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NC State Extension

HVAC (Heating and Cooling Systems)

HVAC (Heating and Cooling Systems)Right-size Heating and Cooling Equipment


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Proper System Sizing

In order to scientifically calculate the right central air conditioner size, you can perform a Residential Load Calculation.

Another way to size air conditioners is to determine the system size that is currently in place. The manufacturers do not list the air conditioner size on the unit. The system capacity will be coded into the model number of the outdoor unit.

For example, model CKL24-1 is a 2 ton unit. Why? There are 12,000 Btu’s per ton. The number 24 in the model number indicates the unit is 24,000 Btu’s, divided by 12,000 Btu’s per ton, equals 2 tons.

24,000 Btu’s/12,000 Btu’s per ton = 2 tons

Use the following conversion information to determine your existing system size (use the system model number NOT the serial number)

  • 18 = 1.5 tons
  • 24 = 2 tons
  • 30 = 2.5 tons
  • 36 = 3 tons
  • 42 = 3.5 tons
  • 48 = 4 tons
  • 60 = 5 tons

When sizing air conditioners, be careful not to make the mistake of getting one that is too large. If the system you install is too large for the space, it will short cycle. In other words, the compressor will not run long enough to dehumidify the space and will limit your comfort. In addition, it will cycle on and off more frequently, increasing operating costs and reducing the system life. Proper air conditioner sizing is critical to optimize performance.

If you’re still unsure which system size is right for you, or if you’re having difficulty downloading the Air Conditioner Size Calculator , please e-mail us or call us toll free number at 1-855-634-5588. Sizing air conditioners by phone is difficult but we can provide you with the tools to get you started.

Heating Square Footage Range by Climate Zone

30 – 35 Btu’s per square foot 35 – 40 Btu’s per square foot 40 – 45 Btu’s per square foot 45 – 50 Btu’s per square foot >50 – 60 Btu’s per square foot

Heating Guide

Use the lower of the two numbers if your home is well insulated and the higher number if it is older or poorly insulated. (Hint: Use the larger of the two numbers above if you’re unsure of your home’s insulation)

Simply multiply the appropriate factor above by your home’s total heated square footage to arrive at your approximate required heating capacity. For example, if you live in the yellow zone, your home is well insulated, and you have 2000 heated square feet, the equation will look like this:

  • 2000 square feet
  • X .40 heating factor (from the chart above)
  • 80,000 Btu actual output

Then, to calculate the output on a gas furnace, multiply its efficiency rating by its listed input rating for the actual Btu output of heat. For example, if a furnace has a listed input rating of 100,000 Btu’s and an efficiency rating of 80%, it will produce

  • 100,000 Btu input
  • X .80 efficiency
  • 80,000 Btu actual output

If the same 100,000 Btu furnace has an efficiency rating of 93% it will produce:

  • 100,000 Btu input
  • X .93 efficiency
  • 93,000 Btu actual output

For this example, using an 80% efficient furnace, the 2000 square foot home above would require a 100,000 Btu input furnace which will produce the necessary 80,000 Btu’s output of heat.

If you’re still unsure which system size is right for you, please e-mail us or call our toll free number at 1-855-634-5588. An experienced design technician will be happy to assist you.

Insulation Variables

Variables such as your insulation, type and number of windows, number of stories, construction type, etc. will greatly affect the required Btu’s per square for both heating and cooling. A general rule of thumb is that if your home is well insulated with newer style windows, you can select the smaller size system that falls within your total square footage.

If your home is two story it will place less of a load on the system in the downstairs area as the second floor acts as additional insulation. If your home is not well insulated, has older style windows, and/or a larger than average number of windows, you will want to select the larger system which falls within your square footage range. The less insulated and more windows within the environment, the more likely you will experience greater air and heat loss.

Page Last Updated: 6 years ago
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