HSPF to HSPF2 Conversion
Convert a pre-2023 HSPF rating into the HSPF2 figure the same heat pump would carry under the current DOE test — so a spec sheet from the old scale can be compared with one from the new.
Calculator
An old-test rating of HSPF 10.0 is roughly HSPF2 8.50 — a documented approximation: the DOE 2023 test measures the same equipment at a higher external static pressure, so the number drops by about 15%.
HSPF2 replaced HSPF when the DOE’s 2023 test procedure took effect. Both rate heating-season efficiency in BTU per watt-hour, but the new test runs the equipment against a higher external static pressure — closer to a real duct system — so the same heat pump measures lower on the new scale. That makes an old HSPF number and a new HSPF2 number non-comparable as printed.
This converter restates an old HSPF rating on the HSPF2 scale, so a unit you already own (or an older quote) can be judged against today’s ratings and minimums.
Formula
The documented approximation used here:
HSPF2 ≈ HSPF × 0.85
Equivalently, HSPF ≈ HSPF2 ÷ 0.85. The 0.85 factor is the commonly cited average for ducted split systems; it is an approximation, not a conversion defined by the standard — the true ratio depends on the equipment’s design and how much the higher test pressure costs it.
Approximation only. HSPF2 is the DOE 2023 seasonal test basis. Where a unit has been formally re-rated, use its published HSPF2 rather than a converted figure.
Worked example
Convert an old rating of HSPF 10:
- HSPF2 ≈ 10 × 0.85 = 8.5
So a heat pump advertised at HSPF 10 under the old test sits around HSPF2 8.5 today — still comfortably above the 7.5 HSPF2 DOE 2023 minimum for split systems, but a weaker number than “HSPF 10” sounds against current labels.
Why the same heat pump rates lower on HSPF2
The conversion changes the number, not the equipment. A heat pump does not become less efficient because the test changed; the old figure simply flattered it by measuring against less duct resistance than a real house imposes. This is why comparing an old HSPF against a new HSPF2 directly makes the newer unit look worse than it is.
Treat 0.85 as a central estimate rather than a fixed rule. Equipment that was already tested near realistic static pressure loses less; designs that depended on the easier test lose more. When both numbers are published for a unit, use the published pair instead of this conversion.
To turn a rating into money rather than another rating, feed the HSPF2 figure into the heating-savings calculation with your own heating load and electricity rate.
Reference table
| Old HSPF | Approx. HSPF2 (HSPF × 0.85) |
|---|---|
| 7.7 | 6.55 |
| 8.0 | 6.80 |
| 8.5 | 7.23 |
| 9.0 | 7.65 |
| 9.5 | 8.08 |
| 10.0 | 8.50 |
| 11.0 | 9.35 |
| 12.0 | 10.20 |
| 13.0 | 11.05 |
Documented approximation (HSPF2 ≈ HSPF × 0.85). The DOE 2023 minimum for split-system heat pumps is 7.5 HSPF2.