The 3 Popular Methods of Calculating AC Size

Properly calculating AC size is an important part of keeping your home cool. There are three methods used to make the AC rating calculation.

The methods range in their effectiveness and accessibility to you. In other words, the first method you can quickly do by hand. While the other two methods you need special software because they’re complex.

In our discussion, we’ll go over each method breaking down the benefits and drawbacks. This includes learning how to best quickly estimate the size of an AC unit. So now, you can confidently buy the right small-size AC unit for your home or office.

But even better, you can have deep discussions with HVAC pros when you go buy a large central AC unit. Because you don’t want to be oversold, or buy an underperforming AC in the blazing heat.

Important Note: the heat lost from a house dictates the size of an AC unit. So naturally, the lost heat depends on a given indoor and outdoor design temperature. 

Required variables of calculating AC size

You’d think calculating AC size would be straightforward. That’s what many websites make you believe. But in reality, there are MANY variables to consider when calculating AC size. And the more variables you consider, the more energy-efficient unit you can buy.

The home variables include the following:

  • Square footage
  • Ceiling heights
  • Arrangement of rooms and home shape
  • Number of home stories
  • Orientation of home to the Sun (East, West, North, or South)
  • Building materials (e.g. bricks or wood)
  • Overhang distance of roof over walls
  • Number of chimneys
  • Number and usage of ceiling fans
  • Type of wall insulation
  • Type of duct insulation
  • Air filtration (e.g. cracks and unsealed edges)
  • Local climate
  • The topography around home (e.g. trees and buildings)
  • Number, types, size, and placement of windows
  • Number and location of doors
  • HVAC equipment performance

Not surprisingly, many of these variables aren’t ever considered in AC calculations. Even by so-called professionals. It’s because the analysis becomes overly complex. And the cookie cutter approach works good enough given a certain size home.

To point out though, many of these variables never stay constant. For example, think about smart landscaping. Say you plant a bunch of trees around your home. Then several years down the line, the trees become massive and tower over your home.

Now, your windows will naturally absorb less solar heat. So the AC unit you sized before the landscaping, is now oversized.

Important Note: larger than required AC units can cause problems. These units will often cycle on and off, commonly known as ‘short cycling.’ This is because they blast your home with cool air, versus gradually cooling it down. So, you’ll have huge temperature swings among other issues.

What’s more, your operating cost increases and the life of your AC unit reduces. 

Method #1: Air conditioner sizing using square feet

office building window mounted air conditioners

This method is a rough calculation only, which works fairly well. In fact, I know many contractors who use this method to size AC units because it’s so simple. And the reason this method works so well is because of the following reasons:

  • The gain or loss of heat is proportional to the volume of air in a space.
  • The gain or loss of heat is proportional to the surface area of the walls in a space.

Next, the home assumptions in this calculation method are the following:

  • Wall thickness is constant in all rooms
  • Ceiling heights are constant
  • The temperature difference is constant
  • The surface emissivity is constant
  • Thermal conductivity of walls is constant

The below table of values you’ll use to calculate your AC unit size. And important to know, 1 ton equals 12,000 BTU/hour.

 Defined parameterEquivalent BTU
STEP #1: heat gain from living space1 square foot20 BTU
STEP #2: heat gain from peoplePer person400 BTU
STEP #3: heat gain from cookingPer kitchen1,500 BTU
STEP #4: ceiling heightCeiling height over 8 feetMultiply total BTU by 1.25
STEP #5: exposure to sunHome facing the sunMultiply total BTU by 1.1

To put this method into action, we’ll go over an example using the following information:

  • (4) family members
  • 2,750 square foot home with 12-foot ceilings
  • (1) kitchen
  • Home facing the sun in the scorching climate of Phoenix Arizona

Now, the following table is the full calculation for our example with the AC unit sized:

 Family exampleCalculationTotal
STEP #1: heat gain from living space2,750 square feet20 BTU per square foot55,000 BTU
STEP #2: heat gain from people4 people300 BTU per person1,200 BTU
STEP #3: heat gain from cooking11,200 BTU per kitchen1,200 BTU
STEP #4: ceiling height12 feet57,400 BTU x 1.2571,750 BTU
STEP #5: exposure to sunYes71,750 BTU x 1.178,925 BTU
Total78,925 BTU or 6.6 Ton AC Unit

Again, I don’t recommend this method for sizing a central air conditioning system. Just use it to size a portable, window-mounted, or small ductless AC unit. Because you leave out too MANY important variables. So, this method can’t guarantee energy efficiency and maybe not even full comfort. You may also end up with a larger footprint and added cost from oversizing.

Important Note: for ductless air conditioners, you can also use Energy Star’s recommendation. They list the AC unit size you need for various sized rooms, with adjustments included.

Method #2: Air conditioner sizing using ‘Manual J’

This is the tried and true method for calculating AC units. It’s used frequently by contractors on large projects, and it’s approved by ANSI. The ACCA developed this calculation method to provide uniform HVAC system sizing. Because it does no one good if every person sizes an AC unit their own wonky way.

