7-B-i

APPENDIX 7-B. SYSTEM CURVE DERIVATION FOR FURNACE FANS

TABLE OF CONTENTS

7-B.1 INTRODUCTION ................................................................................................. 7-B-1

7-B.2 FURNACE FANS REFERENCE SYSTEM CURVES ........................................ 7-B-2

7-B.3 FIELD STUDIES................................................................................................... 7-B-3

7-B.4 DETERMINATION OF REFERENCE SYSTEM CURVES FOR DOE 

FURNACE FAN TEST PROCEDURE ................................................................ 7-B-4

7-B.4.2 Other reference system curves............................................................................... 7-B-6

7-B.5 DISRIBUTION OF SYSTEM CURVES USED IN LCC ANALYSIS ................ 7-B-6

LIST OF TABLES

Table 7-B.3.1 Single Family Field Data ESP at Cooling Airflow Setting.................. 7-B-3

Table 7-B.3.2 Manufactured Home Field Data........................................................... 7-B-4

Table 7-B.4.1 Evaporator Coil Pressure Data............................................................. 7-B-4

Table 7-B.4.2 Filter Pressure Data.............................................................................. 7-B-4

Table 7-B.4.3 Fraction of Installations with Evaporator Coil in ESP......................... 7-B-5

Table 7-B.4.4 Summary of Weighted Average Reference System ESP Values......... 7-B-5

Table 7-B.5.1 Reference System ESP Distribution Parameters Values by Product 

Class..................................................................................................... 7-B-6

LIST OF FIGURES

Figure 7-B.1.1 Sample of System Curves with a Typical Fan Curve .......................... 7-B-1

Figure 7-B.5.1 External Static Pressure Distribution for Non-Weatherized Gas 

Furnace Fan; Oil Furnace Fan; Electric Furnace / Modular Blower 

Fan; Hydronic Air Handler Fan (Heat/Cool) with Central Air 

Conditioning ........................................................................................ 7-B-7

Figure 7-B.5.2 External Static Pressure Distribution for Non-Weatherized Gas 

Furnace Fan; Oil Furnace Fan; Electric Furnace / Modular Blower 

Fan; Hydronic Air Handler Fan (Heat/Cool) without Central Air 

Conditioning or Weatherized Gas Furnace Fan................................... 7-B-7

Figure 7-B.5.3 External Static Pressure Distribution for Manufactured Home NonWeatherized Gas Furnace Fans; Manufactured Home Electric 

Furnace / Modular Blower Fan with Central Air Conditioning........... 7-B-8

Figure 7-B.5.4 External Static Pressure Distribution for Manufactured Home NonWeatherized Gas Furnace Fans; Manufactured Home Electric 

Furnace / Modular Blower Fan without Central Air Conditioning...... 7-B-8
7-B-1

APPENDIX 7-B. SYSTEM CURVE DERIVATION FOR FURNACE FANS

7-B.1 INTRODUCTION

The system curve of the air-distribution system is a graphical representation of the 

airflow through the supply and return ducts in a house for different static pressures. The airflow 

and pressure drop at which the furnace will operate can be determined by the intersection of the 

system curve of the house and the fan curve of the furnace fan.1 Figure 7-B.1.1 shows an 

example of a plot of system curves intersecting a furnace fan curve.

DOE modeled system curves as quadratic curves, which is standard in heating, 

ventilation, and air conditioning (HVAC) design and fan selection handbooks.2 The curves are 

based on Bernoulli’s equations for fluid flow and are expressed as the following equation:

Q = � P

α

Where:

Q = airflow (cfm),

P = static pressure (in.w.g.), and

α = a constant coefficient.

0

400

800

1200

1600

0.00 0.25 0.50 0.75 1.00

External Static Pressure (in. w.g.)

