The 3 digits in the model number tell you the nominal capacity in thousands of BTU/hr. The numbers you’ll see on residential air conditioners and heat pumps are: I say nominal because the actual capacity is almost certainly going to be different. That means the air conditioner-or heat pump in cooling mode in this case-has a nominal capacity of 48,000 BTU/hour. Just past that string of 5 characters, though, is the part that tells you the nominal size: 048. 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. The first section in the model number gives you info about the type and efficiency of the unit you’re looking at.

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 model number is where you can find the number you’re looking for.

Up near the top of the label, you see the model number (M/N) and serial number (S/N). Then find the lable that gives the data about your AC.

It’ll look something like the one you see above, although maybe not quite so decrepit as that one. Go outside and find the outdoor unit, that metal noisemaker hidden away on the side or the back of the house. The good news is that most HVAC manufacturers make it easy to determine the nominal capacity of your air conditioner. ( Note: This article is about finding the size of your existing AC, not determining what size you need.) But to know if your AC is oversized, first you have to know what size it is.

Why? Briefly, they may not dehumidify as well, short-cycling wears them out quicker, and your home will probably be less comfortable if the air conditioner is too big. 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.

Carrier serial number lookup tonnage