Wall Mounted Air Conditioners: BTUs Needed per Square Foot

Mike Cappuccio, founder of N.E.T.R., Inc., talks about wall mounted air conditioners and some general rules for how many BTUs you need per square foot, and in what situations you may need more or less.

Hi, my name is Mike Cappuccio and I’m the founder of N.E.T.R. in North Andover, Massachusetts. Today I’m here to talk to you about the BTU size of a wall-mounted air conditioner in correlation to the space that it would be going into. Now, a lot of people ask me, how big of a wall-mounted air conditioner do I need for my home? It’s a very loaded question because a lot of it depends on the construction of your home and when it was built and a lot of other factors that are involved in this. But for a rule of thumb for a home that was probably built in maybe the early forties to the early nineties, usually what we see when we run load calculations on these types of homes is it’s about 25-30 BTUs per square foot.

So, what does that mean? Okay. Let’s take a 10×20 room, that equates to 200 square feet. All right, so if we have a 200 square foot room, how many BTUs do we need to cool that space? We need roughly around 5,000 BTUs of a wall-mounted air conditioner to do that space. So as the spaces get bigger, the more BTUs you need to extract that heat from the space, because we’re really not putting cold air into the space. We’re removing the heat from the space and putting it outside at that time. So if we move up now into a 20×20 room, let’s say, okay, 20 times 20 is 400. So 400 times roughly 25 BTUs per square foot, now we need a 10,000 BTU wall-mounted air conditioner.

In terms of BTUs in the HVAC industry, we equate to everything in what’s called a ton. One-ton air conditioners, two-ton air conditioners and three-ton air conditioners. What does that really mean when someone comes into your home and says, “well, you need a two-ton wall-mounted air conditioner”? Well that’s 24,000 BTUs. A ton is 12,000 BTUs. One ton of cooling, again, 12,000 BTUs.

The way most wall-mounted air conditioners are made is they come in a half-ton, one-ton, one-and-a-half, two, two-and-a-half tons and three tons. That’s usually what you see. So again, what does that equate to? Anywhere from 6,000 BTUs to 36,000 BTUs.

Now, as we now get into newer homes, when we start running these load calculations on homes and we start implementing some things into the software and putting them in, we see that the newer homes require less BTUs per square foot. We see those sometimes; they’re around anywhere from 15 BTUs to 20 BTUs per square foot. Well, why is that? Because the construction of the homes are now different. The windows are low E glass. Building codes now are two pane windows, sometimes three pane windows. Everything now is 2×6 construction. The insulation has gotten a lot more higher values into the walls and things like that. So what we’re seeing is the BTUs are obviously lower when you start to use different wall-mounted air conditioners in spots like that, you need smaller equipment.

What you don’t want to do is put too big…I’ve always talked to that in previous videos about “bigger is not better” because when you put too big of a wall-mounted air conditioner in a space, you basically can’t remove the humidity now, because what it does is it just runs and it shuts off real quick. We need the unit to run so it does remove the moisture from the air and it keeps the space nice and comfortable. You don’t want a 75 degree room with 80% humidity. It’ll be very damp and very uncomfortable.

So when you’re looking at these different sizes, there’s also other things that come into play: windows, skylights, things like that you have to be aware of. Again, even, “what way is your house facing?” Is it facing east? Is it facing west? Is it facing north, south? So a lot of those things come into play. So you have to know too, where does the sun come late afternoon? That’s usually when it’s the strongest. Remember that from noontime to 3:00, 4:00 is when the sun is extremely strong. Look, if you have a big 10 foot by 6 foot window in the front of your home, and it’s getting sun at 2:00 in the afternoon, just beating in there, well, that is going to increase the amount of BTUs you’re going to need for that space to extract that heat. So when we’re talking rule of thumb, we’re talking basic windows, three-foot-wide windows by five-foot high, maybe two or three of those in a bedroom, nothing crazy going on. You’ve also got to remember too — attic spaces. I want you to think about when you walk up into your attic in the summertime, it’s 140 degrees. When people finish their attic and they try to make that living space, that is going to require more BTUs to remove that heat from that space. So you’re going to need a bigger unit to do spaces like that because those aren’t just vertical, up-and-down walls. Those are more horizontal with angles and the sun is beating onto the roof at that time. That’s going to be a very, very hot room. So those you’re probably going to see are going to be around 40-50 BTUs per square foot. So just some rule-of-thumb measures to look at when you’re looking at installing these wall-mounted air conditioners in your home.