Airflow (CFM)

Highest 1%

median

Lowest 10%

1200 CFM Generic Single-Stage

Furnace Model (Heating Tap)

Figure 7-B.1.1 Sample of System Curves with a Typical Fan Curve
7-B-2

7-B.2 FURNACE FANS REFERENCE SYSTEM CURVES

In DOE’s proposed furnace fan test procedure, the reference system curve is defined as 

follows: 

Kref = ESPref

Qmax2

 where:

Kref = a constant that characterizes the reference system;

ESPref = Reference System External Static Pressure (ESP); and

Qmax = amaxESPref

2 + bmaxESPref + cmax

The operating point in the maximum airflow-control setting is defined by the reference 

system criteria: ESPref and Qmax. External static pressure (ESP) is the portion of the fan total 

pressure that exists by virtue of degree of compression external to the HVAC product in which 

the furnace fan is contained. ESP does not include the pressure drop across appurtenances 

internal to the HVAC product. In the field, ESP is measured as the difference in pressure 

between the HVAC product inlet and outlet points, and includes the ductwork, inlet filter, and 

coil outside of the unit if applicable.

In the field there are four distinct reference system ESPs associated with specific 

equipment:

a) Units with Cooling Option (Coil Outside of Unit): This includes non-weatherized gas 

furnaces and oil-furnaces. Reference system is measured at the maximum default cooling airflow 

control setting. ESP includes a fraction of units in the field that include the evaporator coil.

b) Heating only units: This includes heating-only hydronic air-handlers.

a Reference system is 

measured at the default heating airflow control setting. ESP does not include evaporator coil.

b) Units with Coil inside Unit: This includes weatherized gas and oil-fired furnaces. Reference 

system is measured at the maximum default cooling airflow control setting. ESP does not include 

evaporator coil.

c) Manufactured Home Units: This includes manufactured home gas or oil-fired furnaces. 

Reference system is measured at the cooling airflow control setting for these units. ESP includes 

a fraction of units in the field that include the evaporator coil.

 

a Units like First Company’s FWA series: http://www.firstco.com/products/fwa_hw.asp.
7-B-3

7-B.3 FIELD STUDIES

DOE gathered field data from available studies and research reports to determine an 

appropriate ESP value to propose for the reference system. DOE compiled over 1300 field ESP 

measurements from 27 studies that included furnace fans in single family and manufactured 

homes in different regions of the country. Table 7-B.3.1 summarizes the field data for single 

family homes at cooling airflow setting. Table 7-B.3.2 summarizes the field data for 

manufactured homes at cooling airflow setting. The average measured ESP values are the actual 

values measured in each study. Some studies did not include the evaporator coil or filter in the 

ESP measurement. To account for this DOE added the filter pressure drop to all adjusted ESP 

values (“Adj. w/o Coil” and “Adj. w/ Coil”). In addition, DOE subtracted the pressure drop of 

the evaporator coil for “Adj. w/o Coil” and added the pressure drop of the evaporator coil for 

“Adj. w/ Coil” when appropriate. See next section for the determination of average filter and 

evaporator pressure drop values.

