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Ever Wondered Why A/C Capacity is Measured in Tons?

September 5, 2013 | Blog

When you talk with your neighbor about your new air-conditioning system, you might describe it as a 3-ton or a 4-ton unit. But the expert A/C technician who installed your system knows that your unit’s size (and it doesn’t weigh 3 or 4 tons, by the way), involves much more than ton capacity. We’ll get to that in a bit.

So why is air conditioner capacity measured in tons?

Back in the late 1800s and early 1900s, ice was harvested from waterways and lakes. It was used to refrigerate perishables and keep buildings and homes cool in the summer (if you were rich enough). That ice was measured in tons.

Later on, after modern air conditioning was developed, British thermal units came into play. Ice cools the air by absorbing heat, which causes the ice to melt. The speed at which it melts is relative to the amount of heat applied, and it has been determined that 143 BTUs of heat are required to melt one pound of ice. When those BTUs are multiplied by 2,000 (a ton is 2,000 pounds), the math tells us that melting a ton of ice requires 286,000 BTUs of heat.

Somewhere along the line, a 24-hour period was arbitrarily selected as the time frame for the ice-melting exercise, The 286,000 BTUs divided by 24 hours equals 11,917 BTUs per hour. This has been conveniently rounded up to 12,000 BTUs for ease of calculation. So, if you have a 3-ton A/C in your south Texas home, that means it should be able to remove about 36,000 BTUs of heat from your living area every hour.

The importance of accurate sizing  

As you can see, this is a general calculation, and your home is unique. That’s why the Air Conditioning Contractors of America developed a set of sizing methods called Manuals J, D and S. These methods allow your contractor to factor in your home’s special features. Factors such as home orientation, insulation levels and the number of people in the family are considered, along with ductwork and the brand and model specifications. These considerations allow your contractor to come up with a system that’s exactly right for your home.

The size of a new air conditioning system should never be determined by square footage alone or any other rule of thumb. At Mathews CCAC Air Conditioning in Corpus Christi, we pride ourselves on our expertise. We keep abreast of all technological developments in the HVAC field and use industry-accepted practices. Contact us for help with any of your home-comfort needs.

Our goal is to help educate our customers in Corpus Christi, Texas about energy and home comfort issues (specific to HVAC systems).  For more information about measuring A/C capacity and other HVAC topics, download our free Home Comfort Resource guide.

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