July 15, 2010

With record-breaking heat on the East Coast threatening health and maxing out the power grid, you might ask yourself if it's time to replace your old central air conditioner.  Weighing the financial and environmental costs of buying a new unit vs. keeping the old one for another year can be difficult. A University of Michigan study determined the optimal time to replace a central air unit to save energy, reduce greenhouse gases and lower utility bills.

Research associate Robb De Kleine with U-M's Center for Sustainable Systems (CSS) conducted a life-cycle analysis of central air conditioning with U-M Professor Greg Keoleian, co-director of CSS, and Jarod Kelly, Alcoa Foundation Conservation and Sustainability Fellow. The study calculated the amount of energy required to manufacture, deliver, use and dispose of typical air conditioning units and estimate how this changes over time. 

About a third of central air conditioners in single family homes are at least a decade old. Meanwhile, the typical central air conditioner purchased today is rated as being about 25 percent more efficient than systems purchased 10 years ago due in large part to more stringent federal energy efficiency standards.

The amount of operating energy saved by using newer, more efficient equipment was compared to the additional energy consumption needed to produce the new air conditioning system and to retire the old one. This tradeoff determined the best replacement time to yield the lowest overall energy consumption from 1985-2025. This same approach was repeated for replacement schedules to achieve the lowest greenhouse gas emissions and consumer costs.

To figure out when to buy a new air conditioner, answer these three questions:

1. Have you maintained your current air conditioner?
   If it has never occurred to you to get an air conditioning tune-up, your air conditioner probably isn't performing as efficiently as it could. Making sure that your air conditioning system is operating at its best can reduce the emissions of greenhouse gases by more than 25 percent. When air conditioning systems are in good working order, they need to be replaced less frequently in order to achieve the lowest energy consumption, greenhouse gas emissions, or consumer costs compared to when they are neglected.
2. If you have maintained the equipment, how old is it?
   Even if you have doted on your air conditioner, it will eventually need to be retired. If your currently have a typical cooling system installed in 1992 or before, the results show it is best to replace it immediately. Because the average air conditioning system sold today is considerably more efficient than the average system sold in 1992. Replacing will significantly reduce cost, energy consumption, and greenhouse gas emissions. Having the new air conditioner installed by a knowledgeable technician according to best practices is necessary to achieve the greatest financial and environmental benefits from replacement.
3. What matters most to you: saving money, consuming less energy per month, or reducing greenhouse gases?
   The number of replacements depends on your climate, but if your goal is to reduce energy consumption associated with manufacturing and operating a central air conditioner over its lifetime, you should replace your unit more frequently than if your priority is to reduce your expenses. The frequency of replacement to achieve the lowest GHG emissions lies between the number of replacements to minimize energy and cost.

For example, in a relatively cool climate such as Ann Arbor, MI, 4-10 replacements are needed to achieve the lowest energy consumption over a period from 1985-2025 depending on the level maintenance and service.  Meanwhile, 2-3 replacements are needed to achieve the lowest GHG emissions, and 1 replacement is needed to achieve the lowest consumer cost. Contrast this with a hotter climate like San Antonio, TX, where 10-15 replacements are required to minimize energy, 3-5 replacement are need to achieve the lowest GHG emissions, and 2-3 replacements are needed to achieve the lowest cost.

The best replacement schedule for a specific case might differ due to factors like the type of home, the user's thermostat settings, and the use of other cooling methods like ceiling fans. In general, if you use your air conditioner less often than the average for your area, you would replace less frequently.  This study used typical cooling requirements for various locations to give policy makers and homeowners some direction on when to replace.

A copy of the report, funded by The Energy Foundation and the Alcoa Foundation Conservation and Sustainability Fellowship Program, can be found at http://css.snre.umich.edu/css_doc/CSS10-02.pdf