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AFUE & HSPF Ratings Explained
SEER2 gets most of the attention because it applies to cooling equipment almost everyone shops for, but heating efficiency is measured on two entirely different scales depending on the equipment type — AFUE for furnaces and boilers, HSPF2 for heat pumps — and neither number means the same thing as SEER2 or converts directly to it, which trips up any comparison across different heating technologies.
Last updated: September 29, 2026
SEER2 gets most of the attention since it applies to the cooling equipment nearly everyone shops for, but heating efficiency lives on two entirely different scales depending on the equipment type — AFUE for furnaces and boilers, HSPF/HSPF2 for heat pumps. Neither number means the same thing as SEER2 or converts directly to it, which trips up any comparison across different heating technologies.
This guide explains what each rating actually measures, current federal minimums, and includes a worked example converting both to a common annual-cost basis for a real comparison.
The Three Scales, Side by Side
The table below shows what each rating measures and its typical range for baseline versus high-efficiency equipment.
| Rating | Applies To | What It Measures | Typical Range |
|---|---|---|---|
| AFUE | Furnaces, boilers (gas/oil/propane) | % of fuel energy converted to useful heat | ~80-84% (federal minimum, varies by fuel/equipment type, non-condensing) to ~98.5% (best condensing) |
| HSPF / HSPF2 | Heat pumps (heating mode) | Seasonal heating output (BTU) per electricity input (Wh) | ~7.5–8.5 baseline; ~9–10.5+ high-efficiency (HSPF2 scale) |
| SEER / SEER2 | Air conditioners, heat pumps (cooling mode) | Seasonal cooling output (BTU) per electricity input (Wh) | ~13–16 baseline; ~20+ high-efficiency (SEER2 scale) |
Exact current minimums are periodically revised — confirm against current DOE/AHRI program information for the specific equipment type, and, for central AC SEER2 specifically, the installation's DOE region.
Which Comparison Applies to Your Situation
The table below shows how to make an apples-to-apples comparison for the most common shopping scenarios.
| Your Situation | How to Compare | Why |
|---|---|---|
| Comparing two furnaces for the same replacement | Compare AFUE percentages directly | Both use the identical fuel-in/heat-out percentage basis |
| Comparing two heat pumps' heating performance | Compare HSPF2 numbers directly (not legacy HSPF) | HSPF2 is the current rating for equipment manufactured under the updated test procedure |
| Comparing a furnace's running cost to a heat pump's running cost | Convert both to estimated annual heating cost using local fuel/electricity rates | AFUE and HSPF2 use different units and can't be compared as raw numbers |
| Choosing between a condensing and non-condensing furnace | Weigh the AFUE gain (roughly 90-98.5% vs. 80-83%) against added plastic-venting/condensate drain complexity | The efficiency gain is real but comes with different installation requirements |
| Checking whether new heat pump equipment meets the current minimum | Confirm the current nationwide HSPF2 minimum, and separately the region-specific SEER2 minimum for its cooling mode | HSPF2 is a single nationwide standard; SEER2 is region-split — the two aren't checked the same way |
| Evaluating a heat pump's cold-climate heating performance specifically | Check its low-temperature capacity retention and balance point spec, not just HSPF2 alone | HSPF2 is a seasonal average and doesn't describe single-temperature performance |
Worked Example — Converting to a Common Cost Basis
AFUE and HSPF2 can't be compared as raw numbers, but converting both to an estimated annual operating cost puts them on equal footing.
Example — Converting AFUE and HSPF2 to a Common Cost Basis
A homeowner is comparing a 95% AFUE gas furnace against a 9.5 HSPF2 heat pump for the same home's estimated 60,000,000 BTU annual heating load, using a local natural gas rate of $1.20 per therm (100,000 BTU) and an electricity rate of $0.15/kWh (3,412 BTU per kWh).
| Step | Formula / Substitution | Result |
|---|---|---|
| Furnace: fuel energy needed | 60,000,000 BTU ÷ 0.95 AFUE | 63,157,895 BTU of fuel input |
| Furnace: annual cost | 631.6 therms × $1.20 | ≈$758/year |
| Heat pump: electricity input needed (HSPF2 as BTU output per Wh) | 60,000,000 BTU ÷ 9.5 BTU/Wh | 6,315,789 Wh = 6,316 kWh |
| Heat pump: annual cost | 6,316 kWh × $0.15 | ≈$947/year |
At these specific local utility rates, the gas furnace comes out cheaper to run annually despite the heat pump's strong HSPF2 rating — this result flips easily with a different electricity- to-gas price ratio, which is exactly why comparing the raw AFUE and HSPF2 numbers directly would have been misleading.
Common Mistakes
Comparing a Furnace's AFUE Number Directly Against a Heat Pump's HSPF2 Number
These use completely different units and scales — a furnace's 95% AFUE and a heat pump's HSPF2 of 9.5 aren't comparable as raw numbers; convert both to an estimated annual operating cost using local fuel and electricity rates instead.
