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AC Sizes Explained: What Size Air Conditioner Do I Need?

AC Sizes Explained What Size Air Conditioner Do I Need

As home architecture changes it is important to know the right way to determine proper air conditioner size so your home will always be comfortable no matter what the sun is doing. Many homes today have more windows, higher ceilings, and greater open space than homes 40 or 50 years ago. All that extra air needs to be conditioned with the right system. 

And older homes have other special factors that also need to be considered when sizing an air conditioning system. A central air conditioning system may not be the right choice but ductless mini split systems will still need proper sizing to conserve energy and match the cooling needs of your home. 

Air Conditioning Basics

When the summer sun beats down and begins heating up your home it isn’t the time for an underperforming air conditioner. This may be hard to believe but “cold” is really just a lack of heat. Air conditioning does not create cold but instead removes heat and humidity from warm, moist air so that you and your family can feel coolly comfortable. 

With system fans, air conditioning moves interior warm air across an evaporator coil that contains refrigerant. As refrigerant travels through the system it absorbs heat inside your home then deposits the heat outside. Additionally, the evaporator coil is capturing moisture from the air and condensing it into water – similar to what happens to your cold glass of iced water on a hot day. And just like the air around a cool glass of water, with the moisture removed the air in your home feels cooler.

Related: Ductless Mini Split vs Central Air: A Quick Comparison

Sizing an Air Conditioner

Sizing an air conditioner correctly can be tricky. There are all kinds of “rules of thumb” but you really don’t want to just “eyeball it” to make calculations for the right size air conditioner. And while many people might believe bigger is better, that isn’t the case with an air conditioning system. An oversized air conditioner removes less humidity which actually makes your home feel warmer and more clammy. The proper AC size for your home is based on a variety of factors — that we will talk about later — and the ideal temperature you want to maintain in your home on a hot day.  

What is a BTU?

There are several things to consider when buying an air conditioning system. The most important is the BTU value. BTU stands for British Thermal Unit. The fancy way this is defined is: one BTU is the amount of energy needed to change a pound of water one degree at sea level. Or more simply, it is the cooling effect an AC unit can provide.

Generally, the BTUs refer to the amount of heat your air conditioner can remove per hour. One air conditioner ton is equal to 12,000 BTUs/hour. So a one ton air conditioning system equals 12,000 BTUs, meaning it can remove 12,000 BTUs of heat from the air in an hour. 

What is a Manual J Calculation?

It is common to simply replace the existing model with the same size air conditioner. But what if your old AC unit wasn’t the right size? What if you added onto your home or changed out the windows? Now you simply have a newer improperly sized system. That is a lot of money for a system that isn’t satisfying your cooling needs.

Manual J calculation was developed by engineers at the Air Conditioning Contractors of America (ACCA) to provide universal standards and best practices in properly sizing HVAC systems. With a scientific approach to system sizing, it ensures you have “just right” temperature in your rooms because your air conditioning is balanced for your home. Once the Manual J calculations have been done for your home your HVAC consultant will know exactly what amount of BTUs you will need to cool your home and make system recommendations.

Related: Air Conditioning 101: Everything You Need to Know

Cooling Loads vs Cooling Capacity

When talking with your HVAC expert you may hear some different terms like load and capacity that could confuse you. We’re here to help you cut through the jargon so you can communicate with your HVAC technician. 

A load is how much cooling a home needs, while the capacity is how much cooling the air conditioning system can supply.

A load is influenced by the house, its orientation to the sun, how many occupants it has, and how many heat-generating appliances are in the home. A cooling load is calculated through a Manual J calculation to make sure that the proper air conditioning system is installed with the right cooling capacity to remove heat from your home. 

Factors to Consider

We talked about the air conditioning sizing “rules of thumb” above, and one of those rough estimators is square footage. Square footage should not be the only factor to consider when sizing your air conditioning system. Your neighbor may have the same square footage as your home but you might have more windows facing the afternoon sun or a shade tree on the south side. Those kinds of differences are going to change how you calculate the size of your air conditioner. 

Other factors that a Manual J calculation considers for an air conditioning system: 

Home shape: A compact home takes less energy to cool while a sprawling ranch home will have more area for the cool air to dissipate. 

Windows: Windows are not good insulators. If you have large windows throughout your home they will impact the amount of BTUs needed for sufficient cooling. It will also matter if those windows are full sun in the afternoon or have a shade tree near them. 

Insulation: A well-insulated home will need fewer BTUs to keep your home cool than an older uninsulated house that loses air through the walls. 

Climate: In a warmer climate an air conditioner will work harder because the hot outside air and the desired indoor air temperature are so widely different. Homes in northern regions are going to need fewer BTUs to keep the home at the desired temperature than homes in the southern regions. 

Ceiling Height: The total square footage of your home is not the only factor to consider. The total amount of air volume inside your home has an impact on the BTUs required to keep your home at the optimal temperature. 

Orientation: How the sun hits your home is a significant factor in determining the BTUs required for your HVAC system. 

Related: How Much Does It Cost to Install AC in My Home?

What a Good Air Conditioning System Provides

When sized correctly a good HVAC system provides: 

Problem Solving Air Conditioner Issues

It is the first hot day of summer. No problem. You turn on the AC expecting relief and instead you get . . . nothing. Before you assume that you need a new air conditioning system and start running a cooling load calculation, there are a few reasons your AC unit might not be working

  1. Dirty Filter: No matter if you have a central AC or a window air conditioner, a dirty air filter restricts the airflow through the system causing it to struggle.
  2. Blocked registers: Likewise, blocked registers will keep your AC system from working at peak performance. Look around for errant furniture, toys, or dirty registers that could be reducing airflow.
  3. Annual Checkup: Did you remember to have your system checked this year? An annual inspection of your HVAC system can catch problems before they start, making sure the moving parts move and the refrigerant is at top condition for the summer heat.

Additional reading: Why Is My Air Conditioner Not Working? 10 Possible Reasons

How Jacobs Can Help

If you are looking for a new air conditioning system, our consultants at Jacobs Heating & Air Conditioning will evaluate your home and help find the best system for your home and lifestyle. If you need air conditioning maintenance we are ready to help you save money and keep surprise breakdowns away. You can schedule your next tune-up and keep the stress and hassle out of maintaining your system. No matter what you need we’re always just a phone call away. You are welcome to review our statement on COVID-19 and how we’re taking precautions to protect you, our team, and our communities.

About the Author

Amanda Jacobs portrait

Amanda Jacobs, Internal Project Manager

Amanda Jacobs is an Internal Projects Manager and 3rd generation member of Jacobs Heating & Air Conditioning. She received her MBA from Seattle University and has worked for a leading HVAC training and consulting firm. When not talking HVAC on the Jacobs Blog, you can find her on the golf course or whipping up her famous vegan chili.