In the air conditioning world, a ton doesn’t mean 2000 pounds of weight. Instead, it refers to the cooling capacity of an air conditioning system. While this measurement may seem strange today, it actually relates back to the history of cooling and refrigeration before modern air conditioning was invented.
Cooling Prior to Air Conditioning
Before air conditioning was developed, a common method of keeping a building or perishable food cold during the summer was by cutting ice out of frozen lakes or rivers during the winter and transporting it long distances to be stored in ice houses, until it was sold for cooling and refrigeration during the warmer months. The process was labor intensive and costly.
Ton as a Measurement of Cooling Capacity
When air conditioning was first invented, its performance was compared to the amount of heat it would take to melt a ton of ice, and the measurement eventually became common in the industry.
At 32 degrees, ice requires 143 British thermal units (BTUs) of heat per pound to melt completely into water. When cooling a large building, it often required tons of ice to maintain a comfortable temperature during the day. In order to melt a ton of ice over 24 hours, the ice would have to absorb 286,000 BTUs of heat, or about 12,000 BTUs per hour.
In modern air conditioning, a ton is equivalent to 12,000 BTUs of heat removed per hour. For instance, a 4-ton system removes 48,000 BTUs per hour from a building.
Determining the Cooling Requirements for Your Home
A cooling system that’s not sized properly may run inefficiently, require frequent maintenance and have a reduced life span. To properly determine your home’s cooling needs, have a qualified HVAC contractor perform a Manual J load calculation and recommend compatible systems.