SEER2, HSPF2, and AFUE Explained
Every rating plate and AHRI certificate speaks in acronyms. Here is what each number actually measures, why the M1 test method shaved points off the old SEER and HSPF figures in 2023, and the minimum ratings you can and cannot legally install.
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In This Guide
What Each Rating Measures
All three ratings describe the same idea from different angles: how much useful heating or cooling you get for the energy you put in. What changes is the type of equipment and the unit of energy going in.
- SEER2 & EER2 — cooling efficiency. BTU of cooling delivered per watt-hour of electricity. Higher is better.
- HSPF2 — heat-pump heating efficiency over a season. BTU of heat delivered per watt-hour. Higher is better.
- AFUE — furnace fuel efficiency. The percentage of the fuel's energy that ends up as usable heat in the home. Higher is better.
The "2" suffix (SEER2, EER2, HSPF2) marks ratings produced under the DOE's current M1 test procedure, effective January 1, 2023. AFUE was not changed by M1 — a furnace tested at 96% AFUE reads 96% before and after. The M1 revision only touched the airflow-driven ratings for air conditioners and heat pumps.
SEER2 and EER2 (Cooling)
SEER2 (Seasonal Energy Efficiency Ratio 2) is a seasonal average. It models a full cooling season across a range of outdoor temperatures and part-load conditions, then reports total cooling output divided by total electrical input.
Definition
A 15.2 SEER2 unit delivers 15.2 BTU of cooling for every watt-hour it consumes, averaged across the season.
EER2 (Energy Efficiency Ratio 2) is the same ratio but at a single, fixed peak condition: 95°F outdoor, 80°F indoor dry bulb / 67°F wet bulb. It tells you how the unit performs on a design-day scorcher, not over a mild season. A unit can carry a high SEER2 (great part-load efficiency from a variable-speed compressor) yet a modest EER2 (it works harder at peak). For hot, dry climates and utility rebate programs, EER2 is often the number that matters most.
SEER2 vs EER2 in one line
SEER2 is the season-long "fuel economy" number; EER2 is the "how it does towing a load uphill on a 95°F day" number. Both matter, and both dropped slightly under M1.
HSPF2 and COP (Heat Pump Heating)
HSPF2 (Heating Seasonal Performance Factor 2) is the heating-mode twin of SEER2. It captures total heat delivered over a heating season, including the penalty for defrost cycles and any electric-resistance backup heat that kicks in when the compressor can't keep up.
Definition
Because backup strip heat and defrost drag the average down, HSPF2 numbers are much lower than SEER2 numbers — a typical residential heat pump lands around 7.5–9 HSPF2.
COP (Coefficient of Performance) is the instantaneous, unitless version of the same concept: heat output divided by energy input at a specific condition. A COP of 3.0 means the heat pump moves three units of heat for every one unit of electricity it draws — impossible for a resistance heater, whose COP is fixed at 1.0. Manufacturers publish COP at rating points (commonly 47°F and 17°F outdoor). Watch the low-temperature COP: that is what tells you how the unit behaves on the coldest mornings, when capacity and efficiency both fall off.
Field reality
HSPF2 is a national-average assumption. A homeowner in Minnesota will see far more strip-heat runtime than the rating implies, while the same unit in Georgia may beat it. Always pair the HSPF2 number with the manufacturer's low-temp capacity and COP tables when you size a cold-climate job.
AFUE (Gas & Oil Furnaces)
AFUE (Annual Fuel Utilization Efficiency) is the simplest of the three: the percentage of fuel energy that becomes usable heat over a year, with the rest lost up the flue and through cycling. It does not include the electricity the blower and inducer draw — that is a separate consideration.
- 80% AFUE — standard-efficiency, single flue, atmospheric or induced-draft. 20% of the fuel goes up the chimney.
- 90–98.5% AFUE — condensing furnaces. A secondary heat exchanger pulls latent heat out of the flue gas, which is why they need a PVC vent and a condensate drain.
The 90% line is the practical dividing line in the field: below it, the furnace vents hot dry combustion gas through metal (Category I); at or above it, the flue gas cools enough to condense, so you get a plastic vent, a condensate trap, and a very different install. If you are working on the drainage side of a condensing furnace, the condensate rules matter as much as the efficiency rating.
Quick gut check
PVC/CPVC intake and exhaust plus a condensate line = 90%+ condensing furnace. Single metal B-vent with no drain = 80% class. The venting tells you the AFUE class before you ever read the plate.
