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CHAPTER 4. ENERGY USE CHARACTERIZATION 

TABLE OF CONTENTS 

4.1 INTRODUCTION ........................................................................................................... 4-1

4.2 DEVELOPMENT OF FLEOH ESTIMATES BY BUILDING TYPE AND 

GEOGRAPHIC REGIONS ............................................................................................. 4-1

4.3 DEVELOPMENT OF ENERGY USE ESTIMATES FROM FLEOHS ......................... 4-3

4.3.1 Rated Thermal Efficiency for Energy Use Estimates ....................................... 4-3

4.3.2 Calculation of Seasonal Energy Consumption ................................................. 4-6

4.3.3 Estimated Boiler Usage Weights by State and Building Type ......................... 4-6

4.4 ENERGY ANALYSIS SUMMARY RESULTS ............................................................ 4-7

REFERENCES ............................................................................................................................ 4-8

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LIST OF TABLES 

Table 4.2.1 Boiler Full Load Equivalent Operating Hours by 11 Sub-Census Divisions by 

Building Type ...................................................................................................... 4-2

Table 4.2.2 Augmented Census Division and Component States ............................................... 4-3

Table 4.3.1 Large Gas-Fired Hot-Water Commercial Packaged Boiler Efficiency Levels ......... 4-4

Table 4.3.2 Large Oil-Fired Hot-Water Commercial Packaged Boiler Efficiency Levels .......... 4-4

Table 4.3.3 Fraction of Floorspace Using Boilers by Region and Building Type ....................... 4-7

Table 4.4.1 National Average Site Energy Consumption Estimates by Boiler Class and 

Efficiency Level ................................................................................................... 4-7

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CHAPTER 4. ENERGY USE CHARACTERIZATION 

4.1 INTRODUCTION

The engineering analysis in TSD chapter 3 provides estimates of contractor costs for 

commercial packaged boilers that meet the rated thermal or combustion efficiency levels as 

required by American Society of Heating, Refrigerating and Air-Conditioning (ASHRAE) 

Standard 90.1-20071

 (the baseline level 0 for the life-cycle cost [LCC] analysis) or higher 

standard levels under consideration by U.S. Department of Energy (DOE) and as would be 

determined under the relevant test procedure, Hydronics Institute (HI) Boiler Testing Standard 

(BTS) 2000. 2 However, to perform the LCC analysis as discussed in chapter 5 of the TSD and 

the national impacts analyses discussed in chapter 7 of the TSD, DOE must develop estimates of 

annual energy consumption for users of commercial packaged boilers that correspond to the 

baseline and subsequent higher-efficiency levels as possible standards. 

DOE estimated the energy consumption for the 10 commercial packaged 

boiler equipment classes analyzed in the engineering analysis (TSD chapter 3). For the Notice of 

Proposed Rulemaking analyses, these energy consumption estimates were based on the use of 

Full Load Equivalent Operating Hour (FLEOH) estimates developed for 7 types of commercial 

buildings and 11 geographic regions of the continental United States, and the rated thermal 

efficiency of the boilers as estimated by DOE. For high-efficiency condensing boilers, where the 

rated efficiency depends on the return-water temperature, DOE developed a simplified model of 

the annual efficiency of a condensing boiler used to serve a typical hydronic coil supplying heat 

to a building heating, ventilation, and air-conditioning (HVAC) air distribution system. This 

chapter describes the development of annual energy use estimates for the selected equipment for 

analysis from TSD chapter 3. 

DOE examined various standard levels for each class of equipment, including the 

baseline level (current Energy Policy and Conservation Act of 1975 level), the ASHRAE 

Standard 90.1-2007 efficiency level, and up to four efficiency levels above the ASHRAE 

Standard 90.1-2007. For equipment classes where ASHRAE Standard 90.1-2007 defines 

combustion efficiency and where chapter 3 identifies higher efficiency standard levels using 

combustion efficiency, DOE developed corresponding estimates for thermal efficiency for each 

combustion efficiency level identified. From the thermal efficiency estimates and the FLEOH 

data, DOE developed annual energy consumption estimates for each efficiency level by State and 

building type. Finally using building stock estimates developed from the DOE Commercial 

Building Energy Consumption Survey (CBECS)3

, DOE was able to generate national energy use 

estimates by boiler class and efficiency level. 

