So you’re getting a new air conditioner! Well, what size should you get? Unfortunately, too many contractors use only one input for sizing an air conditioner. Yup. To them, it’s just a matter of floor area. Forget the windows, the insulation levels, and the air leakage. All you need to do is make sure you have one ton of air conditioning capacity for each 500 square feet of floor area. Heck, even Good Housekeeping did better than that.

With that warning, let’s look at some tips you can use to make sure you get a properly sized air conditioner.

1. Know how much cooling you need

For new homes that haven’t been built yet, there’s only one good way to size an air conditioner. You need to have a load calculation done. (The same applies to heat pumps, furnaces, boilers, or any kind of heating and cooling equipment.) For homes in the U.S., the standard way to do this is to use Manual J, the load calculation protocol from the Air Conditioning Contractors of America (ACCA).

For existing homes, you can use Manual J as well. As long as you get accurate information for the house to plug into the software, the results will help you size the air conditioner appropriately. But you may have a better option.

If your current air conditioner is still working well, you can measure the runtime when your home is at “design conditions.” That means you see how many minutes per hour the AC runs when you keep the thermostat at your desired setpoint (or 75°F per ACCA) and the outdoor temperature is at the outdoor design temperature for your location.

Even if—or especially if—you’re still years away from replacing an air conditioner, you can use this technique to see how well your current system cools the house. I did this in the condo I lived in a decade ago.

2. Don’t oversize

One of the results of using floor area to size an air conditioner is that it almost always results in oversizing. Air conditioners that are oversized have several drawbacks:

• Higher upfront cost
• Noisier
• May lead to reduced comfort
• Less ability to dehumidify

Here are the sizing limits from ACCA Manual S.

As you can see, the most oversizing allowed in this standard is 30 percent. For fixed-capacity systems, which is what the great majority of homes have, the limit is only 15 percent. Because of the realities of available air conditioner sizes, you may end up more than 15, 20, or 30 percent above the cooling load. Building codes usually allow you to go to the next available size if the lower size won’t meet the Manual S sizing limits.

3. Don’t be (too) afraid of undersizing

Here’s the thing. A Manual J load calculation still results in a cooling load that’s about 10 to 20 percent higher than the cooling the house actually needs. So if you undersize according to the calculated, you may be just about right. With a smaller system, you can invert that list of disadvantages above and have an air conditioner that’s less expensive, quieter, creates more comfort, and removes more humidity.

I should know. I’ve done it in my own house. I have variable-capacity Mitsubishi heat pumps that I had installed in 2019. We’ve been through plenty of days where we hit design conditions. We’ve also been through a heat wave and an arctic blast, both in 2022. And guess what? We did just fine.

Now, take another look at that table in the previous section. Notice that it also indicates a minimum air conditioner capacity that’s 10 percent below the cooling load.

I’m not saying that undersized systems always lead to comfort and efficiency. Yes, there are cases where a new air conditioner is undersized too aggressively or without proper load calculations. Replacing the air conditioner is one way to solve that problem, but you also could look for ways to reduce the cooling load.

4. Long run-times are a good thing

A properly sized air conditioner should run just about continuously at the design conditions. Why? Because when you follow the ACCA Manual J and Manual S protocols, the air conditioner capacity should be pretty close to the cooling load. (See that table above.)

When the AC is running 60 minutes per hour, those long run-times help in several ways. They create constant mixing of the air in the house. That leads to more uniform temperatures. Long run-times also do better at reducing the humidity. And if you’ve got a good setup with high-efficiency filtration, you get the added bonus of better indoor air quality.

5. Consider multi-stage or variable capacity

A fixed-capacity air conditioner operates at only one speed. It’s either on or its off. There is no in-between. The main reason so many air conditioners get oversized is that contractors don’t want to get called back because it’s not cooling the house. And they don’t have control over all the variables, like air leakage and occupant behavior.

But what if you could size the air conditioner to meet the design conditions and then have another gear to shift into that would do more cooling? Part-load conditions are what our homes experience far more often than design or extreme loads. So, what if you could undersize the air conditioner so that it would meet that lower cooling load?

Such an instrument is the Turbo-Thermo-Encabulator Max. Oops! Sorry. That’s something else. What I’m really talking about here is a multi-stage or variable-capacity air conditioner. They can run at low speed most of the time and the shift or ramp up when necessary.

Beware, though. This more advanced equipment isn’t a panacea. In humid climates, you may be more likely to need supplemental dehumidification with this type of equipment because it often has less capacity to remove moisture. And sometimes it’s done incorrectly, creating more problems. One thing to be aware of if you go this route is that it’s still important not to oversize. Quite a few people believe there are no downsides to oversizing, but that’s just not true.

There you have five good tips for sizing an air conditioner. And let me leave you with one final note. The load calculation doesn’t tell you what size air conditioner you need. It just tells you how much cooling you need. The Manual S protocol takes you that next step to sizing the air conditioner properly.

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Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a popular book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to the Energy Vanguard newsletter and follow him on LinkedIn. Photos courtesy of author.