ASHP vs. GSHP
Rated efficiency values for Air Source Heat Pumps (HSPF and SEER) are calculated based on standard test conditions that often fail to reflect local climate extremes.
To perform an objective comparison against Ground Source systems, engineers must adjust these rated values using climate-specific regressions. This guide provides the coefficients and equations for determining degraded ASHP performance as a function of Outdoor Air Temperature (OAT).
To determine the heating performance at a specific temperature (OAT), first calculate the difference (Diff) using the quadruple-constant regression:
1) Diff = a + (b × OAT) + (c × OAT²) + (d × Rated HSPF)
2) Adj. HSPF = [1 - (Diff)] × Rated HSPF
HSPF COEFFICIENTS
| Rated HSPF Range | a | b | c | d |
|---|---|---|---|---|
| Rated HSPF < 8.5 | 0.1392 | -0.00846 | -0.000107 | 0.0228 |
| Rated HSPF ≥ 8.5 | 0.1041 | -0.008862 | -0.0001153 | 0.02817 |
Variables: OAT = -11°F | Rated HSPF = 8.0
Diff = 0.1392 + (-0.00846 × -11) + (-0.000107 × (-11)²) + (0.0228 × 8) = 0.40171
Adj. HSPF = [1 - 0.40171] × 8 = 4.79
Cooling performance degradation is adjusted using a three-constant regression against the rated SEER value:
1) Diff = a + (b × OAT) + (c × Rated SEER)
2) Adj. SEER = [1 - (Diff)] × Rated SEER
SEER COEFFICIENTS
| Rated SEER Range | a | b | c |
|---|---|---|---|
| Rated SEER < 13.5 | -0.5655 | 0.005414 | 0.01039 |
| Rated SEER ≥ 13.5 | -0.5864 | 0.005668 | 0.01029 |
Variables: OAT = 85°F | Rated SEER = 20.0
Diff = -0.5864 + (0.005668 × 85) + (0.01029 × 20) = 0.10118
Adj. SEER = [1 - 0.10118] × 20 = 17.9