Tonnage in the HVAC profession refers to a specific amount of heat energy the air conditioner must extract from your home every hour to keep it cool and comfortable. It’s measured in BTUs (British Thermal Units) and a “ton” of cooling equals 12,000 BTUs extracted per hour. Your contractor will use both his own expertise as well as sophisticated, industry-standard software to determine your home’s BTU requirements, known as "sizing the house," then recommend an air conditioner that offers that cooling capacity.
You shouldn’t accept anything less.
In the past, calculating the required BTU tonnage per hour with precision was often not a high priority, particularly in new construction. In an era when energy costs were much lower than today, builders frequently installed oversized A/C units as a matter of course to standardize the make and model and cover for all variables in a home. This standard procedure was so widespread that contractors who perform air conditioning upgrades of older units nowadays report that more than 50 percent of the existing installed air conditioners are larger than they need to be. Not only do these oversized units consume unnecessary amounts of energy and produce high utility costs, ironically, they also cool less effectively and often have higher rates of component failure and shorter service life, too. Undersized units aren’t a bargain, either. They won’t extract enough BTU tonnage to adequately cool the home. In addition, a unit that’s too small for the thermal characteristics of the house will run extended “On” cycles, using excessive amounts of electricity.
Getting a Handle on the Cooling Load
In the summer, your home carries a heavy burden. It’s called the cooling load and it’s the sum total of a range of factors that influence how much cooling power in BTU tonnage is required to keep your home at a comfortable indoor temperature. This comfort goal is called the “design temperature” and it’s usually around 78 degrees. To maintain that temperature, the contractor has to know the number of BTUs of heat energy that need to be taken out of the house every hour by the air conditioner.
The cooling load is influenced by factors including the:
- Local climate
- Square footage of the house
- Orientation of the home and the amount of solar exposure it receives
- Total area of windows and the type of glass
- Amount and type of insulation
- Condition of the ductwork
- Number of major appliances
- Amount of lighting and the type of bulbs
- Number and age of occupants
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