What Is a Ton of Cooling? - Smart Inspector Science
Ever hear of a two- or three-ton central air conditioner? Does this mean a two-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 rated tons of cooling is coded in the model number. It also might be listed on the tag.
A quick reminder: the Btu
Btu stands for British thermal unit, the amount of energy needed to raise the temperature of one pound of water one degree. 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 one “ton” of cooling to the amount of energy needed to melt one 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; 2000 pounds times 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 1) 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 name-tag 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.
Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com (htoyh.com), he provides high-quality books and marketing materials that help professional home inspectors educate their customers. Copyright © by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.
To learn more, attend Tom’s technical presentations at educational sessions for ASHI chapters and local groups. Tom can also provide his knowledge for your educational event; contact him at Tom@htoyh.com. Tom is also a guest speaker at ASHI Inspection World.