Choosing the correct size air conditioning unit is a frustrating and confusing proposition for many homeowners. Getting an air conditioning unit that is too big or too small can reduce your comfort, increase operating costs, decrease efficiency, and even lessen the lifespan of your unit. Even when you get price quotes from multiple companies, you will likely get recommendations and rationales for various size units. The following guidelines will give you important insight into the process of sizing air conditioning units.
Load Calculation
Your HVAC company will use a formula called the Manual J Residential Load Calculation to determine the optimal size for your air conditioning unit. This involves a rather complicated calculation using a number of variables, including:
- Your home’s construction materials.
- The number of windows.
- The size of the rooms.
- Your home’s insulation levels.
Fortunately, you can get a close approximation using a much simpler formula. You can get an estimate of the required tonnage of your air conditioner by dividing the square footage of your home by 600. Do not include the square footage of your basement or garage in the calculation. The following is an example:
Square Footage of Home/Size of AC Unit
1,500 sq. ft./2.5 tons (1,500/600)
1,800 sq. ft./3.0 tons (1,800/600)
2,100 sq. ft./3.5 tons (2,100/600)
2,400 sq. ft./4.0 tons (2,400/600)
Greater than 2,400 sq. ft./5.0 tons
It is important to keep in mind that new, energy efficient homes may require a smaller unit depending on the home’s particular style, construction, and orientation.
Your heating and air company will do the long-form load calculation to fine tune the size of your air conditioning unit; however, your rough estimate should be within about one-half ton of the actual size. You should be wary if your heating and air company recommends a unit that is more than one-half ton smaller than your calculation. If this happens, you should advise the contractor to make sure there is sufficient air return for the extra cooling capacity. You should also ask for a written money-back guarantee in case you are not satisfied.
Another way to determine the appropriate size air conditioner is to look at your existing unit. The unit’s capacity is coded into the model number of the unit. For example, if your air conditioner’s model number is CKL18-1, it is a 1.5-ton unit. The number 18 indicates that the unit is 18,000 BTUs. There are 12,000 BTUs in one ton; therefore, your unit is 1.5 tons. The following is a handy conversion guide:
Number of BTUs/Number of Tons
18,000/1.5
24,000/2.0
30,000/2.5
36,000/3.0
42,000/3.5
48,000/4.0
60,000/5.0
It is important to ensure that you are looking at your air conditioner’s model number and not the serial number.
Many homeowners make the mistake of getting an air conditioner that is too big for their home thinking that it will cool quicker or be more efficient. In fact, getting a unit that is too large will cause the system to short cycle. This means that the compressor will not run long enough to dehumidify your home. The unit will cycle on and off more often, which will reduce the life of your system and increase operating costs.
Types of Central Air Conditioning Units
When selecting a central air unit, you have the choice of installing the entire unit outside of the home or installing the compressor and condenser outdoors and the blower or evaporator indoors on the furnace. The split configuration is the most economical. This allows the furnace blower to deliver the cooled air throughout the house through your existing air duct system.
Choosing a Window Air Conditioner
When selecting a central air unit, you have the choice of installing the entire unit outside of the home or installing the compressor and condenser outdoors and the blower or evaporator indoors on the furnace. The split configuration is the most economical. This allows the furnace blower to deliver the cooled air throughout the house through your existing air duct system.
If you opt for a window air conditioner over a central heat and air unit, the first step in selecting the right size unit is to determine the square footage of the area that you want to cool. To do this, you multiply the width of the room by the length. For example, a 10-foot by 10-foot room would be 100 square feet. You can then use the following guide to determine the appropriate BTU rating for your unit:
Square Footage/BTU Rating
150 to 350 sq. ft./5,000 to 8,000 BTUs
351 to 550 sq. ft./8,000 to 12,000 BTUs
551 to 1,050 sq. ft./12,000 to 18,500 BTUs
1,051 to 1,600 sq. ft./18,500 to 25,000 BTUs
Certain conditions may decrease or increase your particular cooling needs. For example, a heavily shaded room may require less cooling. You may need a larger unit if it is to be located in the kitchen, if the room gets a lot of direct sun, or if the room is normally occupied by multiple people.
Contact us today if you are in need of a reliable heating and air technician in the Denver area. Our professionals look forward to helping you with all of your central air conditioning repairs, no matter the size!
