· by Michael Blair, Nashville's HVAC
Efficiency ratings were restated in 2023, and the new SEER2 numbers are lower than the old SEER numbers for identical equipment. The Department of Energy’s 2017 test procedure rule for central air conditioners and heat pumps made ratings under the new method mandatory from January 1, 2023, and says the new metrics generally have different numerical values than the old ones. The machine on the pad is the same; the test it is rated on changed, and that has muddled a lot of replacement decisions.
Why are SEER2 numbers lower than the old SEER?
The old test measured performance at a static pressure that did not reflect real duct systems. The same rule lists the old minimum external static pressure as 0.10 to 0.20 inches of water column, depending on capacity, and DOE adopted 0.50 inches for conventional central systems after commenters argued that figure is more representative of field conditions. That is much closer to what equipment actually faces in a house.
The result is a lower number for the same unit. We don’t know of a published conversion that fits every model, but the federal minimums show the scale: the International Code Council’s guidance on the 2023 efficiency requirements describes the northern minimum for split air conditioners as “14.0 SEER or 13.4 SEER2,” a gap of a little over four percent. Read a SEER2 figure as the same performance measured on a harder test, and compare it only with other SEER2 figures.
What higher efficiency buys here
NOAA’s Tennessee climate summary puts average summer highs between 85°F and 90°F in western and central Tennessee and describes summers across most of the state as warm and humid. More hours of cooling give an efficient system more time to earn back what it cost, so the same upgrade generally has more to work with here than in a cooler market.
The gains are not linear though. Under DOE’s definition, SEER2 is the heat removed over a cooling season divided by the electricity used, so the electricity for the same cooling falls in proportion to the rating. Going from an old 10 SEER unit to a 15 SEER2 one cuts it by about a third, a little more once you allow for the old unit scoring lower on the new test. Going from 15 to 18 SEER2 cuts it by about a sixth. Get the price difference between those tiers in writing and set it beside that smaller saving.
What matters more than the SEER2 rating?
Correct sizing, duct condition and installation quality can outweigh the difference between two efficiency tiers. ENERGY STAR’s duct sealing flyer says about 20 to 30 percent of the air moving through the ducts of a typical home is lost to leaks, holes and poorly installed ducts, and that leaky ducts can reduce heating and cooling efficiency by as much as 20 percent. That is the same order as the step from 15 to 18 SEER2, so a high-efficiency unit on leaky ducts in an unconditioned attic can give back much of what it was bought for. ENERGY STAR’s duct sealing page lists ducts in an attic, crawlspace or garage among the signs of poorly performing ducts, and testing and sealing them is the work of our ductwork service.
Ask what static pressure the installer measured and what they intend to do about it. The rating assumes 0.50 inches of water column, and a contractor who has not measured your system is guessing at one of the variables that decide whether that number holds up in your house. ENERGY STAR’s guide to HVAC quality installation names the others to ask about: sizing from the house’s actual characteristics rather than a rule of thumb, airflow that meets the manufacturer’s specifications, a refrigerant charge that is tested and adjusted, and a duct leakage test.
- Sizing from a load calculation comes before the rating
- Leaky ducts can cost up to a fifth of efficiency
- SEER2 says nothing direct about humidity control
- Ask for measured static pressure before and after
Deciding what tier to buy
The floor is set by federal law. Tennessee is in DOE’s Southeast region, and under 10 CFR 430.32 a split-system air conditioner below 45,000 Btu/h installed here on or after January 1, 2023 must rate at least 14.3 SEER2, or 13.8 at 45,000 Btu/h and above. The old Southeast floor was 14 SEER, so the minimum here rose even as the numbers on the labels fell. The ICC guidance adds that split systems manufactured before 2023 cannot be installed in the Southeast after that date unless they meet the new standard.
Above that floor, our own view for most Nashville houses is a two-stage or variable-speed unit at a moderate SEER2, sized from a load calculation as on every system replacement we quote. That is judgment from the houses we work in rather than a published finding, and the reason run time matters for indoor moisture is laid out in our post on why a house feels clammy at 72 degrees.
The top tier is easier to justify in a large house you expect to keep for a long time, with ducts that test tight. In a smaller house, or if you might move in a few years, the extra cost can take longer to recover than you will own the system. We would rather price two tiers side by side for your house than quote a payback period in general, so ask us for a free written estimate.
Sources
- U.S. Department of Energy, Test Procedures for Central Air Conditioners and Heat Pumps, final rule, 82 FR 1426 (2017) — Appendix M1 (SEER2) mandatory from January 1, 2023; new metrics have different numerical values; old minimum static pressure 0.10 to 0.20 in. wc., new 0.50 in. wc. as more representative of field conditions; SEER2 definition
- International Code Council, New 2023 Efficiency Requirements for Air Conditioning Systems (2022) — Northern minimum “14.0 SEER or 13.4 SEER2”; Southeast moved from 14.0 SEER to 14.3 SEER2; pre-2023 split systems that do not meet the new standard cannot be installed in the Southeast after January 1, 2023
- eCFR, 10 CFR 430.32(c), energy conservation standards for central air conditioners and heat pumps — Tennessee in the Southeast region; 14.3 SEER2 below 45,000 Btu/h and 13.8 at or above for split air conditioners installed from January 1, 2023; prior Southeast minimum 14 SEER
- NOAA NESDIS 150-TN, Tennessee State Climate Summary 2022 — Summer highs of 85°F to 90°F in western and central Tennessee; warm, humid summers
- ENERGY STAR, Duct Sealing flyer — 20 to 30 percent of duct air lost in typical homes; leaky ducts reduce efficiency by as much as 20 percent
- ENERGY STAR, Duct Sealing — Ducts in an attic, crawlspace or garage among the signs of poorly performing ducts
- ENERGY STAR, HVAC Quality Installation — Size from the home’s actual characteristics, not a rule of thumb; oversized equipment cycles too frequently; poor airflow means overly damp air; charge testing; duct leakage test
FAQ
What SEER2 Actually Means for Your Bill — questions
Is a 15 SEER2 unit worse than my old 16 SEER?
Slightly, by a fraction of a point, though not by the full point the labels suggest. SEER2 tests at a higher static pressure, so the same equipment scores lower than it did under the old method. The federal minimums treat 14.0 SEER and 13.4 SEER2 as the same requirement, about four percent apart, and by that ratio a 16 SEER unit would read a little above 15 SEER2. Compare SEER2 only with SEER2.
Does high efficiency pay back in Nashville?
Warm, humid summers give an efficient system plenty of running hours, which helps it here more than it would further north. The biggest cut in electricity comes at the bottom of the range: replacing an old 10 SEER system cuts the electricity for the same cooling by about a third or a little more, while stepping from 15 to 18 SEER2 cuts it by about a sixth. Whether a higher tier earns back its extra cost within the life of the equipment depends on the price difference, the house and how long you stay.
Does SEER2 tell me anything about humidity?
Not directly, which is a real limitation in Middle Tennessee. SEER2 is an energy-efficiency ratio, the heat removed over a cooling season divided by the electricity used. Moisture removal is part of that heat, but the number does not tell you how well a system keeps indoor humidity down. Ask how the quoted system was sized and how its airflow will be set, since ENERGY STAR ties oversized equipment to frequent cycling and poor airflow to overly damp air.
What is a good SEER2 rating?
Above the federal floor of 14.3 SEER2 for most split air conditioners in Tennessee, our view for most Nashville houses is a moderate SEER2 in a two-stage or variable-speed unit, sized from a load calculation. The top tier makes more sense in a large house you plan to keep for years, with ducts that test tight.