People have a strong need to simplify things, especially when it comes to long technical terms. Simplification often involves the use of acronyms or abbreviations. Such is the case with air conditioning. Those in the business throw out terms like BTU's and EER like everyone knows what they mean. But that's not the case. The purpose of this article is to help the reader understand what a BTU is, and what it means to them when choosing a portable air conditioner.
BTU is short for British Thermal Unit, defined as the amount of heat necessary to raise the temperature of one pound of water by one degree Fahrenheit at one atmosphere of pressure. So it's a measurement of energy, specifically heat. So what does that have to do with air conditioning? Well, it's really a misnomer, since what it refers to is the power of an ac unit, which requires a time reference. So with air condition we really mean BTU's per hour, but everyone just shortens it to "BTU," and it refers to the ability of an air conditioner to change the temperature of air.
The higher the BTU rating, the greater the ability to change air temperature. So a 12,000 BTU unit will be able to cool more air than a 10,000 BTU unit in the same amount of time. Now we're getting somewhere. How much more? The ability to cool air is fairly linear with changing BTU (other factors come into play), but that's not the only consideration. Manufacturers of portable air conditioners often have a chart showing what size room a given btu rating should be able to cool adequately. But remember, they didn't test this in your home! Things like sun exposure, obstructions to air circulation, how cool you want the room, outside temperature, insulation, duct length, and whether or not you are supplementing central air, all come into play. Here's the next potentially confusing thing. Rooms are measured in square feet, which is really the size of the floor only. But rooms are three dimensional, they have height too. Air conditioners don't cool the floor, they cool the air, which occupies the entire room. The volume of a room depends also on the ceiling height.
So what is this all about? OK, finally I'll get to "what it means to you." Let's say you have ten foot ceilings in your home, and you have a 289 square foot (17 x 17) room that is too warm. By the charts, a 10000 BTU unit would be recommended for 300 square feet. But that's for standard eight foot ceilings. With ten foot ceilings, the volume of air is 20% higher, meaning the volume is equivalent to a 346 square foot room with eight foot ceilings. So a unit designed for 350 feet will probably be a better choice to cool your room efficiently, i.e. 11,000 BTU instead of 10,000.
Now that is a useful tidbit! The "other factors" mentioned above should also be considered when determining how many btu's to get. Basically, it's not a bad idea to upsize the unit a little over the size charts if you can afford it. I've seen many arguments both ways on this, but in my opinion, upsizing should allow the unit to cool more quickly, allowing it to run less. It also gives the option of using it in a larger room if needed
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