Do you know what size your air conditioner is? In the world of building science, you’ll hear a lot of talk about why oversized air conditioners are a bad idea. Why? Briefly, they don’t dehumidify as well, short-cycling wears them out quicker, and your home will probably be less comfortable if the air conditioner is too big. But to know if your AC is oversized, first you have to know what size it is.

First, let me clarify. I’m not talking about finding the size of air conditioner you need. You do that with a heating and cooling load calculation. What I’m talking about is how to find the size of an air conditioner when you’re standing in front of it. Today, I’ll tell you four ways you can do that.

1. Determine the nominal size from the model number

The good news is that most HVAC manufacturers make it easy to determine the nominal capacity of your air conditioner. It’s in the model number. Go outside and find the outdoor unit, that metal noisemaker hidden away on the side or the back of the house. It’ll look something like the one you see above, although maybe not quite so decrepit. Then find the label that gives the data about your AC. It’ll look like the first image below.

Up near the top of the label, you see the model number (M/N) and serial number (S/N). The model number is where you can find the number you’re looking for. Not all manufacturers do this, but most will give you a 2- or 3-digit section that tells you how many thousands of BTU/hour your air conditioner can move out of your home.

The first section in the model number gives you info about the type and efficiency of the unit you’re looking at. In the case of this Lennox model (which, by the way, is not from the outdoor unit shown at the top of this article), the 13HPX tells you it’s a heat pump with an efficiency rating of 13 SEER. (That’s a nominal rating, too, which is a subject for a different article.) The X here indicates the refrigerant, R-410A in this case.

The digits you need

Just past that string of 5 characters, though, is the part that tells you the nominal size: 048. That means the air conditioner—or heat pump in cooling mode in this case—has a nominal capacity of 48,000 BTU/hour. I say nominal because the actual capacity is almost certainly going to be different.

The numbers you’ll see on residential air conditioners and heat pumps are:

M/N	Tons
018	1.5
024	2
030	2.5
036	3
042	3.5
048	4
060	5

The 3 digits in the model number tell you the nominal capacity in thousands of BTU/hr. Since each 12,000 BTU/hr is equivalent to 1 ton of air conditioner capacity, it’s easy to figure out how many tons of nominal capacity your AC has.

Pretty simple, eh?

2. Using the manufacturer’s data

The second and third methods are basically the same, but the source of the info is different. In the first method, you find the nominal size by looking at the model number of the outdoor unit. By gathering a little more information, you might be able to find the actual capacity of your air conditioner. What you need is the model number of the indoor coil, which is the evaporator coil for an air conditioner or heat pump in cooling mode.

If you’ve got a typical central air conditioner, it’s a split system. That means it’s got two major components: the outdoor unit, where the compressor is, and the indoor unit, which has a blower and the evaporator coil. If the indoor unit is connected to a furnace, you want the model number off of the box with the coil in it. Just look for the part that has two copper pipes (the refrigerant lines or lineset) entering and a plastic pipe (the condensate line) leaving.

When you have both the indoor and outdoor model numbers, you might be able to find the data for your air conditioner on the manufacturer’s website. The easiest way to find it there, though, is usually to let Google do the work for you.

For example, using the outdoor model number in Image #2 below, I typed “Lennox 13HPX-048 data” into the Google search bar and found what I needed. It was the ninth link on the first page, and I had to navigate in a few pages from where it dropped me, but I found what I needed in their Product Specifications document for the 13HPX.

That document has a lot of good data in it, but the part I was looking for was in the table that began on page 10: AHRI System Matches. (AHRI is the Air Conditioning, Heating, and Refrigeration Institute.) It started with the low-capacity units (1.5 ton) and on page 20 finally got to the 4-ton units I was looking for. I’ve included a screenshot of a small part of the table below (Image #3) so you can see the data.

Notice that the actual cooling capacity depends greatly on which indoor unit gets paired with the outdoor unit. It varies from a low of 46,500 BTU/h all the way up to 59,500 BTU/h. The latter number is the capacity of a 5-ton unit, so let this be a lesson to you in not putting absolute faith in nominal capacities.

In fact, the cooling capacities shown in the table aren’t the only thing that affect the actual capacity. I wrote last year about some of the other factors that change an air conditioner’s cooling capacity. Basically, the difference arises because AHRI test conditions aren’t the same as design conditions. You can read the article to get the details on that.

3. Using the AHRI Directory

This method usually gets you the same data you can get from the manufacturer because manufacturers test their equipment to AHRI standards. AHRI just makes you work a little harder to get the info sometimes. If you’ve ever used the AHRI Directory, you know how finicky it is.

As with getting the data from the manufacturer, with AHRI you look up the matching outdoor and indoor model numbers. You won’t always find the system you’re looking for in the AHRI directory, however. Sometimes it’s in there and hard to find. Other times it’s just not there. The latter could be because the model is too old or has been discontinued. It could also be that the two coils don’t match and haven’t gone through AHRI testing.

Here’s a tip for you to make working with the AHRI Directory easier: Enter minimal information to start and gradually add more. For example, when I went there and entered 13HPX for the outdoor model number and Lennox as the manufacturer of the outdoor unit, I got no results. Nothing. So I removed the manufacturer and just searched on 13HPX.

That gave me 1608 results. To narrow that down, I went back and added the size. Searching on 13HPX-048 gave me 260 results. Then I added most of the indoor model number, CBX25UH, and got it down to a manageable 6 results. The key is to be a minimalist! If you try to throw everything you’ve got at the AHRI Directory, it freaks out.

4. Using Preston’s Guide

If you have a really old air conditioner (10 years or more), you’re not going to find it in the AHRI Directory. You probably won’t find it on the manufacturer’s website either. If you can still read the model number on the outdoor unit, you can look it up in another directory called Preston’s Guide. I’ve got a paper copy that covers the years 1960 to 2000. The current edition goes up to 2005. In addition to the old style book, you can also get it in CD-ROM form or buy an annual subscription to their online database.

You need to know three things to look up an air conditioner in Preston’s Guide: the manufacturer, the outdoor unit model number, and the type of system. The last piece of info is needed because they cover several types and configurations of systems, from split-system heat pumps to single-package air conditioners. Once you have that info, you can look it up in the guide and find out the capacity in BTU/h and efficiency in EER or SEER. The book also shows the years that model was made, so you may need that bit of info, too, to home in on your system.

When you’re looking up numbers in Preston’s Guide, you’re not going to get actual capacities because it doesn’t show data for different combinations of outdoor and indoor units. And keep in mind that while it does give the appearance of being more accurate than using the nominal size from the model number, it’s probably not since you haven’t looked up a particular coil combination.

If all else fails, just replace it

Now that you have all these tools at your disposal, you should be able to find the size of just about any air conditioner you happen to run into. I say “just about” because occasionally you’ll find one with a dirty nameplate and when you wipe it off to read it, there goes all the remaining print, too. If it’s that old, though, just go ahead and replace it.

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Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.