You’ve probably heard the word “ton” used in connection with the size of a central air conditioning unit, and may have even committed the common mistake of assuming that ton, in this context, actually means 2,000 pounds. Not so. Actually, in A/C parlance, ton refers to cooling capacity and dates back to the days when folks cooled buildings with ice harvested from lakes and rivers.
How Is an A/C Ton Calculated?
A ton, in this context, refers to how much heat energy an A/C can remove from a building in an hour. A 4-ton A/C – a common residential size – can remove 48,000 British Thermal Units (BTUs). To add more oddity to these terms, a BTU is the amount of heat emitted when a kitchen match burns all the way down.
So what does ice have to do with it? If you have a ton of ice, it takes 143 BTUs per pound to totally melt it. Multiply 143 times a ton (2,000 pounds), and you need 286,000 BTUs to melt the whole ton of ice. Factored out to a full day, the ice will absorb 11,917 BTUs per hour of heat before the whole ton has melted. That number is rounded up to 12,000 BTUs per hour to melt the ton of ice, which equates to a “ton” of A/C cooling capacity.
Sizing an Air Conditioner
An essential step in A/C installation is sizing the system, and while rule-of-thumb methods have been used in the past (such as a ton of cooling capacity for every 600 square feet of space), this is no longer considered accurate or professional.
Modern A/C installation technicians perform a Manual J cooling load calculation, taking into account the size and layout of a home, and a variety of other factors such as air tightness and insulation, windows, lot orientation and even landscaping. Once the cooling load has been established, the technician can proceed to recommend the right size A/C for your home.
To discuss selecting the right size cooling system for your Twin Cities area home, contact us at Marsh Heating & Air Conditioning.
Our goal is to help educate our customers in Minneapolis-St. Paul, Minnesota about energy and home comfort issues (specific to HVAC systems). For more information about cooling capacities and other HVAC topics, download our free Home Comfort Resource guide.
Credit/Copyright Attribution: “Peshkova/Shutterstock”