Is central air conditioning more energy-efficient than window units?

Illuminating answers to environmental questions.
May 3 2011 10:03 AM

Global Cooling

Is central air conditioning more efficient than window units?

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For many urbanites, summer means being constantly dripped on from the air conditioners that fill virtually every city window. These ubiquitous boxes must consume a tremendous amount of energy. From an environmental perspective, aren't central air conditioners better?

Not necessarily. Air conditioning accounts for more than 15 percent of the energy use of the average home—somewhere around 183 billion kilowatt-hours of electricity in the United States per year, according to a 2001 analysis by the Energy Information Administration. Generating that much electricity creates about 119 million tons of carbon dioxide, or roughly the amount of CO2 spewed by 20 million cars each year. Remarkably, the percentage of homes with central air more than doubled, from 27 to 55 percent, between 1980 and 2001.

When deciding on how to air-condition your abode, you have two basic choices: single-room units or a centralized system. Anyone who's been to a big-box hardware store has seen the single-room air conditioners. They're heavy, steel boxes designed to wedge into a window and dangle precariously over passing pedestrians. (In case you were wondering: Yes, they do fall on people.) Inside, there's a condenser, an evaporator, a thermostat, and a couple of fans.

Central air conditioners have a different structure altogether. The condenser typically sits on the roof or in the backyard. A set of pipes runs coolant from the condenser into an air handler, which is usually located near the home's furnace. The air handler blows the cooled air through the home's duct system and into individual rooms.

A central air system's energy efficiency is measured two ways. The more basic rating is the energy efficiency ratio, or EER, which describes the unit's energy consumption while cooling a prescribed volume of 95-degree air over the course of an hour in a specialized laboratory. The seasonal energy efficiency ratio, or SEER, is a bit more complicated and considers the average energy use at various temperatures and humidity levels.

An old air conditioning window unit. Click to expand image.
The energy-efficiency debate over air conditioning

Window units are rated only by EER, and lose out to central A/C by this metric. An Energy Star-certified central unit must have a minimum EER of 12, while window units only need to achieve between 9.4 and 10.7, depending on size, to be certified. (It's possible to buy a window unit rated higher than this minimum, but the same goes for central units. Overall, the central units are still rated higher.)

One reason window air conditioners have lower ratings is that it's impossible to fit much advanced hardware into that little box. While modern central air conditioners can work at a range of speeds, for example, the condensers in most window units only have two: on and off. (This is, incidentally, the primary reason window air conditioners don't get SEER ratings.)

This can make a significant difference in energy use. Air conditioners not only lower air temperature; they also remove moisture. Just how much moisture is removed depends on how much air passes through the air conditioner. A machine that only works at full speed (or not at all) can drop a room's temperature quickly by cooling just a small volume of air to an extremely cold state. In contrast, an air conditioner with variable speed settings can have the same effect by processing a larger volume of air, but cooling each unit volume by a smaller amount. The latter situation results in greater dehumidification because more air runs through the machine.

While this seems like a technical point, it's quite significant, because it means users can set central air conditioners to a higher temperature and feel just as comfortable, because the air is less humid.

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But the window units have their advantages, too. Central air conditioners suffer from "duct losses." As cooled air passes through the system of ducts on its way to the rooms, it warms up and often leaks through the fittings. These losses can decrease an air conditioner's energy efficiency by up to 30 percent.

Window units that block the sun from your room, buzz, and shake violently every time the condenser switches on are basically begging for attention—very few consumers walk out the door without remembering to shut off their window units. The quiet and unobtrusive functioning of central air conditioners, on the other hand, can lead to accidental overuse. Industry observers note that people tend to set their central air conditioner to a comfortable temperature and never change it.

Window units also offer room-by-room control. For singletons or couples who occupy only one bedroom at night, cooling a single room consumes a small fraction of the energy that a central air conditioner would.

Central air units are getting smarter, though, and accordingly more efficient. Programmable thermostats can make up for forgetful central A/C users by shutting the unit off when no one is home. Some central air systems even divide homes into separately controlled zones, so you don't have to cool the kitchen at 3 a.m. just to keep your bedroom a sleepable temperature. And some new central air conditioners have an air handler in each room, skipping the ducts entirely, combining precise control with improved efficiency and less noise.

Because of the construction and behavioral variables, it's impossible to pronounce a winner in this particular competition. As is often the case, the choice is often going to come down to each consumer's behavior. Generally speaking, the newfangled ductless system is going to be the most efficient but priciest option upfront. If you're choosing between an old-school central air system and window units, the latter could be better if you're childless, live in a large home in a dry environment, or tend to be forgetful about adjusting your thermostat.

The Lantern thanks Jennifer Amann of the American Council for an Energy-Efficient Economy.

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