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What the M1 Test Actually Changed
The single biggest change from SEER to SEER2 was the external static pressure (ESP) the equipment is tested against. The old test ran units at a low, unrealistic ESP that most real duct systems never see. The M1 procedure roughly quintuples the default external static pressure to better reflect what happens once a coil, filter, and real ductwork are bolted on.
| Parameter | Old Test (pre-2023) | M1 Test (2023+) |
|---|---|---|
| Default external static (typical ≤ 5 ton) | ~0.10 in. w.c. | ~0.50 in. w.c. |
| Cooling rating name | SEER / EER | SEER2 / EER2 |
| Heat-pump heating rating | HSPF | HSPF2 |
| Reflects real duct load? | Barely | Much closer |
| Furnace AFUE affected? | — | No change |
Higher test static means the blower has to work harder, so the measured efficiency drops. Nothing about the physical equipment changed — a unit that was 16 SEER on the old bench simply reads lower as SEER2 because it is now judged under tougher, more honest conditions. This is why you cannot compare a 2021 SEER number head-to-head with a 2024 SEER2 number; they are two different rulers.
Don't get burned on a quote
A customer comparing a "16 SEER" old brochure to your "15.2 SEER2" proposal may think you downgraded them. They are essentially the same machine. Explain that the rating scale changed, not the equipment.
Converting Old Ratings to New
The DOE published approximate conversion factors so the industry could translate the legacy ratings. These are ballpark multipliers, not exact — actual results vary by model — but they get you close for a field conversation.
Rule-of-thumb conversion factors
- SEER2 ≈ SEER × 0.95 (about a 4.5–5% reduction)
- EER2 ≈ EER × 0.96
- HSPF2 ≈ HSPF × 0.85 (heating took the biggest hit, roughly 15%)
Worked example
A homeowner hands you an old spec sheet for a heat pump rated 16 SEER / 9.0 HSPF and asks how it stacks up against today's numbers.
Step 1: Convert the cooling rating
SEER2 = 16 × 0.95 = 15.2 SEER2
Step 2: Convert the heating rating
HSPF2 = 9.0 × 0.85 = 7.65 HSPF2
Step 3: Translate for the customer
"Your old 16 SEER / 9.0 HSPF unit is about a 15.2 SEER2 / 7.65 HSPF2 machine by today's scale. A modern 15.2 SEER2 system is genuinely equivalent, not a step down."
For the reverse direction on an AHRI certificate, just divide: a 14.3 SEER2 unit is roughly 14.3 ÷ 0.95 ≈ 15.0 SEER on the old scale.
Current DOE Minimum Standards
Since January 1, 2023, split-system air conditioners carry regional minimums — the South and Southwest require higher efficiency than the North because they run more cooling hours. Heat pumps and furnaces use a single national minimum. These are the floors for equipment installed today.
| Equipment | Region | Minimum |
|---|---|---|
| Split A/C (< 45,000 BTU) | North | 13.4 SEER2 |
| Split A/C (< 45,000 BTU) | South / Southwest | 14.3 SEER2 |
| Split A/C (≥ 45,000 BTU) | South / Southwest | 13.8 SEER2 |
| Split heat pump | National | 14.3 SEER2 / 7.5 HSPF2 |
| Non-weatherized gas furnace | National | 80% AFUE* |
| Southwest EER2 add-on (dry climates) | AZ, CA, NV, NM | EER2 floor also applies |
*Furnace rule watch
DOE finalized a rule raising the non-weatherized gas furnace minimum to 95% AFUE for units manufactured on or after the 2028 compliance date, which will effectively end 80% furnace manufacturing. Until then, 80% AFUE remains legal to install. Always confirm the current effective date for your jurisdiction — this one is moving.
One important nuance: for air conditioners the minimums are enforced at the point of installation in the South, so a Northern-spec 13.4 SEER2 condenser cannot legally be installed in a Southern state. Heat pumps and furnaces are enforced at the point of manufacture, so any compliant unit can ship anywhere.
Field Notes & Common Questions
Does a higher SEER2 unit save energy if the ductwork is bad?
Not nearly as much as the label promises. Because M1 now tests at realistic static, an oversized-restriction duct system will still choke a 17 SEER2 unit down toward its low-airflow, low-efficiency corner. Fix the duct static first — a right-sized system on good ductwork beats a premium system fighting 1.0 in. w.c. of total external static every time.
Why is the HSPF2 so much lower than the SEER2 on the same unit?
They use different scales and heating is inherently penalized by defrost and backup strip heat. A 15.2 SEER2 / 7.5 HSPF2 unit is normal — you are not comparing apples to apples, and the heating number already bakes in real-world losses.
Which number do I quote a customer chasing a rebate?
Read the rebate program's fine print. Many utility and ENERGY STAR programs now specify SEER2, EER2, and HSPF2 thresholds together. In hot-dry climates the EER2 minimum is often the hardest one to hit, so verify all three off the AHRI certificate for the exact indoor/outdoor combination you are installing.
Is a 96% AFUE furnace always worth it over an 80%?
In cold climates with high gas usage, the fuel savings usually pay back the price difference. In mild climates with short heating seasons, the condensate handling, PVC venting, and secondary heat exchanger maintenance can outweigh the savings. Match the AFUE to the heating load, not the brochure.
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