Air Conditioning System: How to Choose the Right Size

“So how big does my air conditioner need to be?”

It’s one of the perennial questions asked of HVAC companies by homeowners, and one of the questions that most demands a layman’s answer. Too often technical responses that mention “tonnage” and “load calculation” aren’t clear enough and can obfuscate something that isn’t all that complicated. The following is an attempt to explain, in simple terms, how to buy an air conditioner that is right for your home.

What sizes do air conditioners come in?

Air conditioners are measured in “tonnage.” While the word suggests weight, tonnage actually refers to the system’s ability to cool. One “ton” is the capacity to cool 12,000 British Thermal Units (BTUs) in an hour. A BTU is the amount of energy required to raise the temperature of one pound of water by one degree fahrenheit, so a 1-ton system would have enough energy to cool 12,000 pounds of water one degree in one hour. A 2-ton air conditioner could cool 24,000 pounds of water one degree in one hour.

Central air conditioners in residential settings range from 1.5-5 ton units and increase by increments of 0.5. Any system that requires more than 5 tons is considered commercial. If your space requires more cooling (like 7 tons) you would need to use two units in tandem. Unless you have a huge estate or lack insulation, you probably won’t need more than 5-tons. Some residential customers do elect to have more than one system. This may seem like overkill, but it does ensure some cooling in the blazing summer months in event that one of the units is down.

Why does it matter to get the right size?

When energy costs were low and there was little or no insulation in houses, people used massive, high-powered air conditioners. These units could cool gymnasiums in Phoenix in a matter of minutes. Now energy efficiency reigns supreme, and systems aim to slowly and steadily cool the home. Now size really does matter.

If your air conditioner is too big it will turn on, cool your house rapidly, and then turn off. Since the speed doesn’t allow the unit to dehumidify the room, heat will slowly creep back in and the unit will have to turn on again. This causes a pattern turning on, cooling, turning off, heating, then turning on again. This stop-and-go process uses a lot of energy and runs up your electricity bill.

On the other hand, your air conditioner is too small when it has to work all day to cool down your house. It turns on and keeps working, but takes a long time to get to the right temperature. If your AC system cycles too slowly and doesn’t seem to get the temperature down to the low 70s, it is likely too small. New central air conditioners are meant to cycle more often, but for shorter times. It should not run for more than 30 minutes.

If your air conditioner is the right size it will run through a number of cycles to cool your house gradually. It will retain its efficiency by keeping your house cool by running just enough. It will dehumidify the room to ensure a “chill factor” throughout the day.

So how can I estimate the size I need?

The old school equation for calculating the ballpark figure for tonnage is as follows:

(Square-footage times 30, divided by 12,000) – 1.0 = tonnage needed.

So, for a home that is 1,000 square feet:

1,000 x 30 = 30,000

30,000 / 12,000 = 2.5

2.5 – 1.0 = 1.5-ton unit

This is for an average home in, say, middle America. If you are in a hot, dry climate you will refrain from subtracting 1.0. A 1,000 sq. ft. home in Phoenix will need a 2.5-ton unit rather than a 1.5-ton unit.

Since this is just a rough estimate, there several other factors that are usually managed by a professional. The following are factors they will consider:

• The orientation of your house relative to the sun
• The height of your ceilings
• The number and size of windows and the way they are oriented
• Your quality of insulation in your air ducts and walls
• Any plans for remodeling or additions

Since an AC unit is a permanent fixture, you’ll want to get it right the first time. For most people, this means hiring air conditioning services from Bradley Mechanical to make a professional assessment. Having a basic sense of how tonnage works, the BTUs needed, and the specifications of your house will help you in the process.