Air conditioner capacity is one of the most important considerations for ensuring that your cooling system is both effective and energy efficient. If you’ve been looking for a new air conditioner, you’ve probably noticed the ton is used as a measurement of capacity.

Although a ton is a measurement of weight, in the case of cooling systems, it has nothing to do with the weight of the air conditioning unit. Instead, the use of tons is a convention that comes from early cooling systems that used ice.

Ice-based cooling systems appeared in the 1700s and increased in popularity in the late 1800s. Later versions involved fans and ductwork similar to today’s central air conditioning systems. Due to their size and complexity, though, they were found primarily in public areas such as theaters and department stores. In 1880, the Sanitary Engineer described the cooling system for New York’s Madison Square Theater, which used around 4 tons of ice each evening to keep performance attendees comfortable.

The first electric cooling system began as a project that had nothing to do with keeping shoppers and theater patrons cool. In 1902, engineer Willis Haviland Carrier was looking for a way to improve the printing process for a publishing company in Brooklyn. Fluctuations in heat and humidity were causing the printer’s paper to change slightly during the printing process, which lead to printing errors.

Carrier devised a system to maintain consistent temperatures and humidity levels while also filtering the air. This new electromechanical cooling system, which Carrier called “air conditioning,” created a stable environment that allowed for error-free printing. As word spread about the system, other companies began requesting their own systems and Carrier established the Carrier Air Conditioning Company of America to meet the demand.

Both ice-based mechanical cooling systems and the later electromechanical and refrigerant-based cooling systems have one important thing in common: They remove heat from a space rather than producing cool air.

Naturally, blocks of ice melt as they absorb heat from a room. The heat energy the ice absorbs can be measured in Btu, or British thermal units. This international standard unit of measurement equates to the amount of energy needed to raise the temperature of one pound of water by 1 degree.

It takes 286,000 Btu to completely melt one ton of ice. When the new cooling systems arrived, engineers compared their cooling, or “heat absorption,” power to the old ice-based systems. So a ton of cooling capacity refers to how much heat energy it takes to melt a ton of ice.

Theoretically, this ton of ice could melt over any given time period, depending on how much heat energy the ice absorbs. At some point, though, 24 hours was chosen as the standard time reference. Melting a ton of ice in 24 hours requires 11,917 Btu/hr, or Btu per hour.

That number was rounded up to 12,000 Btu/hr, which is known today as a ton of air conditioner capacity. Modern air conditioners are sized in 1/2-ton increments with residential air conditioners typically at between 1 and 5 tons. For instance, a 2-ton air conditioner can remove 24,000 Btu of heat per hour.

Manuals J and S: the essential guides for accurate sizing

To begin determining your required air conditioner capacity, your HVAC contractor will use Manual J. This manual, put out by the Air Conditioning Contractors of America (ACCA), provides a detailed process for calculating your home’s cooling load. Your cooling load is the amount of heat your system will need to remove to maintain your desired temperature. Building codes require this procedure, so any contractor who meets minimum professional standards will follow it.

While Manual J is important, it’s usually not enough to ensure correct sizing. That’s because A/C systems vary in capacity depending on the actual operating conditions. Manual J assumes certain operating conditions, but conditions in your home are likely to be different. That’s where Manual S comes in. This manual provides manufacturers’ expanded performance tables that account for a variety of operating conditions. Contractors willing to go the extra mile to make sure you get a reliable cooling system will use Manual S when they select your equipment.

To work with contractors who’ll take the time to calculate your ideal air conditioner capacity and then install your system correctly, contact us at Griffith Energy Services. We keep homeowners comfortable in Maryland (Baltimore, Frederick, Hagerstown, Manassas, Westminster and Easton) as well as in Dover, Delaware and Martinsburg, West Virginia.

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