AFUE, SEER2 and HSPF: what the efficiency numbers actually mean
Three different scales, measuring three different things. Knowing which one applies stops you comparing numbers that were never comparable.
The short answer
Three different scales, measuring three different things. Knowing which one applies stops you comparing numbers that were never comparable.
- AFUE is a percentage: how much of the fuel a furnace or boiler turns into heat in your house rather than up the flue.
- SEER2 measures cooling output over a season divided by the electricity it took. Higher is better, with diminishing returns.
- HSPF measures a heat pump's heating season. A unit has both a SEER2 and an HSPF because it does both jobs.
- SEER2 replaced SEER in 2023 using a higher external static pressure in the test, so SEER2 numbers read lower than the old SEER for identical equipment.
AFUE — combustion efficiency
AFUE stands for Annual Fuel Utilization Efficiency and applies to anything that burns fuel: gas furnaces, oil furnaces, boilers. It is a straightforward percentage. An 80% AFUE furnace turns 80% of the fuel's energy into heat delivered to the house; the other 20% leaves through the flue as exhaust.
The meaningful dividing line is 90%. Below it, a furnace vents hot exhaust up a metal flue. At 90% and above — a condensing furnace — a second heat exchanger pulls so much heat out of the exhaust that the water vapour in it condenses. The exhaust is then cool enough to vent through PVC pipe out a sidewall, and the condensate needs a drain.
That is why replacing an 80% furnace with a 96% one is rarely just a swap. The venting changes, and a condensate drain has to go somewhere. It is a normal part of the job, but it belongs in the quote rather than appearing as a surprise.
SEER2 — and why it made every number drop
SEER2 is Seasonal Energy Efficiency Ratio, version two: cooling delivered across a season divided by the electricity consumed. It is a ratio, not a percentage, and it cannot exceed reality — a higher number means less electricity for the same cooling.
In January 2023 the Department of Energy replaced SEER with SEER2. The equipment did not change; the test did. SEER2 runs the unit against roughly five times the external static pressure of the old test, which is much closer to what real ductwork imposes. The result is that the same physical air conditioner scores lower under SEER2 than it did under SEER.
This matters when you are comparing quotes or reading older articles. A 16 SEER unit and a 15.2 SEER2 unit may be the same machine. Compare SEER2 to SEER2, and treat any spec sheet that mixes them as a reason to ask questions.
HSPF — the heat pump's other half
A heat pump both cools and heats, so it carries two ratings: SEER2 for the cooling season and HSPF (now HSPF2) for the heating season. HSPF is heat delivered over the season divided by electricity consumed.
Because a heat pump moves heat rather than creating it, its heating efficiency can exceed 100% in a way a furnace never can — it is not burning anything. That advantage narrows as it gets colder outside, which is the balance point conversation covered in the heat pump guide.
What the ratings do not tell you
Every one of these numbers comes from a laboratory. None of them account for ductwork that leaks into an unconditioned attic, a return that is undersized, or a system that was specified two sizes too large for the house.
This is the honest limit of shopping by rating: a well-installed mid-efficiency system routinely outperforms a premium unit installed badly. The rating tells you the ceiling. The installation determines how close you get to it.
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