The short answer
SEER2 and EER2 rate cooling, HSPF2 rates heat pump heating, and AFUE rates furnaces and boilers. Higher is more efficient. Since January 1, 2023, systems are rated under the tougher “2” test, so a SEER2 number looks lower than the old SEER for the same equipment. Installation quality affects real efficiency as much as the rating does.
SEER2: seasonal cooling efficiency
SEER2 (Seasonal Energy Efficiency Ratio 2) is cooling output over a typical season divided by the electricity used. The “2” version, used for equipment made from 2023, tests with more realistic airflow resistance, so ratings come out lower: an old 14 SEER is roughly 13.4 SEER2, and 15 SEER is roughly 14.3 SEER2. Compare SEER2 with SEER2, never across the two.
EER2: efficiency on hot days
EER2 measures cooling efficiency at a single hot condition. It matters most in hot, dry climates where systems spend long hours at peak load, which is why the Southwest has an EER2 minimum as well.
HSPF2: heat pump heating
HSPF2 (Heating Seasonal Performance Factor 2) is a heat pump’s heating output over a season divided by electricity used. Higher is better. For cold climates, also ask for the heating capacity at low outdoor temperatures – a high HSPF2 does not by itself mean strong performance in deep cold.
AFUE: furnaces and boilers
AFUE (Annual Fuel Utilization Efficiency) is the share of fuel that becomes heat in the home. An 80% AFUE furnace loses about a fifth of its fuel energy up the flue; condensing furnaces reach 90% or more by recovering heat from the exhaust, and vent through plastic pipe.
COP: efficiency at one temperature
COP (Coefficient of Performance) compares heat delivered to energy used at a specific outdoor temperature. A COP of 3 means three units of heat for each unit of electricity. Cold-climate heat pump data often lists COP at several temperatures.
Federal minimums since 2023
The U.S. Department of Energy sets minimum efficiency for new equipment. From January 1, 2023, the main minimums for common home systems are:
- Split central air conditioners (under 45,000 BTU/h): 13.4 SEER2 in the northern region; 14.3 SEER2 in the Southeast and Southwest, where the Southwest also sets EER2 minimums.
- Split heat pumps: 14.3 SEER2 and 7.5 HSPF2 nationwide.
- Gas furnaces: most models sold for homes must be at least 80% AFUE.
Rules can change; your installer should confirm what applies to your equipment and region.
Matched systems and the AHRI certificate
A split system’s rating applies only to a specific combination of outdoor unit, indoor coil and, where relevant, furnace or air handler. Ask the installer for the AHRI reference number for the exact combination quoted – you can look it up in the AHRI directory. A mismatched coil can quietly cost efficiency and affect warranties.
How much should the rating matter?
- Higher ratings save more where systems run more: long cooling seasons, long heating seasons, high energy prices.
- Sizing, duct leakage, airflow and refrigerant charge can erase the gap between two ratings. A well-installed mid-efficiency system can beat a poorly installed high-efficiency one.
- ENERGY STAR labels mark models above the federal minimum, and some rebates require them.
- Ask each company for the yearly running-cost difference between the options it quotes, and how it estimated it.
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Questions
What is a good SEER2 rating?
The federal minimum is 13.4 SEER2 for split central air conditioners in the North and 14.3 SEER2 in the South. Higher-efficiency systems go well beyond that. Whether the extra cost pays back depends on your climate, energy prices and how much the system runs.
Is SEER2 the same as SEER?
No. SEER2 uses a tougher test, so the same equipment gets a lower number. Roughly, 14 SEER equals 13.4 SEER2 and 15 SEER equals 14.3 SEER2.
What does 95% AFUE mean?
About 95% of the fuel the furnace burns becomes heat for the home; around 5% leaves through the vent.
What is an AHRI certificate?
A record from the Air-Conditioning, Heating, and Refrigeration Institute certifying the rating of a specific matched combination of equipment. Ask your installer for the reference number of the system quoted.
Related guides
Updated October 2026 by the Heating and Cooling editorial team. General information for homeowners, not professional advice – rules and equipment differ. When gas, carbon monoxide, electricity or anyone’s safety is involved, call 911 or your utility. How we write our guides · HVAC glossary