As of January 2006, central air conditioning equipment installed in the U.S. must be 13 SEER. SEER stands for Seasonal Energy Efficiency Rating. It is the ratio of cooling produced in BTU, divided by the amount of electricity in watts, which is required to produce a certain amount of cooling. A 10 SEER air conditioner requires 30 percent more watts than a 13 SEER of the same tonnage or output. This does not include electricity needed to deliver the cooling to each room, so blower use is a constant and approximately 3 percent.
There are two main parts to a central air conditioner, indoor and outdoor. A thirteen SEER condenser (outdoor unit) can only be efficient if paired with a 13 SEER coiling coil (indoor unit). Many cooling coils can be upgraded by adding an expansion valve, in fact many manufacturers have done exactly that to their existing product line to meet 13 SEER standards.
A 13 SEER condenser is approximately 50 percent larger, physically; than a 10 SEER condenser. It is also more expensive. When an older air conditioner fails, replacing it will be considerably more expensive from now on. Not only because the outdoor unit simply costs more, but also because the indoor section must be addressed.
This efficiency upgrade does not directly affect warm air furnaces with central air conditioning, if the furnace was properly matched to the air conditioner to begin with. The heating mode is not affected, and the air flow should not need change. The cooling coil which sits on top of the furnace will be effected and possibly the corresponding ductwork.
Most air handlers however, are more specific to the coil and condenser and will be subject to more frequent replacements, if the coil cannot be upgraded or simply replaced. Unlike the furnace, where the cooling coil is a modified fit, the cooling coil inside of an air handler is more or less a custom fit. Replacing the cooling coil itself will depend on the age of the system, as manufacturers have altered their many coil styles over the years, and a new coil may not physically adapt to the old air handler cabinet. Replacing the air handler and coil will most likely require ductwork alterations. When the ductwork does indeed need modification, it would be a good time to upgrade the filter system and insulation values of plenums and accessible ducts.