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

audio, podcast, AC tons, Tons to BTU conversionWhen 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

ice harvestBefore 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

cooling, tons to BTUs, AC TonA 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

sizing HVACIf 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

knowledge is PowerKnowing 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. 


Important Tips & Advice



You often don't think about your heating or air-conditioning system until something goes wrong. But there is so much you can do to help keep your system from breaking down. We regularly post information to help you learn what you can do to keep your system running, to save money and energy.  Search our many posts, videos, and podcasts, for valuable information that is never a sales pitch. Subscribe to receive an email when something new is posted, so you won’t miss any important tips.

Choosing a Repair Company

Whether you’re talking about heating or cooling, repairing or replacing a system, you want a company that can advise you reliably based on what is in your best interest, not based on what is convenient or what earns them the most money (commissions). We'll help you choose the best solution to fit your needs. Check out our many posts at HVAC News You Can Use and our podcasts.

Going Green

We're concerned about the environment and know all the latest technologies - such as geothermal systems, LED lights, and smart thermostats - that can reduce energy and put more "green" in your pocket!  We've provided HVAC systems in certified LEED buildings and written many posts on ways to Go Green.

The Sign of Service

We are a repair company performing a full range of commercial and residential heating, air conditioning, energy management and comfort products service, maintenance, and installation. We are a locally-owned company serving Nashville-Davidson County and the surrounding counties including Williamson, Rutherford, Wilson, Sumner, Robertson, and Cheatham.

Who We Are

Our service technicians are fully trained and experienced.  We can tackle the most complicated system to the simplest of systems – from large commercial plants to single-family homes.  Our ownership group of Eddie Hutton, Tony Anderson, Roger Eldridge,  Swaney Powers and Alan Seilbeck have over 125 years combined experience as leaders in the HVAC industry.


We are proud of our dispatchers that respond to your telephone call. When your system is down they make things happen. Our trucks are loaded with most parts needed for a quick repair. You can call day and night 7 days a week. Our normal weekday hours are 7:00AM to 5:00PM. Click on the button below to schedule a service call online. Emails and forms can only be answered weekdays from 7-5.”  

615-832-8500 Online Service Call


Did you know you should service your HVAC on a regular schedule – just like you would your car?  We have specialized maintenance programs to ensure the clean, energy efficient and safe operation of your HVAC systems.  This helps minimize service interruptions/breakdowns and maximize your equipment's lifespan.  All maintenance contract customers are assigned a primary technician that will get to know your facility and equipment and will treat it like his own.  You’ll receive discounts on repairs or new installations, and get priority service when it is cold or hot outside and you need service promptly.  Plus, maintenance contract customers are never charged overtime rates for emergency services after-hours, weekends or holidays. We’ll teach you how to save on maintenance costs by showing you things you can do yourself and when you need a technician. Here are some good posts:  Repair vs. Replace, Things to Try Before Calling for Service, Springtime Tips that Pay Off.


Our team takes on HVAC projects for office, school, retail and industrial facilities. Check out these case studies:

Replacement of a 250-ton Cooling Tower

Ductless HVAC at Brentwood Middle School