Table 7-B.3.1 Single Family Field Data ESP at Cooling Airflow Setting

Study Sample 

Size

Average ESP (in. w.c.) Notes

Measured Adj. w/o Coil Adj. w/ Coil

Blasnik et al. 19953 40 0.41 0.41 0.61 1

Blasnik et al. 19964 28 0.48 0.48 0.68 1

Parker 19975 9 0.55 0.55 0.75 1

Proctor et al. 19956 40 0.53 0.53 0.73 1

Proctor et al. 19967 36 0.51 0.51 0.71 1

Proctor et al. 19988 15 0.45 0.45 0.65 1

Proctor 19989 36 0.42 0.42 0.62 1

Proctor 200510 78 0.48 0.48 0.68 1

Proctor et al. 200711 4 1.01 0.81 1.01

Proctor 200012 5 0.50 0.50 0.70 1

Proctor 200113 69 0.54 0.54 0.74 1

Proctor 200314 69 0.53 0.53 0.73 1

Proctor 1996a15 8 0.45 0.45 0.65 1

Proctor 1996b16 92 0.31 0.52 0.73 1,2

Wilcox et. al. 200617 51 0.77 0.57 0.77

Dickenhoff 199818 13 0.54 0.54 0.74 1

Baylon et al. 200519 148 0.36 0.57 0.78 1,2

Ueno 2010 (2008 Study)20 4 0.90 0.70 0.90

Ueno 2010 (2009 Study)20 1 1.12 0.92 1.12

Pigg 2008 (2007 Study)21 76 0.73 0.53 0.73

Pigg 2008 (2005 Study)21 37 0.53 0.53 0.73 1

Pigg 200322 31 0.55 0.55 0.75 1

Weighted Average 890 0.50 0.52 0.73

1 ESP measurement includes Coil

2 ESP measurement includes Filter
7-B-4

Table 7-B.3.2 Manufactured Home Field Data

Study Sample 

Size

Average ESP (in. w.c.) Notes

Measured Adj. w/o Coil Adj. w/ 

Coil

Baylon et al. 199523 164 0.18 0.18 0.38 1

Davis et al. 200024 36 0.23 0.23 0.43 1

Davis et al. 200425 100 0.12 0.12 0.32 1

Ecotope 200626 69 0.23 0.23 0.43 1

Baylon et al. 200927 89 0.12 0.12 0.32 1

Weighted Average 458 0.17 0.17 0.37

1 ESP measurement does not include Coil

2 ESP measurement does not include Filter

7-B.4 DETERMINATION OF REFERENCE SYSTEM CURVES FOR DOE 

FURNACE FAN TEST PROCEDURE

Using field data from 3 studies, DOE estimated average filter and coil pressures in order 

to adjust field data that did not include the filter or coil. On average, the pressure drop measured 

for the evaporator coil was 0.20 in w.c. (as shown in Table 7-B.4.1) and the pressure drop for the 

filter was 0.21 in w.c. (as shown in Table 7-B.4.2). 

Table 7-B.4.1 Evaporator Coil Pressure Data

Study Sample Size Average Pressure Drop

(in. w.c.)

Pigg 2008 (2007 Study)21 75 0.20

Pigg 2008 (2005 Study)21 19 0.19

Wilcox et. al. 200617 51 0.21

Weighted Average 145 0.20

Table 7-B.4.2 Filter Pressure Data

Study Sample Size Average Pressure Drop

(in. w.c.)

Pigg 2008 (2007 Study)21 76 0.25

Pigg 2008 (2005 Study)21 37 0.21

Wilcox et. al. 200617 46 0.16

Weighted Average 159 0.21

Using EIA’s RECS 2005 data, DOE estimated the fraction of furnace installations with 

and without a coil in the ESP. For units with a cooling option (coil outside unit), DOE looked at 

all households with either a gas or oil-fired furnace and determined that 72.9% of these 
7-B-5

households had central air-conditioners.b

Table 7-B.4.3

 For manufactured home units DOE looked at all 

manufactured home households with either a gas or oil-fired furnace and determined that 50.2% 

of these households had central air-conditioners. DOE estimated that these two fractions would 

represent the fraction of installations with evaporator coil in the ESP. data shows 

the results for each of the distinct reference systems. None of the heating only units or units with 

coil inside are assumed to have an evaporator coil in the ESP.

Table 7-B.4.3 Fraction of Installations with Evaporator Coil in ESP

Product Description Fraction of Installations with 

Evaporator Coil in ESP

Units with Cooling Option (Coil Outside Unit) 72.9%

Heating only units 0%

Units with Coil inside 0%

Manufacture Home 50.2%

Table 7-B.4.4 presents the final results of this analysis for each distinct reference system 

curve used in DOE’s NOPR furnace fan test procedure. The results take into account the fraction 

of units with the coil included in the ESP. 

Table 7-B.4.4 Summary of Weighted Average Reference System ESP Values

Product Description Airflow Control 

Setting

Weighted Average ESP 

(in. w.c.)

Units with Cooling Option (Coil Outside Unit) Cooling 0.65

Heating only units Heating 0.50

Units with Coil inside Cooling 0.50

Manufacture Homec Cooling 0.30

(All Values Rounded)

The results are determined as follows:

1) for units with cooling option (coil outside unit), 72% of the furnace fans are installed 

with CAC (see Table 7-B.4.3), so 72% * 0.73 in. w.c. + 28% * 0.52 in. w.c., which is 

rounded to 0.65 in w.c.;

2) for manufactured homes furnace fans, 50% of the furnaces are installed with CAC 

(see Table 7-B.4.3), so 50% * 0.17 in. w.c. + 50% * 0.37 in. w.c., which is rounded to 

0.30 in w.c.;

3) for heating only units and units with coil inside the weighted average ESP is rounded 

from 0.52 in. w.c. to 0.50 in w.c.

