Central Air Conditioner Buying Guide

Central Air Conditioner Buying Guide

How to Pick the Perfect Air Conditioner

There are many home cooling options on the market today, but central air conditioning remains the original, tried and true method. When people say “air conditioner,” this is usually to what they’re referring.

Central air conditioners use an outdoor compressor and an indoor air handler to blow cool air throughout ductwork in your home. Although simple on paper, there is much more to understand before buying an air conditioner for your home. By covering some of the nuances, we’ll show you how to choose an air conditioner and stay cool during the dog days of summer.


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A Peak Inside an A/C System

The air handler portion of the AC system sits in your home, often in a basement or service room. A fan draws warm air from the room and passes it across a coil filled with cold refrigerant fluid. This cold fluid absorbs the heat from the air, which turns cool. A blower than blows the cooled air around your house.

The outdoor condenser usually sits on the side of your house. It is connected to the indoor unit via a line set. After the refrigerant absorbs the heat from your home, it flows to the outdoor unit, which dissipates the heat. Important to note is the compressor, which circulates the refrigerant through the system. Now cool again, the refrigerant flows back to the indoor unit to continue the cycle. You can learn more details about how air conditioners work in our Air Conditioners 101 Guide.

The main thing to understand is that central air conditioning is a system of components that work together in cooling your home.

Man installing air conditioner

Staging and Variable Speed Compressors

Now that you know how air conditioners work, let’s talk about efficiency. When shopping for an air conditioner, you’ll have to choose between single-stage, two-stage, and variable speed compressors. What you select will determine your level of A/C efficiency and comfort.

Single-Stage: A single-stage unit can either be off or on at full capacity. It will operate at full capacity regardless of how much cooling is required. Because air conditioning systems are sized to provide adequate cooling on the hottest day of the year, this means single-stage systems are often providing more cooling than is required. This reduces the efficiency of the system and can lead to uncomfortable conditions.

Two-stage: Two-stage compressors help address this mismatch of cooling needs and system output. A unit equipped with a two-stage compressor will have two distinct levels of cooling output, typically called the low and high stages. The high stage will have sufficient capacity to maintain comfortable temperatures in extreme weather, while the low stage will allow for efficient operation during mild conditions.

Variable Speed: Variable speed compressors are the full-out extension of two-stage compressors. Rather than having specific output levels at which to operate, a variable speed unit can produce any output within a range. This range will vary from model to model and will enable the unit to match the required cooling or heating load in real time and maximize comfort and efficiency.

The graph below illustrates how these different types of stages might operate on a typical summer day.


  • The single-stage unit has short choppy cycles with wide temperature swings.
  • The two-stage unit has longer cycles and maintains a more stable temperature.
  • The variable speed unit operates continuously and maintains a nearly constant temperature.

Two-stage and variable speed units often have a higher rated efficiency than single-stage models, but even if all three of these units had the same rated efficiency, the longer cycles of a two-stage unit would save approximately 3 percent over a single-stage model, and a variable-speed model would offer savings of approximately 7 percent.

Understanding Air Conditioner Efficiency Ratings

Air conditioner efficiency is rated in SEER2 (Seasonal Energy Efficiency Ratio). An air conditioner’s SEER2 rating is arrived at by dividing the total cooling output from a piece of equipment in BTUs by the energy it consumes in watt-hours. Most central air conditioners today have a SEER2 rating between 13 and 20, with those at the higher end of this range considered very energy efficient.

It’s important to note, however, that the efficiency of a system and its SEER2 rating depends on all the components in the system and not just the condenser. The SEER2 rating for a condenser is primarily used for marketing and product differentiation and you should always use the rating for a complete matched system when comparing options.

Central Air Conditioning Condensers

What Size Air Conditioner Do I Need?

Central air conditioners are sized based on how quickly they can remove heat from a home in one hour. This is measured in tons. In this context, a ton has nothing to do with weight. Instead, a 1-ton air conditioner can remove 12,000 BTUs of hot air in one hour. A 2-ton air conditioner can remove 24,000 BTUS per hour, and so on.

When buy an air conditioner, you’ll see it sized in tons. How do you know how many tons you need? Although there are different rules of thumb and ways to estimate, you should not rely on them. There is too much room for error and you’ll likely either undersize or oversize your air conditioner.

An ACCA Manual J Calculation is the only trusted way to get the right-sized heating and cooling equipment for your home. Read our HVAC sizing guide to learn more about Manual J calculations and why they’re the best way to go for sizing HVAC equipment.

You now have an understanding of central air conditioner components, efficiency, and key terms. The next step is to browse our central air conditioners. We offer all the components in a package or separately. Remember to contact our HVAC experts with any questions.


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Ken, the eComfort Product Expert
Product Expert
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