At ASM, we are a small, U.S. Veteran-Owned business, and pride ourselves on giving straight, honest answers to peoples’ questions. In order for them to be useful, we base most of our posts on the questions that we get asked most often, and questions on which size central air conditioner to buy are frequent. Most often we’re asked “what size central air conditioner do I need for my house?” There are many factors in your home that affect the efficiency and size of your central air conditioner, such as the height of your ceilings and the type and thickness of your insulation. And with most of these questions, HVAC guys have a tendency to go off on some technical rant about load calculations and R values – at ASM, we are usually no different. But, I have been told by many (mostly my wife) that I need to speak in layman terms and stop going off on these technical tangents. The compromise is that in this article we will figure out how to determine what size central air conditioner you need for your house, and we will do it in two parts: first, we will take a quick look at how to get a general idea of what size central air conditioner is right for your home, and second we will go more in-depth as to how to properly use load calculations to find the right sized central air conditioning system for you and your family.
Part I: A Short, Layman’s Guide to What Size Central Air Conditioner to Buy for your House.
If you are looking for the more in-depth, proper way to calculate which sized central air conditioner to buy for your house, then skip down to Part II below. Instead, this section is a quick way to calculate the right sized central air conditioner for your home without getting into the nitty gritty. It is for the layman who isn’t all that familiar with air conditioners or how they work. If you are absolutely brand new and have no idea what your air conditioner is other than the thing that makes your house cold, then I’d recommend taking a look at the following articles before continuing on:
What different sizes does your central air conditioner come in?
So there are a couple different ways to get a good idea of what sized central air conditioner you’ll need. Both methods are pretty close and are good for getting a ballpark estimate. Before figuring out what sized central air conditioner is right for your house, you need to know what sizes they come in. Central air conditioners come in a variety of sizes, and what size they are is measured in “tons.” Now contrary to what you might think, the tonnage of an AC unit is not actually based on its weight. A “ton” is a measure of an air conditioner’s ability to cool. One ton is the ability of your air conditioner to cool 12,000 BTUs (British Thermal Unit) in an hour. Likewise, a “2-ton” central air conditioner is able to cool 24,000 BTUs per hour. So now you are likely asking, what is a BTU? A BTU is the amount of energy required to heat or cool one pound of water by one degree Fahrenheit. So a 1-ton air conditioner can cool 12,000 pounds of water by one degree every hour. That’s all it means, so don’t let your HVAC contractor tell you any differently!
The central air conditioners that are installed in your home range from 1-ton to 5-ton units and increase in half-ton increments (2-ton, 2.5-ton, 3-ton, etc…). Anything over 5-tons is generally considered a commercial HVAC unit and if your house requires an AC unit of this size, then I’d recommend you install multiple units in tandem. What this means is that if your home needs 6-tons of cooling power for instance, then you’d need two, 3-ton units installed (instead of a 6-ton unit) and probably a zoning system as well. For more information on a zoning system, try: Do you need a zoning system? This seems like overkill, but the nice part about two systems running together is that if one unit completely fails, then you still have one unit running. This might not keep your house at a comfortable 68-degrees but it can at least keep your house from sky-rocketing into the 90’s in the middle of summer. In general, unless you have a mansion or zero insulation in your home then you are unlikely to need anything more than a 5-ton unit. If you want to learn more about BTUs, take a look at Wikipedia’s page on BTU’s:
Why does the right sized central air conditioner matter so much?
Simply put, size matters. At the risk of alienating half of my readers, I am going to skip the obvious joke. It is a common misconception that bigger is better (again, I’ll pause for laughs), and conventional wisdom is wrong in thinking that if a 3-ton unit would work well, then a 4-ton central AC unit couldn’t hurt. Here’s why: this idea was based on older central air conditioners from the 1980’s which were big, used a lot of energy and could cool the Staples Center in about 15 minutes. Remember that electricity was cheap back then, and insulation was minimal at best and nonexistent at worst. Today, energy efficiency is the name of the game and modern central air conditioners run in cycles; slowly but steadily lowering the temperature of your house instead of rapidly.
Too big – if you buy a central air conditioner that is sized too big for your house, then it will run more like that AC unit from the 80’s – it will turn on quickly, cool your house rapidly and then shut off. Your house will still be nice and cool but because it is not going through the entire cycle it was designed for, heat will soon infiltrate your house and it will have to click on again. It will begin cooling your house rapidly and then it will turn off again and again. This constant on-off cycle will defeat the purpose of its energy efficient design and drive up your electric bill.
