x 25) / 12,000 = 2.5 Tons

You would need a 2.5 ton AC unit for your house.

This table should give you an idea of what size AC unit you need, depending on the square footage of your house (located in a temperate climate region):

Of course, all the other factors we previously discussed will also affect the unit you ultimately end up purchasing:

• What geographic region do you live in, and what is the local climate like? Will you need higher or lower power to keep you cool?
• How much insulation do you have and how old is it, and how old is your ductwork? If your insulation or ductwork is old, air can escape, requiring more power to keep your home cool.
• How many windows do you have in your house, and does your house get a lot of shade or sun? Natural sunlight can heat your home up really quickly. Without proper sunblocks on your windows, you might need a stronger AC to keep your home cool.
• How many people live in your house? The more body heat in your home, the more energy you’ll need to keep your house cool. 

Which Type Of AC Unit Do I Need?

There are commonly two different types of air conditioning systems you can install in your home: a packaged unit or a split system. 

Packaged Unit

The packaged unit air conditioner is the one most commonly associated with a central air system. With this system, the evaporator, condenser, and compressor are placed in one single cabinet. This cabinet is most often located on a concrete slab outside or on the roof. This type of packaged air conditioner usually includes electric heating coils or a natural gas furnace. This removes the need for an entirely separate furnace. 

Split-System Central Air

This type of system is most often used in homes that already have a furnace but no air conditioner installed. It’s the most cost-effective way to install air conditioning in a house. 

With a split-system central air conditioning unit, an outdoor cabinet holds the condenser and compressor. An indoor cabinet holds the evaporator. This indoor cabinet may also hold the furnace. 

Buying The Right AC Unit

While it’s fairly simple to come up with a rough estimate on the right size AC unit you will need for your house, ultimately, it’s safer and will save you money in the long run, if you work with an HVAC expert to determine the proper size. 

Installing central air can cost you between $3,779 and $7,429, according to HomeAdvisor.com. So you want to make sure you’re buying the right unit the first time. 

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