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Find the Right Size

Air conditioning size is one of those times when size really matters! If you choose a unit that's too small it will have to work continuously to try to cool the space while leaving you sweating. So then, you would think that bigger is better, right? Unfortunately, that’s not the case. If you’re thinking that unlike the too small unit, the big one will only have to run a little to cool my house, your right, it will run less and your house will be cooler but it will also be clammy and being cold and clammy is not an improvement! If the unit is too large for the space it will turn on in short bursts they cycle off. This is known as short cycling. The problem with this is that the condenser spends so much time off that the coils don’t get cold enough for long enough to condense the humidity from the air. An undersized unit will do the opposite. It runs almost continuously and that will allow the system to dehumidify the space but it won't be able to adequately cool, leaving the space dry and warm. Either way, too big or too small, you’re looking at being uncomfortable and shortening the life of your equipment.

You can see why it's so important to select properly sized equipment. The best way to do this is, of course, to have a licensed professional visit the location to properly calculate for size and load. Anyone can calculate size but load is another matter! Let's start with defining load. Load is the amount of cooling / heating BTUs a system needs to product to make the space comfortable. Some of the factors that influence load are the climate you're in, construction, layout and orientation of the building, size and number of windows and doors, quality of insulation and even number of people who will occupy the rooms. You'll hear the term “manual J” tossed around and here's what that is, it's a trademarked publication by the ACCA (Air Conditioning Contractors of America) that covers the method for calculating load. While these calculations are generally done by HVAC professionals, you may want to gather your own information and a good place to do that is online at free sites that calculate load using Manual J such as loadcalc.net. Be prepared to do lots of measuring before you get your answers.


As HVAC professionals, we believe in getting a professional to do the calculations but as consumers we understand how important it is to do your own research. For that purpose, we are including some handy calculations and BTU chart. The method to calculate size need based on square footage is as follows (remember this is a rough estimate):


     *  First list each room or space being cooled. 
     *  Then calculate the square footage of each room.
         Do this by measuring the length and the width of each space and multiply the two numbers.

     *  Next find the BTUs needed for each room. There are numerous mathematical equations out there for doing this
         but it's a lot easier to just use the chart below which is provided by energystar.gov. 

     *  Write down the BTUs for each room, space or hallway. 
     *  Now you have to factor in some of those load aspects 
             *  If the room in very shady subtract 10% 
             *  If the room is very sunny add 10% 
             *  If it's a kitchen add 4000
             *  Calculations are based on 2 people, so add 600 for each person over 2 
             *  Last step is to add up all your BTUs.


Now that you have figured that out you will probably want to convert this to tons. One ton is equal to 12,000 BTUs so to convert your total BTUs just divide it by 12,000. So if you have total of 57000 BTUs, it comes out to 4.75, so you will need a 5 ton unit.


Room to be cooled (Square Feet)BTUs needed
100 up to 150 5,000
150 up to 250 6,000
250 ups to 300 7,000
300 up to 350 8,000
350 up to 400 9,000
400 up to 450 10,000
450 up to 550 12,000
550 up to 700 14,000
700 up to 1000 18,000
1000 up to 1200 21,000
1200 up to 1400 23,000
1400 up to 1500 24,000
1500 up to 2000 30,000
2000 up to 2500 34,000