Assuming a Condensing Furnace's Only Difference From Non-Condensing Is the Equipment Itself
Condensing furnaces typically need PVC or other manufacturer-approved plastic venting and a condensate drain that non-condensing units don't need — factor in this installation difference, not just the equipment price difference, when comparing the two options.
Using an Old HSPF Number Instead of HSPF2 When Comparing Current Heat Pump Models
HSPF2 numbers run lower than the legacy HSPF scale for equivalent equipment due to the updated, more realistic test procedure — comparing a new model's HSPF2 against an old spec sheet's HSPF number understates the new equipment's actual relative efficiency.
Judging a Cold-Climate Heat Pump Purely by Its HSPF2 Rating
HSPF2 is a seasonal average and doesn't describe how much heating capacity a heat pump retains at very low outdoor temperatures — check the manufacturer's published capacity-retention curve or cold-climate certification specifically if the heat pump will serve as a primary heat source in a cold climate.
Assuming Every Heating Equipment Category Has the Same Regional Minimum Structure as Central AC
Only central air conditioner SEER2 (and, in the Southwest, EER2) minimums are currently split by DOE region — heat pump HSPF2 and furnace AFUE minimums are each set nationally. Confirm which structure actually applies to the specific equipment type rather than assuming a regional split applies everywhere.
Standards and References
| Source | What It Covers |
|---|---|
| DOE 10 CFR Part 430 (AFUE test procedure) | Defines the standardized test procedure furnace and boiler manufacturers use to determine and publish AFUE ratings. |
| DOE 10 CFR Part 430, Appendix M1 (HSPF2/SEER2 test procedure) | The current test procedure, effective for equipment manufactured on or after January 1, 2023, producing HSPF2 and SEER2 ratings under more realistic external static pressure conditions than the legacy Appendix M procedure. |
| AHRI 210/240 | The industry test standard incorporating the DOE Appendix M1 procedure, used to certify and publish heat pump SEER2/HSPF2 ratings. |
| DOE regional minimum efficiency standards (central AC only) | Sets different minimum SEER2 (and, in the Southwest, EER2) requirements by US region for central air conditioners; heat pump HSPF2 and furnace AFUE minimums are each set nationally rather than by region — confirm current figures for the specific equipment type before purchase. |
| ENERGY STAR | Sets voluntary efficiency thresholds above the federal minimum for furnaces, boilers, and heat pumps. |
Final Takeaway
AFUE, HSPF2, and SEER2 each measure a different kind of equipment on a different scale — never compare the raw numbers across categories. Convert to an estimated annual operating cost using actual local fuel and electricity rates whenever comparing a furnace's heating cost against a heat pump's, and always confirm the current federal minimum efficiency requirement for the specific equipment type rather than assuming a previously known figure still applies.
- AFUE measures fuel-to-heat conversion percentage for furnaces/boilers; HSPF2 measures heat pump heating output per electricity input.
- HSPF2 replaced HSPF under the same 2023 DOE test procedure change that replaced SEER with SEER2, and reads lower for equivalent equipment.
- Condensing furnaces reach 90-98.5% AFUE by capturing flue heat, but need manufacturer-approved plastic venting (typically PVC) and a condensate drain.
- Never compare AFUE and HSPF2 (or SEER2) numbers directly — convert both to estimated annual cost using local utility rates.
- A heat pump's HSPF2 is a seasonal average and doesn't describe its capacity at any single outdoor temperature — check cold-climate specs separately.
- Only central AC SEER2 minimums are region-split by DOE; heat pump HSPF2 and furnace AFUE minimums are each set nationally — confirm the current figure for the specific equipment type before purchase.
Related calculators
Use these calculators when you need to turn this reference information into project quantities:
- AC & Furnace Size Calculator
Estimate central air conditioner tonnage and furnace BTU output/input from your square footage, climate zone, and insulation.
- Heat Pump Size Calculator
Estimate heat pump tonnage, balance point, and backup/auxiliary heat needed for Standard or Cold-Climate heat pump types.
- SEER Savings Calculator
Compare an existing system's annual energy cost against a new, more efficient unit, with a simple payback period.
Related resources
- SEER vs SEER2 Ratings Explained
Explains the difference between the old SEER rating and the current SEER2 (plus EER2 and HSPF2) ratings — the 2023 DOE test procedure change behind it, why SEER2 numbers run lower for the same equipment, and how regional minimum efficiency standards apply.
- Heat Pump vs Gas Furnace: Which Is Best
Head-to-head comparison of electric heat pumps and gas furnaces covering installed cost, running cost, efficiency (COP vs AFUE), cold-climate performance, balance point, backup heat, and dual-fuel hybrid systems.