Too small – if your central air conditioner is sized too small, then it will run constantly and spend most of the day trying to catch up to where it should be. Again, your house will still be nice and cool (unless it’s several sizes too small). The problem here is that modern central air conditioners, unlike your air conditioner from the 80’s, will have a hard time cooling your entire house if they are too small because they are made to work slowly over time and don’t have the power to cool your house rapidly. They will run constantly and again run up your electric bill.
Just right – a properly sized central air conditioner will run through the required amount of cycles to keep your house comfortable, but will not run so much that it looses its efficiency. For more tips on how to lower your electric bill, try: How to lower your electric bill in the summer.
How to calculate the right size central air conditioner for your house.
Now that you know what sizes your central air conditioner comes in, you can figure out how to calculate the right sized central AC unit for your house. There are two different ways to get a ballpark size estimate for your house. First, the old-school equation: ((House square footage times 25, divided by 12,000) – 0.5) = required tons. So for instance, a 1,500 square foot home would look something like this:
- 1,500 X 25 = 37,500
- 37,500 / 12,000 = 3.1
- 3.1 – 0.5 = 2.6 So you’d need a 2.5 or a 3-ton sized central air conditioning unit.
The second way to find out what size central air conditioner you might need for your home is to use the chart below. If you are looking for a quick ballpark value, then this is the way I’d go because it also takes into account the average temperatures in your region. For instance, the required size of a central air conditioner in Bozeman, Montana differs greatly from the one you’d need in our service area of Santa Clarita, California. Start at the bottom of the graph by finding your area on the map of the continental US and note what color you are (Alaska use blue, Hawaii use orange). Then go to the top and find your zone color. Go down your zone until you find the square footage of your house, and note the required size on the left.
As you can see, the properly sized central air conditioner for a 1,900 square foot home in Montana is quite different from the required size in Santa Clarita, California. Hopefully, this section helped you get a quick idea of what size central air conditioner is right for your home. But keep in mind that this is just a ballpark estimate, and there are several factors that this doesn’t account for such as how many windows you have, what kind of insulation you have and how high your ceilings are. In order to account for this, you’ll need to keep reading and take a look at our next section.
Part II: What Size Central Air Conditioner Do I Need for my House? The In-Depth Guide.
If you disregarded my advice and skipped straight to this section on what size central air conditioner to buy for your home, but you still don’t have that much experience with central air conditioners or if this is all new to you, then I’d recommend that you scroll up and read Part I first.
Introduction to the “Manual-J” – The Proper Way to Calculate the Right Sized Central Air Conditioner for your Home.
If you are here to see the real deal on how your HVAC contractor should be computing a properly sized central air conditioner for your house, then prepare to nerd out! The proper way to calculate the required size for your central air conditioner is by running a load calculation known in the HVAC industry as a “Manual-J.” (For more information on what the word HVAC means, take a look at: what does HVAC stand for?) A manual-J calculation is a detailed analysis of your home’s central air conditioning and heating needs. There are two types of manual-J calculations: a whole house load calculation and a room-by-room load calculation.
Whole house load calculation vs. room-by-room load calculation.
A whole house load calculation, or a “block calculation” is a manual-J used to determine the amount of cooling required for an entire house as a whole, and includes the heat transfer between walls, number of windows and their efficiency rating, type of insulation and how much of it, heat transfer through your concrete slab, the number of people in your house (each person gives off around 250 BTUs per hour), how many sky-lights you have, the amount of lighting you have and what type, what type of ducting and the ducting location (in your attic or inside your home’s insulation), as well as a few other factors.
A room-by-room load calculation is a manual-J that is used to calculate all of the above inputs, only it is done for each individual room and then added together as a whole. In general, a room-by-room load calculation is the best way to get an accurate idea of what size central air conditioner is best for your house, but each method is useful for different things.
When to use a house load and when to use a room-by-room load calculation.
When to use each is less a matter of science and more a matter of art. In general though, a whole house calculation is best used if your HVAC contractor is only replacing your central air conditioning unit but not your ducting, having already determined that the current ducting in your home is sufficient.
