Air Conditioners - Which size do you need?

Written by Jason Hutchinson
5 Min read
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Table of contents

As we start to get into the sweltering heat of summer, I'm sure lots of you are trying to cool down and considering investing in AC units, but a lot of the lingo is confusing, and it's hard to have any idea if you want a 2.5 ton or a 3 ton, or you might not even understand the difference at all, and that's okay! The unit size is the most important thing, so we'll help you go over all the little bits that you'll need to get your AC unit perfect for your house.

The first and most important tip, bigger isn't necessarily better.

A really important part of the process is to carefully do the maths to figure out what size you need, maths is not estimates or guesses, but precise maths so you get the perfectly sized AC for your needs.

Manufacturers can give you an estimate a lot of the time for a general range of what you're looking at, but the specific maths needs to be a lot more precise and will usually be a paid bit of work.

Square Footage Estimations

Some examples of the estimates that manufacturers will give you will tell you that a 1,200 to 1,500 square foot house will take an air conditioner that is 30,000 BTU and 2.5 tons. The slightly larger units come in at around 36,000 BTU and 3 tons, for homes between 1,500 and 1,800.

There is another rule of thumb that tends to be a lot more inconsistent but can still give you a rough idea, but that you need a ton of air cooling capacity for several square feet in your home. I've seen this number argued all over the internet but a pretty common answer is 600 square feet per ton of air cooling capacity. This means that a 2.5-ton air conditioner would be able to effectively cool a 1,500-square-foot space.

Having said that, estimates aren't the way to size up a house though and it makes it more effort and some professional help to figure out how much you'll need.

Estimating the Right Size Air Conditioner

The HVAC industry has a rough estimate that you can find online that'll help as a general guideline, doing a little bit of research on your own is always helpful so that when an HVAC professional comes over with something more precise, then you can know what you're talking about.

How are Air Conditioners Measured?

We use a measurement of BTU to measure how an air conditioner works. The BTU is the number - in tons - of British Thermal Units (BTU) it can extract in an hour.

The general standard for central air conditioning begins at a minimum of a ton and a half and then goes up in half tonnes, so ton and a half, two tonnes, two and a half, etc. If we go off the same measurement we were using before then each half a ton also covers about 750 to 1,000 square feet of land.

Location

The HVAC industry also takes into account the area that you live in, so all of America is split up into a temperature bracket of zones 1-5 that also help to determine what size of air conditioner you will need.

Here are a couple of examples so you can see how it will affect the air conditioner size that you need.

For a 1,200-square-foot home, you would probably need a two-ton or a two and a half tonnes air conditioner for a hotter climate. However, if you bring that down to a colder climate then two and a half tonnes unit will be oversized and you'll probably be too cold.

Again, 1,800 square feet with all of the usual variables and nothing too crazy that will drastically change the maths, will need a 3 tonnes unit in a cold climate. If you bring that same home with the same variables into a hot climate then it will probably need a three and a half tonnes unit to keep it cool; effectively.

What Kind of Variables will Change the Size?

Appliances

Cooling rooms that have appliances in them will take extra power, for example, the kitchen and ovens, the laundry room and washing machines, etc.

What is your house made of?

Whether your house is made of brick, wood, stone or anything else will be a variable in the size of the air conditioner that you'll need.

Height of the Ceilings

Much taller rooms with higher ceilings will take more power to be able to air condition.

Climate

As previously mentioned, the area that you live in and the temperature in that area will make the air conditioning unit need to be bigger for a hotter climate, and maybe a little smaller for a colder one.

Insulation

You can calculate the amount of insulation that your home has and this will impact the size of the unit needed.

Electronics

In the same way that the appliances work, having lots of electronics in a room will increase the amount of insulation needed.

Windows

The number of windows that you face and the direction they face impact lots of things, like the amount of direct sunlight that reaches inside your house and the amount of shade inside your house receives, so this will also be a variable.

The Best Way to Start Measuring

You will be able to find how much size you need if you can understand it and can get the hang of the maths. There are even heat load calculators that you can find online which can help the homeowner to figure out the amount of air conditioning space that you'll need on your own.

But what is a heat load?

The heat load is essentially the combination of every variable possible and is usually measured in BTU, that measurement that we spoke about earlier, or sometimes kilowatts. Instead of heat load, it can also be called heat gain.

This bit of maths is really important because the capacity of the air conditioner always has to be greater than the heat load!

Every single thing that surrounds the house has some kind of an impact on the temperature of the home and the amount of air conditioning capacity that you'll need to effectively counteract it and make sure you feel comfortable in your own home.

This is the maths that you'll need to get the right heat load:

  1. Find out the square footage of the area that you want to be kept cool, and multiply it by 31.25
  2. Calculate the amount of heat gain coming through the windows and multiply that number by 1.4
  3. Times the number of occupants by 600, the average person generates 600 BTU
  4. Find the power wattage for every electronic appliance in the home and multiply it by 3.4.
  5. Do the same with the power wattage for each light in the home and multiply by 3.4.
  6. Bring all of these numbers together to figure out, theoretically what the total heat load should be.

All of this information is extremely overwhelming and a lot to take in, and it's very easy to make a silly mistake and ruin all the measurements, this is why it's so important to take get a professional to do these measurements for you.

Final Thoughts

Now that you can do a little bit of research, use this to be confident in any price or estimate that a professional will give you, whilst you won't know for definite what size is best for you, it'll at least help to stay in the know and to get a better idea of what you're going for.

How Many Square Feet will a 3-ton AC cool?

If we use the general rule of thumb that each ton can cool around 600 square feet, then a three-ton cooler will be able to cool around 1,800 square feet of land. Remember though that this isn't exact maths and lots of other variables come into play.

Appliances made simple. 

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