SEER2 ratings are simply an updated version of the original SEER ratings. The actual air conditioning units do not change, SEER2 is simply a revised testing standard / calculation that is designed to more accurately mimic real world operating conditions as air conditioners are tested and rated.

The result is a lower number than what an air conditioner would receive with a standard SEER rating system, despite the equipment being identical. For example, a York YC2D is rated at 13.4 SEER2, which equates to 14 SEER with the old standard.

This is important to know when comparing estimates, because if quote is showing you a 14 SEER unit, and the other is showing you 13.4 SEER2, they are in fact the same. SEER2 ratings will be roughly 4.7% lower than a SEER rating, due to the new testing conditions. All new York models will reflect a SEER2 rating, per federal requirements.