Be careful – many HVAC contractors would just assume replace your central air conditioning unit and not mess with the ducting, but the modern way of ducting a home is a lot more involved than the old way which was basically to run a tube to each room and call it good. For a contractor, it is just easier to replace the central AC unit itself and skip the ducting because the money is mostly in installing the HVAC unit itself, and it also requires the least amount of time. Ducting is a hassle and takes a decent amount of time, during which the contractor is paying his men $25-$50 an hour.
A room-by-room calculation is best if both the central air conditioner and the ducting need to be replaced. I swear that this is not a shameless plug but is instead a recommendation – unless ASM installed the ducting in a house or business ourselves, we always recommend that our clients replace their old ducting as well. There is just too much energy (and therefore money) lost in ducting if it isn’t done properly. There isn’t a lot of money in it, but it is an essential part of doing the job right. Imagine, you have a room with large windows that is on the sunny side of your house. This room will heat up far more quickly than a room on the dark side of the house with no windows, right? The sunny room will probably need extra ducting and another register (an AC vent) added to ensure that it is cooled properly. If you have enough rooms like this, then you will probably need a larger size central air conditioner as well. Just something to keep in mind.
Which should you use to calculate the right size central air conditioner for your house?
I know that you aren’t going to like this answer, but to do a true manual-J calculation you will need to buy some software and I wouldn’t really recommend getting that in-depth unless you are an HVAC contractor. Remember that you are paying your contractor to come to your house and do a manual-J calculation for you! If you are absolutely determined to do it yourself, then you can purchase a pretty good Manual-J product here:
Otherwise, I would recommend two things: first, if you are going to do it yourself then start small and start with a house load calculation; and second, don’t try and tackle a full, comprehensive Manual-J by yourself. It is not an intelligence thing, it’s an experience thing so don’t let your ego get the better of you! You can mess up a lot of things in one of these calculations (I know because I’ve done it myself), all of which could lead to you buying the wrong sized central air conditioner for your house. Here is my recommendation: go to the following website and enter in the values for your home. It will give you a Manual-J estimate for your house that is far more accurate than the beginner methods I discussed above in Part I and should be more than enough for what you are looking for. The “home type” section takes into account how many stories and the approximate height of your ceilings for the most common types of homes:
Final Tips for Sizing your Central Air Conditioner.
1. Make your HVAC contractor do a room-by-room Manual-J for your house. Remember, no HVAC contractor, no matter how experienced, can look at your house and say what size central air conditioner is best for your house. The better ones who have been doing this long enough can give you an answer after asking you some questions about your insulation and sticking their head into your attic, but be wary of anyone who looks at your house and says, “yea, you’re gonna be a 3-ton unit.” It doesn’t work that way. Just because the old unit was 3-ton doesn’t mean the new one will be!
2. Get multiple estimates. If you are not in Santa Clarita or the Greater Los Angeles area, then here is a good article on how to choose an HVAC contractor. If you are in Santa Clarita, then I’d still recommend you read our article on how to choose an HVAC contractor, then contact ASM if you think that we are a good match for you. I will tell you right now that we are not always the cheapest, but you get what you pay for in this business. Wherever you may be located, don’t skimp on your contractor – it will end up costing you more in the long run!
3. Replace your ducting too unless you are sure it is sufficient and done properly. Improper ducting is like having a strong heart with clogged arteries – the system still doesn’t work. Hotter rooms should have multiple ducts and registers.
4. Size matters – there is a such thing as too big of an air conditioner and too small.
5. Who you get to install your central air conditioner is more important than which air conditioner you buy or what size it is. See item 2 above for tips on how to choose a reputable contractor.
6. Once you have figured out who should install your AC and what size central air conditioner to buy, then you should start looking at brands. Not all air conditioners are the same, so do your research. Here are a few good articles to start with:
I hope you have found this article helpful. For more answers to questions you might have, visit the ASM air conditioning blog.
If you found this article helpful, All Systems Mechanical offers online and in-person professional consultations. If you have questions about your project, or something your contractor is telling you doesn’t sound quite right, purchase an ASM Technician for an hour and put your mind at ease – it will save you money in the long run. More here: ASM Air Conditioning Consultation Services
If you live in Southern California, including San Diego or Santa Clarita and the Greater Los Angeles area, we’d be happy to help ensure that your central air conditioner is installed properly, efficiently, on time and on or under budget. It’s easy – just click the link below: