A correctly sized heat pump or central air conditioning system can save money and make building interiors more comfortable for occupants. A system that is either too small or too large may not provide adequate cooling and may have a shorter life. It can also result in increased energy costs.
For Rough Estimation Only
Calculating the proper tonnage accurately requires you to consider a variety of factors, including climate, number and size of windows, ceiling height, number of occupants, foundation type and amount of insulation. Professionals use a complex formula to determine the correct size of a cooling system, but there is a simpler method that can be used to get a "ballpark" estimate. Before purchasing a system, however, it is best to have a professional compute the tonnage by using the "Manual J" method, which is the formula preferred by the Air Conditioning Contractors of America.
Heating and cooling professionals typically use the following terms:
- British Thermal Unit: Commonly abbreviated as Btu or BTU. One Btu represents how much heat is needed to increase the temperature of one pound of water at sea level from 58.5 degrees Fahrenheit to 59.5 degrees Fahrenheit. It is normally expressed by contractors in terms of Btu per hour.
- Tonnage: In reference to air conditioning, a ton measures how much heat removed by the system would be needed to melt 2,000 pounds, or one ton, of ice in a 24-hour period. The result is then expressed in Btu per hour. According to the standard calculation, it takes 288,000 Btu to melt a ton of ice in 24 hours, or 12,000 Btu per hour. Therefore, 12,000 Btu per hour equals one ton of cooling,
Although window units are typically rated in Btu, heat pumps and central air conditioning systems are almost always rated in tonnage.
How to Estimate Tonnage Required
To calculate the estimated tonnage for a house with 2,000 square feet, 15 windows and 3 exterior doors, occupied by 5 people:
2,000 x 25 = 50,000 base Btu
5 people x 400 = 2,000
15 windows x 1,000 = 15,000
3 exterior doors x 1,000 = 3,000
50,000 + 2,000 + 15,000 + 3,000 = 70,000 Btu
70,000/12,000 = 5.83 estimated tons required
This calculation does not include features that might reduce the required tonnage, such as insulation or energy-efficient windows. Therefore, a professional calculation might show a lower requirement.
Determine the square footage of the area to be cooled. The sales contract or home's blueprint may show this information. Otherwise, measure the length and width of each room, multiply these measurements to get the square footage, and add the square footage of each room together. Alternatively, you can take the exterior measurements of the entire house and subtract the square footage of any area that will not be cooled, such as an attached garage.
Many contractors use a rough estimate of one ton of air conditioning for every 400 square feet, according to the U.S. Department of Energy. Although this calculation is often inaccurate, using it for comparison purposes might help you catch math errors. To illustrate, using this method, a house with 2,000 square feet would need a 5-ton unit. If other calculations indicate the need for a 0.5-ton or 50-ton unit, it is a sign that there is an error in the calculation.
Multiply the square footage by 25 to obtain the base Btu value. Add 400 Btu for every occupant and 1,000 Btu for every window or exterior door. The result is the total Btu value.
Determine the estimated tonnage by dividing the total Btu by 12,000.
Bill Goodwin has more than three decades of experience in commercial, residential and industrial building maintenance and construction. He has been certified in refrigerant recovery, recycling and transition. Goodwin has been the author of numerous home improvement articles from 2008 to the present.