The Simple Guide to BTU Values For Air Conditioners
Almost as soon as you start to research buying a new air conditioner you start to come across TLAs or “three letter acronyms”. This can all get a little confusing so in this article I’m going to explain the meaning of one of these common terms you’ll have definitely come across and how you can use it to help you make the choice of air conditioning unit that is perfect for you. This term is the BTU.
British Thermal Units – BTU
The BTU was established during the mid-1800’s but didn’t become widely recognised and used in publications until the later part of that century, when it was fully acknowledged and accepted by engineers.
The BTU is a measure of power. It can measure steam generation and heating, but for this article I’ll focus on its application and relevant to air conditioning units.
In terms of the unit itself 1 BTU is equivalent to the energy needed to heat a pound of water by one degree Fahrenheit. When we talk about BTU in terms of cooling it is the opposite i.e. the amount of heat energy used to reduce a pound of water by one degree Fahrenheit. To help put this into perspective 1 BTU is the amount of heat generated when you burn a standard match all the way down. Remember to let it go or you’ll burn your fingers! 🙂
This is not a great deal of energy so BTU values for air conditioners are normally quoted in thousands. It is worth mentioning at this point that the BTU value on these units actually means BTU per hour. This is the amount of energy the unit can extract from the room in an hour.
The common sizes for portable air conditioners are 8,000, 10,000, 12,000 and 14,000 BTUs.
What About Air Conditioning Units Which Are Sized In Tons?
The other method of sizing air conditioner requirements is in tons. This measure is normally used for quantifying the capacity of central air conditioning or larger commercial systems and is used for sizing much larger areas i.e. a complete home or office rather than individual rooms.
The origin of the ton as a measure is actually fairly interesting (to me anyway!). Before the invention of air conditioning buildings were cooled using ice which was harvested and sold by the ton.
A ton of cooling energy is the energy (heat fusion) required to completely melt a short ton of ice (2000lbs) in a 24 hour period. Ice takes 143 BTU/lb of energy to melt completely to water. Multiply this by 2000 to get 286,000 BTUs of total energy. Now divide this by 24 (number of hours in a day) and you get a BTU/h value of 11,917 which is approximately 12,000 BTU/hr, assuming the ice melts at a constant rate over the day. So one ton of cooling is around 12,000 BTU.
How Is The BTU Helpful To People Buying AC Units?
To help us consumers make sense of what that actually means manufacturers convert the BTU values into a value required to cool an area in square ft. This conversion is based on the average room having 2 windows, 1 door and an 8-foot ceiling. It is also assumed that the indoor temperature is cooled to 75°F with a humidity of 50%.
Below is a table of these conversions:
Up to 150 Sq/Ft
Up to 250 Sq/Ft
Up to 350 Sq/Ft
Up to 450 Sq/Ft
Up to 550 Sq/Ft
For larger areas and offices the values are often quoted in tons required to cool the area. There is also a handy chart that takes into account the climate in different places across the US.
The table above allows you to select where you live, understand how large your house is and then estimate the capacity of unit or system you’ll need using a rule of thumb calculation.
The BTU to area conversion is based on averages, therefore there are some adjustments that need to be considered when estimating the cooling power requirements for a particular room:
- The number of people normally in the room – people give off heat so increase the BTU requirement by 600 BTU per person after the second person who will be living or working in the room.
- Sunlight Exposure – for a room that is shaded reduce the requirement by 10%. If it gets a lot of sun, increase by 10%. (click here for a useful site that shows the sun’s movement over the course of the day at your location)
- Room Type – if you are cooling a kitchen add 4000 BTU.
Using an air conditioning system which is not suitable for the room size will mean that the rate warm air is entering the room through windows, walls, ceilings and air gaps etc. will be greater than the cool air being produced by the a/c system. This will result in the air temperature not being reduced and the room not being cooled to the desired level, no matter how long you leave the unit running for.
- The BTU is used by air conditioner manufacturers to rate the ability of the air conditioning unit to remove heat from the air.
- Rules of thumb allow tables to be drawn up which indicate the BTU value that is required for a particular area i.e. room or house size.
- Even if you use the best portable air conditioner if it doesn’t have a BTU rating that is suitable for the room size it will not cool the room to your desired level and you will simply be wasting energy (and money!).
- Adjustment factors can be used to either increase or decrease the standard BTU to Sq/Feet rules of thumb.
- If a room is well insulated well insulated it will require a lower BTU rating.
Hopefully this has helped you understand what a BTU is and how to use it when selecting your portable, window or central air conditioning units. I’ve also written an article on how to calculate air conditioner running costs and another explaining what SEER and EER mean. These might be useful for you to read.
Please leave a comment below if you found this useful or you have anything you can add to the article.