7-A-i

APPENDIX 7-A. DEVELOPMENT OF HSPF IMPROVEMENT DEGRADATION 

FACTOR

TABLE OF CONTENTS

7-A.1 Introduction............................................................................................................7-A-1

7-A.2 Development Methodology ...................................................................................7-A-1

LIST OF TABLES

Table 7A.1. Coefficients for %HSPF Decrease (used in equation 7A.1)................................7-A-2

Table 7A.2. Regional HSPF Improvement Degradation Factor..............................................7-A-4
7-A-1

APPENDIX 7-A. DEVELOPMENT OF HSPF IMPROVEMENT DEGRADATION 

FACTOR

7-A.1 Introduction

Heat pump annual heating performance is affected by climate. There are two reasons for 

this. First, both the efficiency of compressor heating and the available output capacity of 

compressor heat are reduced with cooler outdoor temperatures and greater refrigerant lift. 

Second, because the heat pump rating is a rating of the total heating system, including electric 

backup heat, a greater fraction of the heat provided by the heat pump in cold climates occurs at 

colder temperatures when the compressor-heat capacity is low and where the remainder of the 

heat provided is electric resistance heat. Electric resistance heating has an effective COP of 1.0--

lower than the compressor heating COP--so when the fraction of heat that is electric resistance is 

increased; the average COP (and the corresponding seasonal HSPF for that climate) decreases. 

DOE, in establishing the HSPF rating procedure, provided climate data for six climate 

regions (I – VI) so manufacturers could develop regional HSPF ratings to inform consumers. 

However, manufacturers do not typically provide ratings at other than the standard rating 

condition established for Region IV in the DOE test procedure.a

7-A.2 Development Methodology

 For the residential heat pump 

preliminary analysis, DOE received comment that simply scaling the heating load with the rated 

HSPF would not address the climate dependence of heating performance. For the NOPR 

analysis, DOE developed a methodology to estimate the improvement in seasonal heating 

efficiency for a heat pump for a specific climate based on a nominal HSPF improvement. This 

methodology is described in this appendix.

For the residential analysis, DOE received comment that simply scaling the heating load 

with the rated HSPF would not address the climate dependence of heating performance. To 

address this, DOE developed a regional HPSF scaling factor which was incorporated in the 

residential energy analysis. This methodology was based on a published study of how heat pump 

seasonal heating performance is affected by climate.

1

The simulation analysis showed that the 1% design temperature and rated nameplate 

HSPF are sufficient to achieve reasonable estimates of the climate dependent heating seasonal 

performance. The analysis showed that a significant decrease in nameplate HSPF (~50%) will 

occur in severe climates like Minneapolis, MN, but that in very mild climates like Miami, FL, 

the achieved heating seasonal performance exceeds the nominal HSPF. The analysis also 

suggested that there was a greater percentage discrepancy in climate-specific seasonal heating 

performance and the nominal HPSF rating for higher nominal HPSF ratings. The study 

 This study used residential building 

simulations to calculate seasonal heating performance factors for residences in 15 U.S. climates 

for four HSPF ratings. The climates selected had from 149 (Miami, FL) to 7876 (Minneapolis, 

MN) heating degree days. 

 a Canada, with a cooler average climate, requires ratings for region V for compliance. However, data available for 

certified products seems to indicate that a region V rating allowed for compliance is simply 15% lower HSPF than 

the region IV HSPF rating used by the U.S. 
7-A-2

developed a regression model to calculate the percentage degradation in climate-specific HSPF 

as a function of the ASHRAE 1% heating design day temperature and the nominal HSPF rating. 

Two equations were provided, one to represent HSPF values <8.5 and one to represent HPSF 

values 8.5 or higher. These are shown in equation 7-A.1 below.

b

%HSPF Decrease = a + b*Tempi + c*Temp2

i + d*HSPF Eq. 7-A.1

Where

Tempi = 1% ASHRAE Heating Design Temperature for Climate i. in °F

HSPF = Nominal HSPF Rating

and a, b, c, d are coefficients as shown in Table 7A-1.

Table 7A.1. Coefficients for %HSPF Decrease (used in equation 7A.1)

Coeff: HSPF < 8.5 HSPF => 8.5

a = 0.1392 0.1041

b = -0.008460 -0.008862

c = -0.0001074 -0.0001153

d = 0.02280 0.02817

In the analysis of residential heating energy with higher standards, the starting point is the 

RECS estimate of heating energy, not building load. Thus, for the same RECS observation in 

the same climate, it is not the intrinsic reduction in heating seasonal performance due to a colder 

climate that is important in the energy savings analysis. For example, if all HSPF ratings were 

simply reduced by 15 percent for a cold region, it would have no bearing in the energy savings 

estimate for a higher HSPF. Rather, it is the how that percentage reduction in a specific climate 

differs as a function of nominal HSPF rating that is important. 

The results of the simulation study for heat pumps were also used to develop the Heating 

Fuel Comparison Calculator spreadsheet tool (heatcalc.xls), available on the DOE/EIA website.2

 

The heatcalc.xls tool provides estimated climate specific HSPF values for various locations in 

the U.S. based on a nominal HSPF rating. DOE modified the heatcalc.xls calculator to provide a 

mapping between regional seasonal heating performance (climate specific HSPF) and nominal 

rated HSPF values for each of 237 TMY2 locations using ASHRAE Heating Design Day data3

DOE then developed HSPF improvement degradation factors (DFHSPF_clim_i ) for each 

nominal HSPF and for each climate location. The HSPF degradation factors are calculated for 

nominal HSPF improvements above 7.7 HSPF and are based on the ratio of relative 

improvement in climate specific HSPF (based on the corresponding climate specific HSPF value 

corresponding to a particular HSPF rating divided by a climate specific HSPF value 

corresponding to an HSPF of 7.7) divided by the ratio of the improvement in the nominal HSPF 

ratings. See Equation 7A-2. Once the climate specific degradation factors have been calculated 

. 

 b The same study reports a similar analysis for SEER ratings, but notes that while the seasonal heating performance 

was strongly depending on climate, seasonal cooling performance was less affected. The analysis did not appear to 

have taken into account multistage equipment or variable speed air handlers
7-A-3

for a range of HSPF values above the baseline, one can use the DF factor to estimate the 

improvement in climate specific HSPF from the improvement in nominal HSPF. This 

improvement in climate specific HSPF can, in turn, be used to better estimate the reduction in 

heat pump heating energy for specific climates due to a change in nominal HSPF ratings.

































