Size matters when you’re buying a central air conditioner. Buy too small and it will struggle to keep the room at a comfortable temperature; buy too big and the room will cool too quickly without removing enough humidity from the air. Buy just right and you’ll be comfy and save money too. Modern central air conditioners run in cycles; slowly but steadily lowering the temperature of your house instead of rapidly. They are energy efficient and take advantage of insulation.
If your central air conditioner is sized too small for your house, it doesn’t have the power to cool your house rapidly, especially if the outside temperature is very high, and in a satisfactory way. It will run constantly and spend most of the day trying to catch up to where it should be. Modern central air conditioners, unlike your air conditioner from the 80’s, are made to work slowly over time. If it’s too small, it will run continuously and run up your electric bill.
If the central air conditioner is sized too big , then it will run more like that AC unit from the 80’s – it will turn on quickly, cool your house rapidly and then shut off. It may not dehumidify as well, and your home will probably be less comfortable. Short-cycling wears them out quicker. Each time the compressor stops working, the power consumption is about 6 times higher than during operation. This on / off cutting cycle decreases efficiency and often increases your electric bill. Also it causes your compressor a shorter lifespan. It is not profitable to have an AC unit above required.
Before figuring out what sized central air conditioner is right for your house, you need to know that the size an AC is measured in “tons.” The tonnage of an AC unit is not actually based on its weight. A “ton” is a measure of the air conditioners’ ability to cool.
The label of the AC has a model number (M/N) and serial number (S/N). The model number is where you can find a 2 or 3 digit section or the nominal size. That tells you how many thousands of BTU/hour your air conditioner can move out of your home. Since each 12,000 BTU/hr is equivalent to 1 ton of air conditioner capacity, it’s easy to figure out how many tons of nominal capacity each AC has. f the nominal size is 048, that means the air conditioner—or heat pump in cooling mode in this case—has a nominal capacity of 48,000 BTU/hour.
A BTU (British Thermal Unit) is the amount of energy required to heat or cool one pound of water by one degree Fahrenheit. In other words, BTU is the measure of the cooling capacity of an air conditioner, its ability to remove heat from a room in one hour. Therefore, a “2-ton” central air conditioner is able to cool 24,000 pounds of water by one degree every hour. The higher the number of BTUs, more heat can be removed.
The central air conditioners installed in houses range from 1-ton to 5-ton units and increase in half-ton increments (2-ton, 2.5-ton, 3-ton, etc…). Anything over 5-tons is generally considered a commercial HVAC unit and if your house requires an AC unit of this size, then it is recommended to install multiple units in tandem.
What this means is that if your home needs 6-tons of cooling power for instance, then you’d need two, 3-ton units installed (instead of a 6-ton unit) and probably a zoning system as well.
You should know how to calculate the BTU air conditioner, because if done improperly, you may end up buying an air conditioner with insufficient BTU or above what you require. As a rule of thumb, an air conditioner needs 20 BTU for each square foot of living space. There are two different ways to get a ballpark size estimate for your house.
First, the old-school equation:
((House square foots multiplied by 25, divided by 12,000) – 0.5) = required tons.
So for instance, a 1,500 square foot home would look something like this:
1,500 X 25 = 37,500
37,500 / 12,000 = 3.1
3.1 – 0.5 = 2.6 So you’d need a 2.5 or a 3-ton sized central air conditioning unit.
Calculate the volume of the room: length x width x height = response / 2 (the response divided by 2)
For example: 2.5m x 3m x 2.5m = 18.75 / 2 = 9,375
With the response 9,375 (which is between 9,000 and 12,000 BTU) match the BTU air conditioning to the nearest from the available ones. Thus, you should get a 9,000 BTU air conditioning if you do not like air too cold, and a 12,000 BTU, if you do not like the heat at all.
But don’t buy by BTU alone. There are other considerations in your home that can affect the efficiency and size of your central air conditioner, such as the height of your ceilings, the size of your windows and doorways, and the type and thickness of your insulation. To measure your room, multiply the length of the room by the width.
- Reduce capacity by 10 percent if the room is heavily shaded.
- If the room is very sunny, increase capacity by 10 percent.
- If more than two people regularly occupy the room, add 600 BTUs for each additional person.
- If the unit is used in a kitchen, increase capacity by 4,000 BTUs.
A properly sized central air conditioner will run through the required amount of cycles to keep your house comfortable, but will not run so much that it loses its efficiency. Now that you know what sizes central air conditioners comes in, you can figure out how to calculate the right sized central AC unit for your house.