7.6-i

APPENDIX 7.6. OVERALL AIR-MOVING EFFICIENCY

TABLE OF CONTENTS

7.6.1 INTRODUCTION ................................................... 7.6-1

7.6.2 RESULTS ......................................................... 7.6-5

LIST OF TABLES

Table 7.6.2.2 Coefficients for Overall Air-Moving Efficiency of Condensing 

Generic Model Furnaces ........................................ 7.7-6

LIST OF FIGURES

Figure 7.6.1.1 Overall Air-Moving Efficiency (non-condensing, cooling mode) ........ 7.6-2

Figure 7.6.1.2 Overall Air-Moving Efficiency (non-condensing, low-fire mode) ........ 7.6-2

Figure 7.6.1.3 Overall Air Moving Efficiency (non-condensing, heating mode) ......... 7.6-3

Figure 7.6.1.4 Overall Air-Moving Efficiency (condensing, cooling mode) ............ 7.6-3

Figure 7.6.1.5 Overall Air Moving Efficiency (condensing, heating mode) ............ 7.6-4

Figure 7.6.1.6 Overall Air-Moving Efficiency (condensing, low-fire mode) ............ 7.6-4
7.6-1

APPENDIX 7.6. OVERALL AIR-MOVING EFFICIENCY

7.6.1 INTRODUCTION

The overall air-moving efficiency is a variable in the calculation of blower motor

electricity consumption. The overall air-moving efficiency is a ratio of the air power divided by

the electric power used by the blower motor. Air power is the power embodied in the air due to

its motion and pressure increase. Air power at any operating condition can be calculated for any

furnace from pressure, airflow, and supply air outlet area, as explained in Chapter 7, section

7.5.3, Overall Air-Moving Efficiency.

In addition to the pressure and airflow data that are available for all furnace models, one

manufacturer reports fan motor electricity consumption as well.1, 2, 3, 4, 5, 6 The model series used

for these calculations are Lenox G23 and G40 for non-condensing furnaces and Lenox 90UGF,

G26Q, and GHR26Q for condensing furnaces. For these models, DOE calculated air power and

overall efficiency for each point in the fan operating tables for each of the models.

The relationships between air power and overall efficiency are only valid for these

particular furnace models and vary by airflow capacity. To generalize the relationship of overall

air-moving efficiency to airflow in order to calculate efficiency for any furnace type, the

Department transformed airflow to a percentage of airflow at free flow. Free flow signifies no

pressure difference or drop. The free flow percentage transformation allowed all of the curves to

be plotted together independently of cfm measurement. Figures 7.6.1.1 through 7.6.1.6 show the

overall air-moving efficiency as a function of airflow, where airflow is indicated as fraction of

free flow.
7.6-2

0%

5%

10%

15%

20%

25%

30%

40% 50% 60% 70% 80% 90% 100%

Airflow (as percent of free flow)

Overall Air Moving Efficiency

Lennox Series G23 & G40

Cooling Data

Predicted Efficiency

Figure 7.6.1.1 Overall Air-Moving Efficiency (non-condensing,

cooling mode)

0%

5%

10%

15%

20%

25%

30%

40% 50% 60% 70% 80% 90% 100%

Airflow (as percent of free flow)

Overall Air Moving Efficiency

Lennox Series G23 & G40

Low Fire Heating Data

Predicted Efficiency

Figure 7.6.1.2 Overall Air-Moving Efficiency (non-condensing,

low-fire mode)
7.6-3

0%

5%

10%

15%

20%

25%

30%

40% 50% 60% 70% 80% 90% 100%

Airflow (as percent of free flow)

Overall Air Moving Efficiency

Lennox Series G23 & G40

High Fire Heating Data

Predicted Efficiency

Figure 7.6.1.3 Overall Air Moving Efficiency (non-condensing,

heating mode)

0%

5%

10%

15%

20%

25%

30%

40% 50% 60% 70% 80% 90% 100%

Airflow (as percent of free flow)

