· Michael Blair
Efficiency ratings were restated in 2023, and the new SEER2 numbers are lower than the old SEER numbers for identical equipment. That is a change in the test, not a change in the machine, and it has confused a great many replacement decisions.
Why the numbers moved
The old test measured performance at a static pressure that did not reflect real duct systems. SEER2 tests at a higher external static pressure, closer to what equipment actually faces in a house.
The result is a number typically around four to five percent lower for the same unit. A 15 SEER2 system is broadly equivalent to the old 15.7 or so, not a downgrade.
What higher efficiency buys here
Middle Tennessee has a long cooling season, which is exactly the condition under which efficiency pays back. The same upgrade returns more here than it would in a cooler market.
The gains are not linear though. Moving from an old 10 SEER unit to 15 SEER2 is a large step; moving from 15 to 18 SEER2 is a much smaller one for a much larger price.
What matters more than the rating
Correct sizing, duct condition and installation quality routinely swamp the difference between two efficiency tiers. A high-efficiency unit on leaky ducts in an unconditioned attic will underperform a mid-tier unit installed well.
Ask what static pressure the installer measured and what they intend to do about it. A contractor who has not measured is guessing at the one variable that determines whether the rating means anything.
- Correct sizing beats a higher rating
- Duct leakage can waste a fifth or more of output
- Variable-speed helps humidity, which SEER2 does not capture
- Ask for measured static pressure before and after
Deciding what tier to buy
For most Nashville houses, the mid tier — a two-stage or variable-speed unit at a moderate SEER2 — is the sweet spot. It captures most of the efficiency and all of the humidity benefit.
The top tier makes sense in a large house with a long expected ownership and good ducts. In a smaller house or a shorter stay, the payback period usually outruns the ownership.