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AC Capacity: Understanding How It's Measured

You don't need to be a heating and air conditioning expert to decide on the best air conditioning equipment to replace or upgrade your existing unit. Understanding how AC capacity is measured, however, and the factors that influence proper equipment sizing will help you make the best choices among the options that your contractor gives you.

The ability of your air conditioner to remove heat from the air inside your home is measured in tons. This can be confusing unless you're aware of the origin of the units of measurement used in specifying AC capacity. The "tons" referred to in AC capacity measurements do not refer to the weight of the equipment. If your home requires a three-ton air conditioner, it does not mean that your HVAC contractor has to figure out how to move and install a unit that weighs 6,000 pounds. Rather than a measure of weight, the AC capacity "tons" are a measure of the number of BTUs of heat per hour that your AC unit can remove from the air in your home.

Measuring AC Capacity

A BTU, or British thermal unit, is a measure of energy. One BTU is the amount of heat required to elevate the temperature of one pound of water by 1 degree Fahrenheit. To give you some idea of how large a BTU is, you can release about one BTU of heat by burning a couple of toothpicks.

Before mechanical refrigeration and air conditioning were invented, people used ice for refrigeration and for cooling buildings. Ice was cut from frozen rivers and lakes in the winter and stored for use in hot weather. The ice was measured and sold in tons.

When mechanical refrigeration and air conditioning systems were being commercialized, it was useful to express their cooling ability in terms that people could easily relate to. The cooling capacity was specified in terms of how many tons of ice would have to be melted in a day to produce the same cooling power as the mechanical system.

In order for a ton of ice to melt in one day, approximately 12,000 BTUs of heat must be added to it every hour. So, a one-ton AC capacity cooling system can move 12,000 BTUs of heat per hour, except that it removes heat from the air rather than adding heat to ice.

Designing Your Home's AC System

Your HVAC contractor will design your air conditioning system to perform efficiently under the climate conditions expected where you live. Some of the factors that will be included in your AC system design are the airflow required to move conditioned air throughout your home, the expected temperature differential between the inside and outside of your home, and the amount of moisture that will have to be removed from your air to keep your home comfortable. Here in the Chicago area, AC systems have to be designed to perform well during our hottest and most humid summer days.

After your HVAC contractor finishes the load calculations for your system, he or she may specify equipment that's bigger than the nominal AC capacity tonnage specified by the design. The nameplate tonnage of air conditioning equipment is determined by manufacturers using standards specified by the Air Conditioning, Heating and Refrigeration Institute (AHRI). The AHRI ratings are based on a system maintaining an indoor temperature of 80 degrees with a standard humidity level.

You'll probably want to keep your home cooler than 80 degrees on hot summer days, and the humidity in the Chicago area isn't the same as the standard used in the AHRI ratings. For these reasons, it's important that your HVAC contractor adjust the equipment requirements for your system to reflect those differences. Your contractor should use ACCA Manual S for finalizing your system design. Manual S is produced by the Air Conditioning Contractors of America. It's the nationally recognized standard for proper AC equipment sizing. Using it will ensure that your system meets code requirements.

Your cooling system should be built with equipment that's sized right specific to your building. If the AC capacity of your equipment is too small for your system, your home or office won't be comfortable on the hottest summer days.

There's a tendency to oversize equipment in AC system design to be sure the building will stay cool under the most extreme summer temperature conditions, but equipment oversizing is a bad idea. An oversized system costs more upfront because of higher equipment and labor costs. It also costs more to operate. Oversized equipment brings the building temperature to the set point quickly, resulting in the frequent starting and stopping of the compressor. A correctly-sized system will have longer run times, resulting in much less wear on the system's moving parts. Think of a properly-sized system as being like a car running smoothly down the highway on a long road trip, while an oversized system is like a car caught in stop-and-go rush hour traffic.

Oversized AC equipment will make it harder to keep your building comfortable. The short run times mean that the system is off most of the time. With the thermostat located on an interior wall, the parts of your house near exterior walls will heat up to an uncomfortable temperature before the system kicks on again. A correctly-sized system keeps air flowing most of the time, minimizing temperature differences between different areas of the building. A smaller system will also run quieter than a system with larger equipment.

Humidity control is one of the bigger factors to consider to keep your home or office comfortable. A room with high humidity feels warmer than a room with low humidity even at the same temperature. The AC system removes moisture from the air by condensing it on the cold indoor coil. In a system that has oversized equipment, the short run times will often keep the coil from getting cold enough to condense the water vapor in the air. The increased humidity in an oversized system not only makes it hard to keep the building comfortable, but it can also lead to mold growth and structural damage due to excess moisture.

With modern energy-efficient construction, buildings often require smaller AC capacity systems than would have been needed just a few years ago. Other factors that can reduce the size of equipment needed for building comfort include frequent filter changes, keeping coils clean and efficient duct design. Right-sized equipment will give your system the AC capacity you need for maintaining comfortable temperatures and humidity levels in your building. Proper sizing leads to higher energy efficiency, lower maintenance costs and longer equipment life, so it's important that your HVAC contractor get this part of your system design right.

Comfort24-7 serves homes and businesses throughout Chicago, Northwest Indiana and Southwest Michigan. Our heating and air conditioning experts can design and install heating and cooling systems for your home or business that will keep your building comfortable and energy efficient throughout the year. Call us for energy-saving tips or for advice on proper maintenance of your HVAC equipment.

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