The good thing about this method is, it uses best sizing practices based on a scientific approach. So, you get a much better approximation of BTUs your home needs to cool down.

Not surprisingly, this sizing method isn’t done by hand. You instead use specialized software. The program will call for many of the sizing variables we initially had discussed.

The following is a simplified breakdown of the program’s steps:

  1. Choose the desired temperature and humidity you want in each room/space.
  2. Calculate your desired temperature and humidity difference between the outside and inside of a given room/space.
  3. Calculate the heating and cooling load using the ‘Manual J’ database and template.

HVAC load calculator is a great free online calculator based on ‘Manual J’ you can use yourself.

Important Note: with this method, you can still oversize your AC unit. 

Tell your AC technician to not include extra safety factors in their calculation. This would be on top of safety factors already baked into the software. 

I recommend requesting the outputted calculation sheet from your HVAC technician. This way you can verify all the inputs and assumptions made in the calculation. 

Method #3: Air conditioning sizing using ‘Manual J’ and extra charts

rooftop hvac
Photo Credit: P199

To turn it up a notch, this method is even more involved than Method #2. This method uses the following:

  • ‘Manual J’
  • Comfort chart
  • Duct sizing chart
  • Psychrometric chart

You use this method for large homes or commercial buildings above around 9,000 square feet. At this square footage, you want to nail the size of your central air conditioning unit the best you can. There’s no room for errors.

For this method, you need to complete the following steps:

  1. Use the comfort chart from the ‘American Society of Heating & Ventilating Engineers.’ You want to identify the comfort region inside your air-conditioned space.
  2. Use the ‘Manual J’ method to calculate the heat gain in all rooms. This calculation will consider both inside and outside design variables.
  3. Calculate the mass flow rate of air into each room. This is the ratio between fresh air and re-circulated air. Thereafter, figure out the air conditioning unit size from the cooling load value.
  4. Calculate the duct size in each room.

Important Note: even a perfectly sized AC unit isn’t perfect. Say we calculate the perfect AC unit using the best method avaiable. Our calculation will give us the optimal operating range for your AC. But not surprisingly, you probably won’t operate within this range.

It’s no different than with cars. A car may have a max fuel efficiency at a constant speed of 60 miles per hour. But most people’s driving is erratic with excessive gassing and breaking. And this is the conundrum of trying to size the perfect AC unit.

Limitations with the AC calculation methods

None of the methods are perfect, even when done by the pros. Yes, even if you consider every variable that comes with calculating AC size. And I’ll explain why.

I live in California, and year after year, the temperature is increasing. Just this last year, we hit a temperature of 112°F.

What’s more, these insane temperature spikes are becoming more common in odd months. So, do you design for these temperature anomalies we’re experiencing today? If you do, your AC unit will be overly large for the rest of the year when temperatures are normal. But if you don’t, you’ll bake those several months where the temperature skyrockets.

To make matters worse, what if extreme temperature cycles become permanent. Do you look into the future and account for them today? Because who wants to replace their costly AC unit every 5 years? I know, I don’t!

At the same time, replacing an AC unit in kind isn’t as clear-cut as you may think either. For example, you buy a new home, and you simply check the AC nameplate and then buy an identical replacement unit. The problem is, the existing AC unit size was maybe incorrect from day 1.

Or, what if you remodeled your home by adding extra windows and appliances? You can’t just swap out your existing AC unit any longer, without doing new calculations.

And finally, AC units are becoming MUCH more efficient today. Today, companies sell SEER 20 systems. Where ten or so years ago, SEER 14 systems were commonly sold. So comparing an older unit isn’t a apples to apples comparison.

Important Note: SEER stands for Seasonal Energy Efficiency Ratio. The higher the SEER rating, the more efficient an AC unit will run. For example, a 20 SEER AC unit compared to a 10 SEER AC unit will have twice the efficiency. So you can cut your energy bill for cooling in half.  

Conclusion

Calculating your AC size isn’t too difficult as long as you have all the information you need. But that’s the problem. Most of the time, you have very little information.

As we learned though, for smaller AC units, you can get away with Method #1. I’ve done it plenty of times. But for large units, like central cooling units, you need to hire a pro. Or at the very least, get a hold of the software yourself, and learn the ins and outs.

If nothing else, learn about the method limitations. This way, you understand what can go wrong in sizing. For example, say you size your central AC unit based on square footage alone. I’ve seen one too many online charts that hinge on this sizing oversimplification. In the end, you’ll only hurt yourself by burning up in the summer heat or having a deep hole in your pocket.

And not to beat a dead horse, but for huge purchases like central AC units, don’t rely on cookie-cutter charts. Also, don’t blindly take an HVAC technician’s word. Especially if they don’t show you how they calculated your AC unit size. Just be smart, and you’ll then stay cool without paying an arm and a leg.

What are your thoughts on calculating AC size? What are your thoughts on the limitations of AC sizing?

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