Excerpt from This Old House Magazine

Excerpt from This Old House Magazine article on “Air Conditioners Really are Getting Better” by Max Alexander
All in the Sizing
No matter how efficient your air conditioner is, it needs to be sized correctly for maximum comfort and minimum energy expense. The key is proper balance between the condenser, where the refrigerant is pressurized and cooled, and the evaporator, where indoor air is cooled and dehumidified before being circulated through the house’s ductwork by the air handler.
Condensers are sized by the refrigeration ton, or “ton” for short. That’s the amount of refrigeration needed to freeze one ton of water in 24 hours — 12,000 Btus per hour. As a general rule, it takes a ton of air-conditioning to cool 1,000 square feet of well-insulated space. When recommending size, a savvy air-conditioning contractor will take into account the home’s layout, insulation levels, air leakage, sun exposure, and general climate.
Undersizing a system can overwork the condenser and clog it with frost, which shuts the system down (for a time). But according to Richard Trethewey, This Old House’s plumbing, heating, and cooling expert, oversizing is the more common problem because contractors tend to jack up the tonnage in hopes of avoiding future complaints. “More is not better,” Richard warns. The house cools down so quickly that the thermostat shuts off the air handler before it has a chance to fully circulate and dehumidify the inside air. “Your house ends up feeling cold and clammy,” he says. His advice: Tell your contractor that you intend to keep the summer thermostat at 75 degrees — perfectly comfortable if the inside air has been sufficiently dehumidified.
Richard also recommends investing in a two-stage compressor, which works at full power only on the hottest days. The rest of the time, it doesn’t compress (and, by extension, cool) the refrigerant as much, so the air handler operates longer and has more time to squeeze humidity out of the indoor air. “You save energy and you’re more comfortable,” he says. “How can you beat that?”
Still, before you run out and replace your old system with a new and improved one, do some homework. The amount you should invest depends on how many days you’re likely to run it and your cost of electricity. “You don’t want to spend $2,000 for a system that saves you $100 a year,” Richard says, especially since condensers last only about 15 years. And when it’s time for an upgrade, make sure to replace the evaporator at the same time. If you try mixing old and new components, you’ll probably end up with a ruined compressor and no AC when you need it most. And that’s definitely not cool.

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