Why is Air Conditioner Capacity Measured in Tons?

Aug 20, 2022Air Conditioning, Blog

You may have noticed before that HVAC systems are categorized in “tons”, such as 2.5 ton, 3 ton, or 4 ton units. That may be confusing, as usually a “ton” equals 2000 pounds. The weight of a residential HVAC unit is far less than a ton. So what does the term “ton” refer to in heating and cooling?

In the HVAC industry, a ton is a measurement of a unit’s cooling or heating capacity.

When you are installing a new AC system for your San Jose home, you’ll want to know the size of your home, and how many tons your new HVAC system needs to be to cool your house efficiently.

Origins of Tonnage as a Measurement of Cooling Capacity

The measurement of cooling capacity in tons dates to the early 1800s. In this time period, refrigeration and indoor cooling was done using blocks of ice, which were farmed from rivers in cold regions. The ice was held in iceboxes until they were ready to distribute and use.

When ice is below the freezing point, it absorbs heat from the nearby air. As ice reaches it’s melting point of 32 °F, it no longer raises in temperature, but begins to melt. The amount of consistently applied heat energy it takes to melt 2,000 pounds of ice over a period of 24 hours is referred to as a British Thermal Unit (BTU).

This measurement was codified in 1824 in Thomas Tredgold’s book, Warming and Ventilating of Public Buildings. An excerpt from this book reads:

“I take as the measure of the effect of a fuel, the quantity, in pounds avoirdupois, which will raise the temperature of a cubic foot of water one degree of Fahrenheit’s scale.”

What Does Tonnage Mean in HVAC Today?

One BTU is the amount of heat energy it takes to melt one ton of ice over a 24-hour span. By putting that formula in reverse, one BTU is also the amount of heat energy it takes to raise the temperature of one cubic feet of water by 1 °F.

One pound of ice at 32 °F takes 143 BTUs to melt it. Multiplying that by 2,000 (a physical ton is two thousand pounds) results in 286,000 BTUs to melt one ton of ice in 24 hours.

Why Does Air Conditioning Sizing Matter?

Selecting the right sized central AC system is important, because you want to fit it closely to the size and conditions in your home.

If you install an undersized cooling system, there will be inefficiencies in keeping your home cool when the temperature outside is blazing hot.

Counterintuitively perhaps, there are also inefficiencies when you purchase an oversized HVAC system. Air conditioning units extract the humidity from the indoor air, making the indoor climate seem cooler. Oversized units do not have enough time to remove the humidity during the cooling cycle, making the indoor air clammy. You are also paying more for the same cooling rate with a properly sized HVAC system.

How To Calculate The HVAC Unit Size You Need

Here’s a rough calculation that you can do to determine how many tons of cooling power you need to keep your home cool in the summer months.

Multiply the square feet of your home * 25. This is how may BTUs it takes to cool that area. Next divide that number by 12,000 to give you the total tonnage of the air conditioning unit you need for your home.

To sum up:

(Total sq ft * 25) / 12,000 = tons of HVAC unit

This is a rough calculation, and there may be more nuance to selecting the perfect unit. Other factors to take under consideration are: layout of your house, shade and foliage around your house, how many stories in your home, state of your home’s insulation.

Your HVAC contractor should follow the guidelines set forth in the ACCA’s (Air Conditioner Contractors of America) Manual J.

The HVAC technician should also have Manual S as a reference. This guide contains manufacturer performance table for various units, and can inform the HVAC contractor about peak capacities of different AC units when specific conditions are present.

Getting HVAC Installation in the South Bay Area

Our professionally trained HVAC technicians can make the proper measurements and design the perfect air conditioning system for your home. At Ventwerx HVAC Heating & Air Conditioning, we provide the expertise you need to make sure your home’s temperature is set correctly year-round. To get a no-obligation heating and cooling estimate for your home in San Jose, Morgan Hill or Gilroy call us at (408) 422-2987.