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Dear Pablo: Air conditioning uses a lot of energy but I am wondering if it is actually better (more sustainable) to live somewhere hot, where you need AC, or somewhere cold, where you need to heat?
This is a great question. Living closer to the poles comes with long, cold winter nights and requires a lot of heating while living closer to the equator often comes with unbearable heat that demands air conditioning. Overcoming the extremes of outside temperature involves either adding heat, typically with open combustion in a natural gas or oil-burning furnace, or removing it by means of an air conditioner's heat pump. Let's take a look at which could be a more sustainable option.The answer to our question cannot be based on a specific pair of buildings, but will rather be a general answer based on two identical theoretical buildings. This will allow us to eliminate variables and narrow down the answer. Now imagine these buildings in a cold part of the US and a very hot part of the US.
Heating and cooling requirements can be estimated based on a measure called Heating Degree Days (HDD) and Cooling Degree Days (CDD). These measures essentially indicate the difference between the daily average temperature and a base temperature (usually 65° F) added up for the entire year. So, if every day averages 1 degree above the base temperature, the CDD value would be 365 and if every day were to average one degree below the base temperature the HDD would be 365. For our cold location we will select Northern Minnesota, and for the hot location we will select the Southern tip of Texas.
Living In The ColdLiving in a cold climate requires a lot of heating. Traditionally this came from burning wood, but today most people heat with natural gas. Natural gas furnaces run 78%+ efficient, with high-efficiency models turning 96% of the fuel into usable heat. Our selected location in Northern Minnesota has over 9000 HDD per year, but less than 1000 CDD so we can consider the cooling requirements here negligible. Homes up North are unlikely to have AC and residents are probably just happy to have a few warm days.
Living In The HeatHot climates require air conditioning and a lot of it. A cooling system uses a heat pump, working with compressors, condensers, heat exchangers and refrigerant gases to pump heat energy out of your house. AC units are rated by Coefficient of Performance (COP), which is the ration between heat energy removed from the house and the energy input into the system. It may seem surprising but a typical COP is in the 3-4 range, meaning that 3-4 times more heat energy is removed than is put in as electrical energy. Electricity has to come from somewhere and most power plants are around 30% efficient at turning fossil fuels like natural gas and coal into electricity. This means that the overall efficiency, from fossil fuel energy to cooling your house, is around 100%. Our selected location in Southern Texas has less than 1000 HDD per year, so we can consider the heating demand negligible, but it does have over 3500 CDD per year.
The Conclusion: Which Is Better?We are comparing a location where there are 9000 HDD to a location with 3500 CDD. If bringing heat into a house were as efficient as removing it, it would be clear that living in the South is more energy efficient. But recall that natural gas furnaces are 78-96% efficient while air conditioning is around 100% efficient (including power plant efficiency). This means that someone residing in the cold North uses even more energy to keep their home at a comfortable 65° F than their Southern equivalent.
Of course the answer is not this simple. Different regions have different insulation requirements and architectural styles so it isn't fair to assume two identical buildings. A building in the South may be built with a lot of thermal mass, brick, tile, or adobe, so that the home stays comfortable throughout the day until sundown allows cooling by natural ventilation. In the North a home might be ultra-insulated and designed to maximize solar heat gain through triple-glazed windows or Trombe walls.
In addition to the building's energy use there are other environmental impacts to consider. Parts of the South are arid and have little naturally available water, especially if a perfectly manicured lawn is desired. In addition to landscaping, water scarcity also means that a lot of food needs to be trucked or flown in from far away. If the hot region happens to have high humidity, a lot of the AC's energy use is actually working to remove moisture from the air, decreasing its efficiency.
Finally, some people chose to live in the warmer regions in the winter and the cooler regions in the summer. These "snow birds" will reduce their energy use from both heating and cooling by avoiding the seasonal extremes but incur the environmental impact of maintaining two homes and traveling back and forth between the two.
In the end it is probably better to live in a moderate climate rather than one of the extremes in our example here. Some locations require very little heat of cooling, and if your home is designed properly it may not require either. Just remember, if it's energy efficiency that you want: Location, Location, Location!
Pablo Päster is a weekly columnist for TreeHugger.com and Principal Environmental Consultant at Hara Software. Send your questions to Pablo(at)TreeHugger.com or submit the via this form and connect to his RSS feed.
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