= ÷ no al

j no al

c i

j c i

HSPF j c i HSPF

HSPF

HSPF

HSPF

DF

7.7 _ min

_ min

7.7 _ lim_

_ lim_

_ _ lim_ Eq. 7-A.2

Where 

DFHSPF_j_clim_i =Degradation Factor for specific HSPFj for specific climate i.

HSPFj_clim_i =Climate Specific HSPF in location i corresponding to a nominal HSPFj being 

evaluated.

HSPF7.7_clim_i = Climate Specific HSPF in location i corresponding to a nominal HSPF of 7.7. 

HSPFj_nom = Nominal value of HSPFj being evaluated.

HSPF7.7_nom = nominal HSPF of 7.7 (equal to 7.7)

The RECS observations do not correspond to particular locations. DOE developed 

aggregate regional degradation factors for each of the 17 RECS geographic regions (13 census 

divisions with four large states--New York, California, Texas, Florida--broken out, plus those 

four states independently). To do this it used the climate specific HSPF ratings calculated for the 

237 TMY2 locations and the fraction of U.S. population represented by those locations, 

normalized for the total population weight for all of the states within a given RECS geographic

regions (See Appendix 7-C for calculation of population weights). This provided insight into the 

climate-specific HSPF variation at the geographic resolution provided by RECS. 

DOE used the household weights assigned to each RECS observation with a heat pump to 

further develop regional HSPF degradation factors for the northern region, the hot-humid region, 

and the hot-dry region. These regional degradation factors were then incorporated into the 

energy analysis of heat pump heating energy savings. Table 7A-2 shows the average degradation 

factors by region. Because the HSPF improvement degradation factors are simple ratios, it is 

possible to use these same factors to calculate the relative improvement in going from any pair of 

nominal HSPF ratings. It is also noted that the values shown in Table 7-A-2 take into account 

the relative number of RECS households with heat pumps in each of the 17 regions and are not 

purely the result of the differences in climates. 
7-A-4

Table 7A.2. Regional HSPF Improvement Degradation Factor

HSPF Rating Regional HSPF Improvement Degradation Factor

Rest of Country Hot-Humid Hot-Dry National

6.8 1.0286 1.0217 1.0188 1.0226

7.0 1.0223 1.0169 1.0146 1.0176

7.2 1.0159 1.0121 1.0104 1.0126

7.4 1.0095 1.0072 1.0063 1.0075

7.7 1.0000 1.0000 1.0000 1.0000

7.8 0.9968 0.9976 0.9979 0.9975

7.9 0.9936 0.9952 0.9958 0.9950

8.0 0.9905 0.9928 0.9937 0.9925

8.1 0.9873 0.9903 0.9917 0.9899

8.2 0.9841 0.9879 0.9896 0.9874

8.3 0.9809 0.9855 0.9875 0.9849

8.4 0.9777 0.9831 0.9854 0.9824

8.5 0.9622 0.9844 0.9945 0.9816

8.6 0.9583 0.9814 0.9920 0.9785

8.7 0.9543 0.9785 0.9894 0.9754

8.8 0.9504 0.9755 0.9868 0.9723

8.9 0.9465 0.9725 0.9842 0.9692

9.0 0.9425 0.9695 0.9817 0.9661

9.1 0.9386 0.9665 0.9791 0.9630

9.2 0.9347 0.9635 0.9765 0.9599

9.3 0.9307 0.9606 0.9739 0.9568

9.4 0.9268 0.9576 0.9714 0.9537

9.5 0.9229 0.9546 0.9688 0.9506

9.6 0.9190 0.9516 0.9662 0.9475

9.7 0.9150 0.9486 0.9636 0.9444

9.8 0.9111 0.9456 0.9611 0.9413

9.9 0.9072 0.9427 0.9585 0.9381

10.0 0.9032 0.9397 0.9559 0.9350

10.1 0.8993 0.9367 0.9533 0.9319

10.2 0.8954 0.9337 0.9508 0.9288

 
7-A-5

REFERENCES

 

1. Fairey, P., D.S. Parker, B. Wilcox and M. Lombardi, "Climate Impacts on Heating 

Seasonal Performance Factor (HSPF) and Seasonal Energy Efficiency Ratio (SEER) for 

Air Source Heat Pumps." ASHRAE Transactions, American Society of Heating, 

Refrigerating and Air Conditioning Engineers, Inc., Atlanta, GA, June 2004. (Also 

available at http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-413-04/)

2. DOE/EIA 2009. Heating Fuel Comparison Calculator (Heatcalc.xls V. HEAT-CALC-VsnD_1-09.xls). <www.eia.doe.gov/neic/experts/heatcalc.xls> (Accessed October 10, 2010)

3. ASHRAE 2009. 2009 ASHRAE Handbook of Fundamentals. American Society of Heating 

Refrigerating and Air Conditioning Engineers, Atlanta, GA.