Ever hear of a 2- or 3-ton central air conditioner? Does this mean a 2-ton air conditioner weighs 4,000 pounds? Or is it a random term that tech folks use to impress us?
No, It Doesn't Weigh 4,000 Pounds
A "ton" is a unit of refrigeration/cooling capacity. A ton of cooling is equivalent to transferring 12,000 Btu of energy per hour. So, this means a ton of air conditioning cools a home by removing 12,000 Btu from the air in one hour. Likewise, a 4-ton unit removes 48,000 Btu.
On the nameplate of an exterior condenser/compressor AC unit, the rating for tons of cooling is coded in the model number. It also might be listed on the tag.
A Quick Reminder: BTU
Btu stands for British thermal unit, the amount of energy needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit. A reasonable equivalent is the energy you get from completely burning one kitchen match (Illustration H099).
Where Did "Ton" Originate?
Way back when engineer Joe Cool (and what a great coincidence that "Cool" was his last name) developed the standards for measuring mechanical cooling, ice was commonly used for cooling. You know, in the old days before air conditioning, food was stored in the icebox. So, Joe decided that the cooling capacity measurement should relate to ice (Illustration A047C).
A standard was set, equating 1 "ton" of cooling to the amount of energy needed to melt 1 ton (2,000 lbs.) of ice over a 24-hour period. With the change of phase from ice to water, 144 Btu of latent energy is required: 2,000 lbs. x 144 Btu/lb. = 288,000 Btu. Therefore, 288,000 Btu over 24 hours = 12,000 Btu/hour.
What Transfers The "Ton" of Energy?
In a refrigeration cycle, two coils, a compressor and a control/metering device mechanically transfer heat (Illustration A032). The refrigerant is compressed and then cooled/condensed into a liquid. The liquid evaporates/boils in the A coil (Photo) with the energy from the warm air circulated over the coil. The warm air is cooled as it passes through the A coil and back into the home.
When the nametag designates an AC unit as 3-ton, you know that's equivalent to melting 3 tons of ice in 24 hours or 36,000 Btu per hour. Use this to impress your clients so you both understand the "tons" of cooling for their home.