 

b For simplicity, electric furnaces are excluded since they are mostly associated with heat pumps. Also, RECS does 

not provide information to distinguish which households have hydronic air-handlers. Adding electric furnaces and 

hydronic equipment will increase the fraction of households with central air-conditioners, since this equipment tends 

to be located in warmer climates.

c Manufactured home external static pressure is much smaller due to the fact there is no return air ductwork in 

manufactured homes. Also HUD requirements stipulate that the ductwork for cooling should be set at 0.3 in. w.c.
7-B-6

7-B.4.2 Other reference system curves

Manufacturers Rating for Cooling – 0.5 in.w.c. at cooling airflow setting [manufacturer 

product literature]

DOE test procedure for Cooling - 0.1 to 0.2 in.w.c. for conventional split systems [Subpart B 

Appendix M of Title 10 Part 430 of the Code of Federal Regulations (CFR)]

DOE test procedure for Furnaces – 0.12 – 0.58 in.w.c. minimum static pressure values depend 

on equipment type as follows [Subpart B Appendix N of Title 10 Part 430 of the Code of Federal 

Regulations (CFR), which references ANSI/ASHRAE 103, table 4 and 5]:

a) Gas furnaces and Oil furnaces (w/ temp rise greater than 65 deg) - 0.18 to 0.33 in. w.c. 

(depending on input capacity) 

b) Oil furnace (w/ temp rise less than or equal to 65 deg) – 0.38 - 0.58 in w.c. (depending on 

input capacity) 

c) Electric furnaces - 0.12 to 0.25 in.w.c. (dependant on Standard Air Quantity (SCFM))

Canadian Furnace Fan Standard – 0.3 inches WC (Recommended practice) and 0.6 inches 

WC (Common Practice) at heating airflow setting [CSA. C823-11: Performance of air handlers 

in residential space conditioning systems. May 2011.]

HUD for Manufactured Home with comfort cooling certificate – 0.3 inches WC at cooling 

airflow setting [Title 24 of the HUD code PART 3280--Manufactured Home Construction and 

Safety Standards, Part 3280.715 (a) (3) (ll)]

7-B.5 DISRIBUTION OF SYSTEM CURVES USED IN LCC ANALYSIS

For the LCC analysis, DOE used the field data above to generate normal distributions for 

the various equipment installation variations as shown in Table 7-B.5.1. Figure 7-B.5.1 to 

Figure 7-B.5.4 show the external static pressure distributions used in the analysis.

Table 7-B.5.1 Reference System ESP Distribution Parameters Values by Product Class

Product Classes Household 

Has 

Central AC

Average 

ESP 

(in. w.c.)

Standard 

Deviation 

(in. w.c.)

Non-Weatherized Gas Furnace Fan; Oil Furnace Fan; 

Electric Furnace / Modular Blower Fan; Hydronic Air 

Handler Fan (Heat/Cool)

Yes 0.73 0.12

No 0.52 0.12

Weatherized Gas Furnace Fan Yes 0.52 0.12

Manufactured Home Non-Weatherized Gas Furnace 

Fans; Manufactured Home Electric Furnace / Modular 

Blower Fan

Yes 0.37 0.06

No 0.17 0.06
7-B-7

Figure 7-B.5.1 External Static Pressure Distribution for NonWeatherized Gas Furnace Fan; Oil Furnace Fan; 

Electric Furnace / Modular Blower Fan; Hydronic 

Air Handler Fan (Heat/Cool) with Central Air 

Conditioning

Figure 7-B.5.2 External Static Pressure Distribution for NonWeatherized Gas Furnace Fan; Oil Furnace Fan; 

Electric Furnace / Modular Blower Fan; Hydronic 

Air Handler Fan (Heat/Cool) without Central Air 

Conditioning or Weatherized Gas Furnace Fan
7-B-8

Figure 7-B.5.3 External Static Pressure Distribution for 

Manufactured Home Non-Weatherized Gas Furnace 

Fans; Manufactured Home Electric Furnace / 

Modular Blower Fan with Central Air Conditioning

Figure 7-B.5.4 External Static Pressure Distribution for 

Manufactured Home Non-Weatherized Gas Furnace 

Fans; Manufactured Home Electric Furnace / 

Modular Blower Fan without Central Air 

Conditioning
7-B-9

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