Overall Air Moving Efficiency

Lennox Series 90UGF, G26Q, &

GHR26Q Cooling Data

Predicted Efficiency

Figure 7.6.1.4 Overall Air-Moving Efficiency (condensing,

cooling mode)
7.6-4

0%

5%

10%

15%

20%

25%

30%

40% 50% 60% 70% 80% 90% 100%

Airflow (as percent of free flow)

Overall Air Moving Efficiency 

Lennox Series 90UGF, G26Q, &

GHR26Q High Fire Heating Data

Predicated Efficiency

Figure 7.6.1.5 Overall Air Moving Efficiency (condensing,

heating mode)

0%

5%

10%

15%

20%

25%

30%

40% 50% 60% 70% 80% 90% 100%

Airflow (as percent of free flow)

Overall Air Moving Efficiency

Lennox Series 90UGF, G26Q, &

GHR26Q Low Fire Heating Data

Predicted Efficiency

Figure 7.6.1.6 Overall Air-Moving Efficiency (condensing,

low-fire mode)
7.6-5

Also shown in each figure is a single curve that best fits the other curves in each plot. 

The Department fit these curves to an equation of overall efficiency as a function of the ratio of

airflow to free airflow. The form of the equation was selected to match the general trends of the

overall efficiency curves of the specific models. The Department found the best fit between the

data points from the manufacturers' literature and the equation by adjusting the coefficients to

minimize the root-mean-square error in overall efficiency.

() ()( ) ( )( ) ηoverall c c Q c Q c Q = + ×− + ×− + ×− 0 11 0 21 0 1/2 31 0 1/3

where:

η = overall air-moving efficiency, overall

Q0 = ratio of airflow to free flow, and 

cccc 0123 ,,, = empirically determined coefficients.

7.6.2 RESULTS

The Department calculated the results separately for condensing and non-condensing

furnaces, and for each model. The coefficients for the six equations are shown in Tables 7.6.2.1

and 7.6.2.2.

Table 7.6.2.1 Coefficients for Overall Air-Moving Efficiency of Non-Condensing Generic

Model Furnaces 

Cooling Heating Low-Fire

Coef0 0.015169 .0096357 0.010040

Coef1 -0.21144 -0.49263 -0.75554

Coef2 0.91056 1.1948 1.4069

Coef3 -0.39518 -0.42712 -0.49662

Table 7.6.2.2 Coefficients for Overall Air-Moving Efficiency of Condensing Generic Model

Furnaces 

Cooling Heating Low-Fire

Coef0 0.012659 0.0090140 0.0079380

Coef1 -0.74379 -0.94689 -0.90500

Coef2 1.7392 1.8971 1.7061

Coef3 -0.83602 -0.79362 -0.65437
7.6-6

REFERENCES

1. Lennox Industries Inc., Lennox Engineering Data: G26 (Elite 90 Series Upflow Gas

Furnace), Bulletin #210027, 1998. Lennox Industries Inc. 

2. Lennox Industries Inc., Lennox Engineering Data: GHR26 (Elite 90 Series

Horizontal/Downflow Gas Furnace) Bulletin # 210093, 1998. Lennox Industries, Inc. 

3. Lennox Industries Inc., Lennox Industries Engineering Data: 90UGF (Elite 90-S Upflow

Gas Furnace) Bulletin #210171., 1997. Lennox Industries Inc. 

4. Lennox Industries Inc., Lennox Engineering Data: G23 (Elite 80 Upflow Gas Furnace)

Bulletin #210026, 1998. Lennox Industries Inc. 

5. Lennox Industries Inc., Lennox Engineering Data: G40DF (Merit Series Downflow Gas

Furnace) Bulletin No. 210320., 2002. Lennox Industries, Inc. (Last accessed July,

2003). <http://www.lennoxcommercial.com/tech_pdf/ehb_g40df_0206.pdf>

6. Lennox Industries Inc., Lennox Engineering Data: G40UH (Merit Series

Upflow/Horizontal Gas Furnace) Bulletin No. 210320., 2002. Lennox Industries, Inc.

(Last accessed July, 2003).

<http://www.lennoxcommercial.com/tech_pdf/ehb_g40uh_0206.pdf>