Why is AC Capacity Measured in Tons? And What is a BTU?

When sizing up a new air conditioner, heat pump, or HVAC unit, it is common to hear the pros throw around terms involving tonnage, saying for example, you need a 3-ton or 4-ton unit.  For reference, 4 tons is about the weight of an adult elephant!  But don’t be alarmed: they’re not talking about the weight of the unit!  This is a throw-back from the 1800’s and the term stuck.  You may also hear pros refer to the capacity of HVAC units in terms of BTUs.  Did you know these terms are related?  Here’s how they originated:

Historical Perspective

Before the modern air conditioner was invented, people used to cool buildings in the summertime using ice harvested from rivers and lakes. Gathering the ice from farther and farther north and shipping it down south, or storing it from winter until summer, meant a lot of ice was lost to melting.  A BTU, which stands for British Thermal Unit, is the amount of heat required to raise 1 pound of liquid water 1 degree.  But, when water is below freezing (less than 32 degrees Fahrenheit), the amount of heat needed to melt ice is 143 BTUs per pound.

How the Math Works Out

A ton weighs 2,000 pounds, so to melt a ton of ice it takes (143 BTU/lb) x (2,000 lbs) = 286,000 BTUs.  How quickly the ice will melt depends on how quickly you heat it.  If you apply the heat uniformly over a 24 hour period, the ice will have absorbed (286,00 BTU) / (24 hours) = 11,917 BTUs per hour.  That number is generally rounded up to 12,000 BTU/hr.  So, a one ton air conditioning unit has the capacity to cool up to 12,000 BTU/hr.

Sizing an AC Unit for Your Home

When comparing costs of different HVAC units, be sure you are comparing similarly-sized units.  Central air conditioning units frequently have the capacity coded into the model number: for example, model RDR36 would be a 36,000 BTU unit, or expressed in tonnage – with 12,000 BTUs being the heat needed to melt 1 ton of ice – the model RDR36 would be a 3-ton unit (= 36,000 / 12,000).

Multiple Factors Affect Sizing

If you have a 2100 sq.ft. home, a 2-ton heat pump will not be adequate; you’ll need maybe a 3.5 or 4 ton unit. Here’s where having a professional who can do a site visit helps. There are many things that contribute to sizing an HVAC system, other than the number of square feet you want to cool.  Here are just a few:

• How well the house is insulated and when it was built
• The numbers and sizes of the windows and doors in the house
• The color of the roof: darker ones absorb more heat
• The height of the ceilings
• The overall shape of the home: long narrow houses lose more heat (through walls) than a more square house of the same size.
• The size of AC’s condenser, and whether it is positioned in the shade or in the sun
• The efficiency of the cooling unit, often expressed as a SEER rating, or seasonal energy efficiency ratio (see our post What’s Your SEER?).

Moral of the Story

Knowing the number of BTUs that will be required to heat and cool your house effectively is not only important in choosing the right size furnace and air conditioning system, but it can also tell you whether your existing system may be too large or too small… yes, a system that is too large will not cool effectively, so bigger is not always better! (read our post Think Bigger is Better?)  Now that you know that tonnage does not refer to the weight of the unit, and how to convert from tons to BTUs, you can more readily compare units. Proving yet again that knowledge is power!

If you live in the Nashville and surrounding area, and need some help figuring out the right BTUs and tons for your home, we’ll be happy to help.  Plus, if you haven’t had your spring tune-up yet, now’s the best time!  Call on us at 615-832-8500.