IRS Form 5695 (2009 version)¹ explains tax credits for some HVAC products. Below are the relevant excerpts and CEE Tiers:

An electric heat pump that has a heating seasonal performance factor (HSPF) of at least 9, a seasonal energy efficiency rating (SEER) of at least 15, and an energy efficiency rating (EER) of at least 13. For property placed in service after February 17, 2009, the property must achieve the highest efficiency tier established by the Consortium for Energy Efficiency (CEE) as in effect on January 1, 2009. However, if you purchased property before June 1, 2009, you can still take the credit if you relied on the manufacturer's certification issued before February 18, 2009, that the property met the standards in effect before February 18, 2009.

A central air conditioner that achieves the highest efficiency tier that has been established by the CEE as in effect on January 1, 2006. For property placed in service after February 17, 2009, the property must achieve the highest efficiency tier established by the CEE as in effect on January 1, 2009. However, if you purchased property before June 1, 2009, you can still take the credit if you relied on the manufacturer's certification issued before February 18, 2009, that the property met the standards in effect before February 18, 2009.

Identifying Performance Requirements
The current and complete CEE Specification may be viewed here. Below is a list of CEE’s highest tiers for each type of equipment addressed in the specification.

CEE’s Highest Efficiency Tiers Effective January 1, 2009

SEER: Seasonal Energy Efficiency Ratio
EER: Energy Efficiency Ratio
HSPF: Heating Seasonal Performance Factor

Find Qualifying Equipment in the CEE HVAC Directory
CEE in partnership with Air-Conditioning, Heating and Refrigeration Institute (AHRI) provide easy access to an online directory that lists AHRI performance certified central air conditioners and heat pumps that are fall within one of the CEE Tiers, according to the claims of the manufacturer. CEE does not independently test or otherwise verify such claims.  To identify systems that meet CEE’s Highest Efficiency Tier, select the highest tier listed on the “CEE Tier” dropdown menu. Your installation contractor can help you with this search. Please note some additional systems may meet these specifications but are not listed in  the CEE Directory because the manufacturer has not submitted data for listing.

Further information:
The ENERGY STAR website also has helpful information about these and other energy efficient products for which tax credits are provided.

The foregoing information was posted on March 4th, 2009 and is published by CEE for informational purposes only. CEE does not undertake to revise or update the information. This information is not meant to substitute for tax advice from a tax professional. Please consult your tax advisor for information regarding your individual circumstances and specific eligibility for this and other tax credits.

¹Section 1333(a) of the Energy Policy Act of 2005, Pub. L. No. 109-58, 199 Stat. 594 (2005) and Section 1121 (a) of the American Recovery and Reinvestment Act of 2009