There are three key questions to ask when considering installing a 13 SEER unit.
Which region you are in?
When was the 13 SEER system built?
Was it a heat pump or straight AC?
You can still install a 13 SEER unit anywhere in the U.S. if it was manufactured before Jan. 1, 2015. If you are in the North region, as specified by the Department of Energy (DOE), you can continue to install 13 SEER AC systems even if they were manufactured after that date, and there is not yet a date for phasing out those units in the North. However, all heat pump systems built after Jan. 1, 2015 must be at least 14 SEER.
If you are in the South or Southwest DOE regions, you can only install 13 SEER units (AC or heat pumps) built before Jan. 1, 2015, and you can continue to do this until the 18-month grace period is over on June 30, 2016. After that date, everything in those regions must be 14 SEER irrespective of when it was built. Also, the Southwest has an EER minimum in addition to the SEER minimum, so if you are in that region you will need to watch for the EER rating as well. It is not clear how the availability of 13 SEER units built before Jan. 1, 2015, will play out, but there are certainly some in the channel for the 2015 cooling season. It is likely that this availability will drop significantly as we move closer to the June 30, 2016, date when they can no longer be used (except in the North).