What Air Conditioner Size Do I Need?

Do you know the correct air conditioner size for your home? Many do not. This is critical information to ensure you get the best comfort possible. An improperly sized system, either too large or too small, will provide inferior comfort. Nobody wants that.

In this article, we'll cover the basics you need to know about proper air conditioner size calculations, and give you the inside track on getting it right the first time.

Relevant Nomenclature: Tonnage & British Thermal Units (BTUs)

When we talk about air conditioner size, we're usually talking about tons. Those are broken down as 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, or 5.0 ton units for residential applications. There are larger sizes, but 6.0 tons and above are generally reserved for commercial applications. When a residential home requires more than 5.0 tons of capacity, the standard procedure is to install multiple smaller systems (e.g. - two 3.0 ton systems instead of a single 6.0 ton).

Not all air conditioners are classified according to tonnage. Some are ranked according to the amount of BTUs they produce. For example, you might see systems classified as 9k, 12k, 18k, 24k, or 36k air conditioners or heat pumps. Basically, this means that the system will produce, at maximum capacity, the volume of cooling listed in BTUs.

The main divide here is between central air systems and ductless systems. Central air systems like packaged air conditioner or split systems are almost always denoted according to tonnage. Ductless mini-splits and window air conditioners are almost always denoted by BTU. This is because the production capacity of the two different designs are not equitable. There are no central systems equal to a 9k window air conditioner and no window units equal to a 5 ton package unit.

Replacing an Existing System

You own a home with an air conditioner. It works great. It keeps you cool. And then it goes out. You have no idea what you need. You just know that you need an air conditioner. Because being hot sucks. The first question a lot of people in this position ask is, "Can I just replace my broken air conditioner with the same size unit?"

Yeah, probably. If nothing significant has changed with your property since you bought or built it, you can probably replace your broken unit with a new one that's the same capacity. Did your 3 ton air conditioner burn out? Buy a new 3 ton air conditioner, get it installed, and you're done.

This is by far the simplest solution to figuring out the right air conditioner size. The original installer did all the heavy lifting, so you don't have to go to a lot of trouble. There shouldn't be any problem with this approach unless....

New Construction, Remodeling, and So Forth

In a new construction there is, obviously, no existing HVAC system to replace. Ergo, you can't just go with what you had, since you had nothing. Unless you're literally building your own home, chances are that your contractor will have an HVAC professional to figure this out. And how will they do that?

Manual J Load Calculation

The only way to know exactly what air conditioner size will work in a given building is by conducting a Manual J load calculation. This is more than just measuring the square footage of the space. It does involve square footage, but also the building facing, the number of doors and windows, ceiling height, insulation, and so on. This is an involved process and not something a typical homeowner will be able to undertake alone.

How can you get this kind of work performed? Local HVAC professionals in your area may offer the service. It might also be performed by energy efficiency specialists, engineers, or others in your region. A quick search online should point you in the right direction.

Educated Guessing

Couldn't you just measure the square footage and make an educated guess? Sure, you could, but that's not a great idea. A central air conditioner needs to be properly sized for a space. An undersized system is going to run constantly and won't keep your space cool. An oversized system will run hard for a short time, but it won't dehumidify properly. Which means your space will be the temperature you want and still uncomfortable. The air will be clammy instead of cool. Trust us, you don't want that.

Variable-Speed Technology

Undersizing is never good. However, the latest HVAC technologies go a long way to ameliorating the worst effects of oversizing. How so?

An air conditioner is designed to cool and dehumidify simultaneously. As it produces cool air, it pulls humidity out of your air. A proper balance of humidity in your interior air will improve your comfort, and improve overall air health.

As we mentioned before, the problem with an oversized air conditioner is that its operating cycle doesn't last long enough to properly dehumidify the air. This problem can be ameliorated somewhat with a variable-speed compressor.

Unlike a traditional one-speed compressor, a variable-speed model can operate at different stages. It initiates its operating cycle at the lowest setting. It can modulate up if necessary, but the ability to run a lower cycle means it doesn't have to run a hard start-stop cycle like an otherwise oversized system might.

But what does this mean for the average person?

It means that when it comes to variable-speed systems like ductless mini-splits, you have some wiggle room in terms of capacity. For example, a variable-speed 24k mini-split is generally considered suitable for 1,000 - 1,200 sq ft. Of course, your particular space that size might have other factors (i.e. - insulation, windows, etc.) that means it could actually work well with an 18k system. You don't know that, of course, since there has never been a Manual J load calculation performed on that detached garage where you want to put a new mini-split. Fortunately, the variable-speed of the 24k means that if it's slightly oversized for its space, that's not a big deal. You should still get quality performance from the unit.

Do You Still Have Questions About Air Conditioner Size?

Now, you should be fully equipped to figure out the tonnage or BTUs you need to keep your home cool or warm, as the case might be. Even if you're not in the market for a new system, this should  be useful knowledge in the future, since we will all inevitably have to replace our home HVAC at least once in our lives. But do you still have questions? At Ingram's Water & Air Equipment, we want to make sure you have all the information you have. Reach out to let us know your questions or just drop them in the comments below!

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