AC Load Calculations: What You Should Know

May 31, 2018

Man cooling off in summer heat Determining the best central AC fit for your home can be a complicated process, especially if you don’t know what you need or what to look for! For the sake of this discussion, we will focus on central air conditioning for homes that have forced air heating systems, employing a furnace and ducting to heat the home. Later in this article, we’ll also touch on ductless AC, but we’ll shelve that discussion for the moment.

We recently discussed how calculating a heat load works and why that’s important for your home, but did you know that there’s a separate load calculation that helps determine what your home needs for air conditioning? In both cases, a properly sized unit will operate more economically and more comfortably than a heater or air conditioner that’s too large or too small for your needs. So let’s take a closer look at what goes into a cooling load calculation, and what to do with that information once you have it. After all, another number to keep track of doesn’t mean much if you can’t use it to help make your home as comfortable as your heart desires!

What’s Included

A heat load calculation and an AC load calculation go through similar processes to arrive at the final number that determines the size equipment your home requires. Check out our January blog on heat loads to get a better idea of what goes into this assessment. In brief, by taking into account the size of your home, its insulation values, and other important data, an HVAC professional can calculate the BTU’s per hour of heating or cooling output from the unit needed to keep your home at a predetermined temperature. For whole-home central AC systems, this calculation typically produces cooling outputs ranging from 24,000 to 60,000 BTU’s per hour to keep your home comfortable in the summer. The higher the load number, the larger the system you’ll need. Let’s see how this translates to the equipment that will be installed in your home.

What to Consider: What’s A Ton?

Once you have this cooling load number, it is used to determine what size air conditioner makes sense for your home. Interestingly, central AC sizing, as opposed to heating, is actually not discussed in terms of BTU’s per hour when it comes to choosing equipment. Rather, the BTU’s are converted to “tons of cooling” by dividing the BTU number by 12,000 ( 1 ton of cooling equals 12,000 BTU’s). We won’t bore you with the details, but the term “tons” in cooling jargon goes back to the 19th century, when large blocks of river ice were often used in cooling applications.

So like it or not, a 2 ton air conditioner (2 x 12,000) provides 24,000 BTU’s of cooling, and on up to the largest common residential AC unit, a 5 ton unit providing 60,000 BTU’s of cooling. So much for the annoying math, the important takeaway is that when you purchase a central AC system, you may find yourself talking about both BTU’s and tons of cooling, and it can be helpful to understand the relationship. This is just one of many good reasons why you need a cooling contractor you can trust when you decide to make a significant cooling investment. No one expects you to perform this calculation or to size the right cooling system for your home. That’s the job of an experienced pro.

What Else Matters?

Knowing what size unit makes sense for your home is the first step in the AC shopping process. From that point, learning more about SEER efficiency ratings and optional comfort features is the next piece of the puzzle. In a nutshell, the higher the SEER (“seasonal energy efficiency ratio”) rating, the lower your electrical bills for cooling. For central AC, currently available equipment ranges in efficiency from approximately 13 SEER to as high as super efficient 25 SEER rated systems. Depending on the particular situation, anything from this wide efficiency range might make sense for a particular application. For example, a family that’s planning to stay put for many years while the kids go thru school, is more likely to choose a system in the high-end range for greater long-term savings, while a short-term residence or a rental unit might be a better home for a more moderately priced system.

In addition to knowing what size air conditioner makes sense for your home and deciding what efficiency is a good match for your plans and budget, it’s also a good idea to learn a bit about enhanced comfort options such as two-stage and variable output air conditioners. Briefly, these types of systems are capable of running at lower output levels and quieter noise levels when the weather is not quite so boiling hot outside. The result is a highly efficient system that distributes cooling more evenly around your home, with a quieter, longer run cycle that also allows your furnace filter to do a better job filtering your indoor air.

Well, How About Ductless AC?

As promised in the beginning of this article, there’s another air conditioning and heating option well worth mentioning: Ductless air conditioners and heat pumps. We won’t dive into great detail here, but be aware that ductless AC systems, in appropriate applications, can provide the ultimate in quiet, super efficient zoned cooling (or heating) for homes with no ducting system, or for homes with problem areas that the central system cannot address. Sizing procedures and efficiency options for ductless systems are fairly similar to central systems, with the noteable exception that the equipment is sized for specific areas in the home, as opposed to the entire living area.

At the end of the day, understanding how your new AC system is sized, and learning more about your efficiency and enhanced comfort options, will help you make the best decision for your particular situation. Talk with a trusted HVAC pro and obtain quotes for any cooling options that make sense for you to consider for your home.