7 Factors That Determine the Size of Your Air Conditioner

An air conditioner that’s too big will waste money and energy. An air conditioner that’s too small won’t be able to stand up to the unrelenting summer temperatures. So, how do you make sure your air conditioner is just right? After decades of experience, many in the HVAC industry have it down to a fine science. Below are seven factors experts consider when determining the perfect sized air conditioner for your home.

Air conditioner sizes 101

First off, air conditioners are measured in tonnage. This doesn’t refer to the air conditioner’s weight, but to its cooling ability. One ton is equal to 12,000 BTUs (British Thermal Units) of cooling per hour. BTUs are the amount of energy needed to heat or cool one pound of water by one degree Fahrenheit. In other words, a one-ton air conditioner can cool up to 12,000 pounds of water by one degree every hour.

Most residential air conditioners range in size from 1.5 tons (18,000 BTUs) to 5 tons (60,000 BTUs). Anything over 5 tons is considered a commercial HVAC unit.

1.      Square footage

The square footage of your home has a lot of influence over the size of your air conditioner—but it’s not the end all be all. Depending on a home’s construction, an air conditioner weighing one ton (12,000 BTUs) can cool between 300 and 800 square feet. A general rule of thumb in Arizona is to assume that one ton can cool 400 feet. So, a 2000 square foot home would need a 5-ton air conditioner. However, this is more of a basic guideline that may not be accurate in all cases. A good contractor will take into account not only square footage, but other factors as well.

2.      Where you live

The location of your home is a major factor when it comes to sizing an air conditioner. It may seem obvious, but a home in a temperate region isn’t going to need the same size air conditioner as a home in a hot desert climate. The United States is divided into several climate zones. Arizona ranges from being classified as “cold” in the northern portion of the state to “hot-dry” and “mixed-dry” in the central and southern regions. Phoenix is located within the “hot-dry” zone. By comparing which climate zone you live in with the square footage of your home, a contractor can get a ballpark estimate of how big (or how small) your air conditioner should be.

3.      Sunlight and shade

The amount of sunlight and shade your home receives has a large impact on how warm your home feels throughout the day. This in turn influences how much cooling power you’ll need for maximum comfort. Homes that face east or west tend to feel the warmest. In the morning, you have the sun facing right at your front windows, while in the afternoon the sun will face the back windows. In contrast, homes that face north or south tend to feel cooler, even at the warmest points of the day.

A large tree in the front yard or a physical sunshade or awning can make a big difference in terms of indoor temperature. According to the EPA, scientists estimate that trees and vegetation can reduce a building’s cooling consumption by up to 25% annually. A contractor will take any existing shade structures and the direction your home faces into account when sizing your air conditioner.

4.      Your windows and doors

Air leakage from windows and doors can have a major impact on how well your air conditioner cools your home. If your windows are badly sealed or have formed gaps along the edges, hot air from the outdoors will be able to enter your home unfiltered. In this case, you’ll need a larger air conditioner to keep up with the air leakage. Energy-efficient double-pane, triple-pane, or low-e windows are less prone to energy loss than standard single-pane windows. If your home has energy-efficient windows, your air conditioner won’t need to be as large.

5.      The age and state of your home

Newer homes are built according to building codes that dictate a minimum amount of insulation in walls and ceilings, but older homes may have been constructed before these codes were in place. Without adequate insulation, an older home could require a larger A/C unit to keep the home cool. Along with lack of insulation, failing ductwork can silently leak air, causing your home to cool less efficiently. A contractor will need to take any existing ductwork, and its status, into account when sizing a new air conditioner for your home.

6.      The number of people in your home

A family of two isn’t going to need the same cooling power as a family of six. More people equals more body heat, which raises the temperature in your home. Each occupant adds an estimated 400 BTU per hour to the cooling load. For a general figure, most calculations account for the number of occupants in your home as equal to the number of bedrooms plus one, unless you specify otherwise.

7.      The size of your current air conditioner

One way to find the right size air conditioner for your home is to evaluate your current air conditioner. The size of your current system will either help the contractor know where to start in terms of sizing an air conditioner for your home, or it’ll reveal that your home has had the wrong size system all along. Some professionals state that up to 80% of the homes they service have an oversized size air conditioner. Oversizing air conditioners is very common, and many homeowners incorrectly assume bigger is better, but it can pose major problems, such as short cycling and a shorter system life span.

You won’t find the tonnage of your current air conditioner listed clearly on the system itself. But fear not—system capacity is often coded into the model number found on the outdoor unit. You’ll find the model number (or M/N) printed near the top of the label, underneath the manufacturer name. After a 5-digit string of letters and numbers, you’ll find a number between 018-060. This reflects the nominal size of the air conditioner in BTU/hour. An air conditioner with a nominal size of 048, for example, has a capacity of 48,000 BTU per hour. Nominal size converts to tonnage according to the scale below:

018 = 1.5 tons

024= 2 tons

030= 2.5 tons

036=3 tons

042= 3.5 tons

048 = 4 tons

060 = 5 tons

Get the Air Conditioner That’s Just Right with Chas Roberts

As you can see from the factors above, sizing an air conditioner comes down to a lot more than just the size of your home. With over 75 years of experience, the experts at Chas Roberts know the ins and outs of evaluating your home and finding the perfect air conditioner. Whether you’re in the market for a new system or have other HVAC questions, contact us today.