Sometimes those who deal with British Thermal Units on a daily basis do not stop to explain what they mean by the term BTU to everyone else. That is when you might need help.
The BTU or British Thermal Unit is the amount of energy required to increase the temperature of a pound of water by one degree when measured on the Fahrenheit scale at an atmospheric pressure of 760 mm of mercury. The term is used to measure energy, in specific, heat.
What does the BTU have to do with air conditioning?
It is actually used incorrectly in the field. It is used in reference to the power of the cooling unit, a measure requiring a factor of time as well.
When used in the AC field, the term is actually a measure of BTU per hour, but often shortened to simply BTU. It refers to the unit’s ability to lower the air temperature. Thus, 10,000 BTU units are able to cool more air than 8,000 BTU units.
The next question is how much more can it cool. Here you encounter another potential point of confusion. Rooms are measured in square feet, by the amount of floor space.
Rooms, as you know, also have height. The cooling units are not just responsible for the floor, but the air that fills the whole room.
Thus, while unit’s ability to cool air is fairly linear in changes with BTU, the room’s volume also depends on ceiling height. A room that is 20x20x8 contains 3200 cubic feet of air. Increasing the ceiling height to 10 feet increases the volume to 4,000 cubic feet.
This is important to the consumer buying a portable air conditioner. The charts manufacturers use in recommending cooling equipment assume an 8 foot ceiling. However in homes with higher ceilings (or cathedral ceilings) the recommendation based on floor space can be off. Increasing the ceilings from 8 feet to 10 feet increases the volume by 20%.
The smaller portable ac units that the chart would recommend is simply too small for the room. Thus, unless you choose a larger unit to accommodate the additional air in the room, you are likely to be too warm.
To determine the actual cooling of the unit, multiply the manufacturer’s floor space by the actual height of the and divide by eight. This will give you the floor space the volume of air in a room would occupy in a room with eight foot ceilings.
There are also other factors one should consider when determining the size unit to buy. Sun exposure, the desired final temperature, average outside temperatures and whether supplementing a central unit all figure into the equation.
It is better to buy a unit that is a little too large than one that is too small. While a 10000 BTU air conditioner room size unit may be recommended by the chart, up-sizing to a 12000 BTU unit allows for quicker cooling and the unit to run for shorter lengths of time. If you move, you also have the option of cooling a larger room if necessary.