Air Conditioners Really Are Getting Better
New air conditioners perform significantly better, last longer, and run quieter than past models.
Central air-conditioning has always been the convenience people love to hate.
Despite the fact that it's standard equipment in most American homes — 88 percent of new construction and millions of retrofits every year — the systems have long been decried for the alarming amounts of electricity they devour, their ozone-depleting refrigerants, and the unseemly levels of noise generated by their condenser units, which rumble away just outside the house.
Happily, new AC technology means we can chill out about those worries. With improvements in fan-blade shape and compressor technology, some models make one-twentieth the noise of old units. A new refrigerant, known generically as R410A, is free of the chlorine that eats away the ozone layer. And energy use is on the decline. Twenty years ago, a typical system might use 6,000 watts of electricity per hour to cool an average-size house. Today, that same house can be cooled with as little as 1,710 watts per hour, an astounding 250-percent increase in operating efficiency.
WHAT IT IS
A two-part (aka "split") system for cooling indoor air that consists of a condenser outside the house and an evaporator inside the house.
HOW IT WORKS
Refrigerant circulates in a closed loop between the evaporator and the condenser, capturing heat from the house and carrying it outside to a fan that blows it away.
WHAT TO LOOK FOR
SEER 13 or higher: SEER (seasonal energy-efficiency ratio) is like miles per gallon for a car: The higher the SEER rating, the better the system efficiency and the lower your electric bills. By law, the minimum SEER is 10; next year, it will be 13.
R410A refrigerant: Not only is it safe for the ozone layer, the compressors that use it are quieter than those using other refrigerants.
Low decibel rating: The quietest condensers have a rating of 68.
Diagnostics port: Permits a technician to quickly pinpoint and diagnose problems electronically.
Two-stage compressor: Operates at full power only on the hottest days.
WHAT IT COSTS
$2,000-$4,000 (A SEER-13 system with condenser and evaporator for a 2,000-square-foot house, installed)
The Refrigeration Cycle
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.
The Refrigeration Cycle
Refrigerant exits the compressor (1) as a hot, highly pressurized gas and is pumped through the condenser (2), coils of tubes embedded in a matrix of thin metal fins. A fan (3) sucks outside air past the fins, cooling the coils and condensing the refrigerant into a liquid. The compressor pushes the cold liquid through a filter/drier (4) that removes moisture and other contaminants. From there it enters the air handler inside the house. An expansion valve (5) turns the liquid to a mist as it enters another coil of tubing, the evaporator (6). A blower (7) moves warm air from the house across those chilly coils, which absorb the heat and condense any humidity out of the air. (Condensate collects in a pan; a tube takes it outside.) As this drier, cooler air passes into the ducts and through the house, the refrigerant warms up, turns back into a gas, and returns to the compressor, where the cycle starts over again.
Install It Right
Richard Trethewey has seen even professional HVAC contractors make installation mistakes over the years. Here are a few of the most common:
Refrigerant overdose: "There's a cowboy mentality that says jam a little more gas in, but you need a precise amount and no more, or the system won't cool properly."
Overly long pipes: "Refrigerant lines are typically limited to 50 feet. Longer than that means the refrigerant can't complete its cycle, and the condenser will run forever."
Insufficient clearance: "The condenser usually needs about two feet around it in every direction, so enough air can circulate and pull the heat out."
Inside scroll compressors, two spiral-shaped scrolls work to compress gas so it can cool more easily.
The Inside Scoop
On a Scroll
"Scroll technology" is the latest buzzword in the world of refrigeration and air-conditioning. That's because scroll compressors run quieter and use less energy than the piston-driven compressors that had been the norm. And as with most good ideas, this one is utterly simple.
The heart of the mechanism is two identical spiral-shaped scrolls, like the ones above, that fit one inside the other. Oneis stationary, the other is attached to an electric motor. The eccentric motion of the motor means the scrolls are in contact with each other at all times, but only at discrete points. Refrigerant gas trapped in the gaps between those points is pushed from the outside of the spiral into its center. compressing the gas so it can be cooled more easily.
Another virtue of scroll compressors is that they actually work better over time. As the scrolls wear down, they fit together more tightly and allow less gas to escape.
More Cool Air for Less
The two parts of a central air-conditioning system, the condenser and the evaporator, should be maintained annually to maximize their efficiency and minimize your electric bills. Some of these tasks can be easily accomplished by a homeowner; others require a skilled technician. Here are a few things you can do before the cooling season begins.
At the Condenser
Pop open the unit's side covers and:
1. Gently brush dirt off the cooling fins with a soft-bristled paintbrush or spray it off with a hose. (Never use a pressure washer, which can bend the fins.)
2. Realign crushed or bent fins with a fin comb, available from AC dealers and supply houses.
3. Rake up and remove loose leaves or debris. Pine needles are particularly adept at clogging the fins.
4. To ensure maximum air flow through the condenser coils, prune back vegetation and remove any obstacles within 2 feet of the condenser.
At the Evaporator
Shut off the power, remove the cover, and:
1. Scrub out the evaporator pan with soap, hot water, a little bleach, and a sponge.
2. Pour bleach through the pan's condensate tube to kill mold and algae, which can clog the tube and cause the pan to overflow. Check the tube's outside end to make sure it's draining freely.
3. Wipe down the coils.
4. Vacuum dust off the blower fins.
5. Change the air handler's filter. Make sure the replacement matches the blower's capacity in cubic feet per minute (cfm).
Call In An Expert
You'll need a technician to check the refrigerant pressure, test for leaks, clean and tighten electrical connections, measure air flow through the evaporator, and make sure everything is functioning properly. It's critical that whoever does this work follows the manufacturer's specs for the amount of refrigerant in your system. "Overcharging," adding too much refrigerant, is as harmful as not having enough.
Place a chlorine tablet, the kind used for swimming pools, in the evaporator pan. It will prevent the growth of algae or mildew.
Where to Find It
Our thanks to:
The Air Conditioner Contractors of America
Trane air-conditioning distributor:
The Wallwork Group
West Caldwell, NJ
Copeland Scroll Compressors
Emerson Climate Technologies
St. Louis, MO