energy efficiency of central air conditioners and heat pumps

For those of you living in warmer climates a central air conditioning unit maybe the single biggest source of energy consumption in your home.  For this reason it is one of the best places to begin when trying to reduce your energy costs. This is true for businesses as well.  According to the EPA about one-seventh of all the electricity generated in the US is used to air condition buildings. Heating and cooling costs the average homeowner about $1,000 a year - nearly half the home's total energy bill. If your central air conditioning unit is more than 12 years old, there is a good chance that replacing it with an ENERGY STAR qualified model could cut your cooling costs by as much as 30%

There are several different metrics used to measure the efficiency of air conditioning units. These are summarized below:

• Seasonal Energy Efficiency Ratio (SEER) - Central air conditioning units are usually measured using a ratio called SEER.  In technical terms, SEER is a measure of equipment the total cooling of a central air conditioner or heat pump (in Btu) during the normal cooling season as compared to the total electric energy input (in watt-hours) consumed during the same period. New requirements took effect in 2006 which raised the SEER requirement by 20%. The higher the SEER number the more efficient the unit is. The most efficient models will have a SEER number of 14.5 or greater.
• Energy Efficiency Ratio (EER) - is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (95oF). The higher the EER, the more efficient the system. This metric represents the cooling output divided by the power input for a hypothetical average U.S. climate.
• Heating Seasonal Performance Factor (HSPF) - is the efficiency measurement used to gauge the efficiency of the heating mode of heat pumps. The higher the number, the greater the efficiency and cost-savings. Today's models of heat pumps are required to have a 6.8 HSPF

The table below provides SEER ratings for both central air conditioners and air source heat pumps. Both SEER and EER are included in the ENERGY STAR specification because each rating indicates the energy efficiency of the product under different operating modes. SEER rating more accurately reflects overall system efficiency on a seasonal basis and EER reflects the system’s energy efficiency at peak day operations. Both ratings are important when choosing a product. Heat Seasonal Performance Factor (HSPF) is the most commonly used measure of a heat pumps heating efficiency. The higher the HSPF, the more efficient the heat pump. In technical term, HSPF represents the total heating output of a heat pump (including supplementary electric heat) during the normal heating season (in Btu) as compared to the total electricity consumed (in watt-hours) during the same period. HSPF is based on tests performed in accordance with AHRI 210/240 (formerly ARI Standard 210/240)1.

Keep in mind that even an efficient air conditioning unit will waste a great deal of energy if it is bigger than what your home actually requires.  Make sure your contractor does a complete load calculation so you know exactly how big a unit to get.  Also, don't be penny wise and pound foolish.  When installing a new air conditioning unit be sure to look at getting the most up-to-date thermostat you can find to go with it.  Modern thermostats with digital timers allow for very precise scheduling of your home temperature and can ensure that you don't waste a lot of energy cooling the rooms when no one is there.

Sometimes finding a heating and air conditioning contractor who really understands energy considerations is difficult. A great resource to go to in that case is the Air Conditioning Contractors of America (ACCA) web site at ww.acca.org.  On their site you can find a Contractor Locator which includes a special designation to identify contractors who sell and are knowledgeable about ENERGY STAR qualified HVAC equipment. 

Non-AHRI Central Air Conditioner Equipment and Air Source Heat Pump (ASHP) Product List

List Posted August 2, 2010