Yes, smart thermostats are cool -- letting you monitor and control your home’s temperature from your smartphone, PC or tablet -- but they’re really about saving money. That’s because these devices can monitor temperature and humidity outside your home, your comings and goings, and tailor heating and cooling cycles accordingly. Based on our comprehensive testing, Ecobee3 ($249 on Amazon) is the best overall choice in this category because its sensors let you deliver the right temperature on a room-by-room basis. It also knows which rooms are occupied and comes with an intuitive app. Since our initial review, it's only gotten better, and now works with Apple's HomeKit. As a result, we've bumped its rating to 9 stars. Our runner-up is the third-generation Nest Learning Thermostat, which has a larger display than before, and works with a multitude of other smart home devices, including the Amazon Echo. How We Tested and Rated I installed—or had installed—several smart thermostats in my apartment, and tested them separately for at least one week.

This included not just changing the temperature, but also evaluating its accompanying app and web portal, if available. If the thermostat worked with other smart home devices or Web-connected services (such as IFTTT), I tested that functionality as well. As you'll see in our comparison table (above), we break down the features you should look for in a smart thermostat. The Ecobee is the only device we tested that supports multiple zones. All of the smart thermostats reviewed offer motion activation, except for the Carrier Cor, which is also the only one that requires professional installation. Other Smart Thermostats Reviewed Related Buying Guides:Smart Home GadgetsRobot VacuumsBest Bluetooth SpeakersHave you heard the news? Scientists have found a planet that can support life. Its atmosphere is too hot for year-round habitation, its gases impede breathing, and surface conditions are sometimes fatal. But by constructing a network of sealed facilities, tunnels, and vehicles, humans could survive on this planet for decades and perhaps even centuries.

The planet is called Earth. If you've seen this planet lately, you know what's going on: temperature records shattering, scores of Americans dead. air handling units working principleBy summer's end, the toll will be in the hundreds. best coastal ac unitsIt's not as bad as 2003, when a heat wave killed 30,000 people in Europe. ac to dc converter deviceBut according to global-warming forecasts, within 40 years, every other summer will be like that one. Thank goodness for air conditioning. To keep old folks alive, cities from Washington to Los Angeles are opening artificially cooled buildings to the public. Meanwhile, people are lining up to buy window units. According to the Air-Conditioning and Refrigeration Institute, shipments of air conditioners and heat pumps have tripled over the last three decades.

The percentage of single-family homes built with central air has gone from 36 to 87. The percentage of cars built with air conditioning has risen from 61 to 98. In 1970, only 42 percent of occupied mobile homes had it. By 2003, that percentage had more than doubled. It's a heartwarming—or, more precisely, a heart-cooling—story. Unfortunately, the story doesn't end there. Air conditioning takes indoor heat and pushes it outdoors. From a cooling standpoint, the first transaction is a wash, and the second is a loss. We're cooking our planet to refrigerate the diminishing part that's still habitable. All over the country, power consumption is breaking records, and air conditioning is a huge reason why. We use about one-sixth of our electricity to cool ourselves. That's more than the total electricity consumption of India, a country whose population exceeds 1 billion. To get the electricity, we burn oil and coal. We also run air conditioners in our cars, which reduces urban fuel efficiency by up to four miles per gallon, at an annual cost of 7 billion gallons of gasoline.

More burning of oil and coal means more greenhouse gases. Based on government data, Stan Cox, a scientist at the Land Institute, calculates that air-conditioning the average U.S. home requires 3,400 pounds of carbon-dioxide production per year. The effects of this are particularly bad at night. Over the last five summers, very high minimum daily temperatures—those that score in the top 10 percent historically—have been far more widespread in this country than during any other five-year period. This is what's killing people. Outdoor air used to cool at night, allowing us to recover from the day's heat. To fuel our own air conditioning, we're destroying nature's. The hotter it gets, the more energy we burn. In 1981, only one in three American households with central air used it all summer long. By 1997, more than half did. Countries once cooled by outdoor air now cool themselves. In Britain, 75 percent of new cars have air conditioning. In Canada, energy consumption for residential cooling has doubled in 10 years, and half the homes now have central or window units.

Kuujjuaq, an Eskimo village 1,000 miles north of Montreal, just bought 10 air conditioners. According to the mayor, it's been getting hot lately. Instead of fixing the outdoors, we're trying to escape it. On every street in my neighborhood, people have torn down ordinary homes and put up giant air-conditioned boxes that extend as far as possible toward the property line. They've lost yards and windows, but that's the whole idea. Outdoor space is too hard to control, so we're replacing it with indoor space. From 1991 to 2005, the median lot size of single-family homes sold in the United States shrank by 9 percent, but the median indoor square footage increased by 18 percent. If you can't stand the heat, go hide in your kitchen. Seven years ago, when my wife and I moved into our house, we built a garden and patio in the back yard. Now, overcome by heat and mosquitoes, we're thinking of replacing them with something a bit more climate-controlled. We still want to look at nature.

We just don't want to feel it. And for better or worse, we'll probably succeed. Two months ago, we saw Al Gore's movie, An Inconvenient Truth. Walking out of the air-conditioned theater, we agonized over what we could do to fight global warming. The conversation ended when we realized that our most useful contribution would be to cancel the renovation. Wrapping ourselves in a climate-controlled bubble can't make global warming less true. But in the short run, it can make it a lot less inconvenient. That's the problem in Washington today. Policymakers aren't facing global warming, because they aren't feeling it. They gave themselves air conditioning in the 1920s and '30s, long before the public got it. White House meetings and congressional hearings on climate change are doomed hours beforehand, when the thermostats are set. One minute, you're watching video of people sweltering in New Orleans. The next minute, you're watching senators dispute the significance of greenhouse gases. Don't ask whether these people are living on the same planet.