4.2 DEVELOPMENT OF FLEOH ESTIMATES BY BUILDING TYPE AND 

GEOGRAPHIC REGIONS 

DOE uses the energy use characterization analysis to assess the per-unit energy savings 

potential of commercial boilers at different efficiency levels. This analysis estimates the energy 

use of commercial boilers at specified efficiency levels by using previously calculated FLEOH 

metrics by building type and by climate across the United States. FLEOHs are effectively the 

number of hours that a system would have to run at full capacity to serve a total heating load 

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equal to the annual heating load on the equipment. Boiler FLEOHs are calculated as the annual 

heating load divided by the equipment capacity. In the case of the FLEOHs used in this analysis, 

the FLEOH values were based on simulations documented for the “Screening Analysis for 

EPACT-Covered Commercial HVAC and Water-Heating Equipment” (hereafter called the 2000 

Screening Analysis)4

 and used 7 different building types and 11 different U.S. climates. This 

resulted in 77 unique FLEOH values defined for commercial boilers. 

In the 2000 Screening Analysis, these 11 U.S. climates were in turn mapped to 11 

geographic regions of the United States using defined weighting factors for each climate 

developed for each region. These regions consist of seven of the nine U.S. Census divisions, 

each composed of groups of States with no overlap, and four sub-census divisions. The subcensus divisions were developed by breaking the Pacific Census division into northern and 

southern geographic regions, approximately at the Oregon-California border, and breaking the 

Mountain Census division similarly into northern and southern geographic regions. The 

separation into sub-census divisions is done to reflect climate and energy cost differences 

characteristic of the northern and southern regions of the Pacific and Mountain Census divisions. 

For the purposes of this chapter, the set of 11 geographic regions is referred to as the sub-census 

divisions. 

Table 4.2.1 shows the commercial boiler FLEOHs by these sub-census divisions as 

developed under the 2000 Screening Analysis. 

Table 4.2.1 Boiler Full Load Equivalent Operating Hours by 11 Sub-Census Divisions by 

Building Type 

Building Type 

Augmented Census 

Division Assembly Education 

Food 

Service Lodging Office Retail Warehouse 

New England 2042.1 939.8 1591.5 1713.6 739.0 605.3 1094.1 

Middle Atlantic 2011.2 939.4 1525.3 1642.2 767.9 681.5 993.8 

East North Central 2176.8 1137.3 1833.4 1988.4 1101.6 1201.1 2111.6 

West North Central 2156.6 1067.1 1705.8 1917.7 1075.5 1005.1 1484.0 

South Atlantic 1151.7 531.0 898.4 735.4 376.3 342.9 1024.4 

East South Central 1443.2 650.7 1048.0 912.2 388.2 314.5 342.1 

West South Central 1172.2 556.5 978.0 750.3 368.4 257.8 331.5 

Mountain North 1968.1 924.2 1453.2 1474.2 577.5 577.6 698.6 

Mountain South 1001.8 349.0 788.5 373.3 205.8 40.9 36.0 

Pacific North 2640.9 1357.0 1995.3 2202.4 968.6 1234.2 1855.5 

Pacific South 1402.1 642.3 979.7 655.8 320.9 187.1 36.7 

DOE had no further geographic resolution for the FLEOH analysis, but presumed that the 

FLEOHs shown in Table 4.2.1 effectively represent average values for the defined region and 

could be used to represent the typical FLEOHs for States within each region. Each region was 

defined by the group of States composing it. The mapping of States to regions used for this 

analysis is shown in Table 4.2.2. Each mapped State is assigned the characteristic FLEOH shown 

in Table 4.2.1 for the corresponding augmented census division in Table 4.2.2. 

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Table 4.2.2 Augmented Census Division and Component States 

New England Middle Atlantic 

East North 

Central 

West North 

Central South Atlantic 

East South 

Central 

Connecticut 

Maine 

Massachusetts 

New Hampshire 

Rhode Island 

Vermont 

New Jersey 

New York 

Pennsylvania 

Illinois 

Indiana 

Michigan 

Ohio 

Wisconsin 

Iowa 

Kansas 

Minnesota 

Missouri 

Nebraska 

North Dakota 

South Dakota 

Delaware 

Dist. of Columbia 

Florida 

Georgia 

Maryland 

North Carolina 

South Carolina 

Virginia 

West Virginia 

Alabama 

Kentucky 

Mississippi 

Tennessee 

West South Central Mountain North Mountain South Pacific North Pacific South 

Arkansas 

Louisiana 

Oklahoma 

Texas 

Colorado 

Idaho 

Montana 

Wyoming 

Arizona 

Nevada 

New Mexico 

Utah 

Oregon 

Washington 

Alaska* 

California 

Hawaii* 

*

 Alaska and Hawaii are technically not part of the defined U.S. Census divisions, but have been included with the nearest 

augmented census divisions for the purpose of assigning FLEOHs. 

4.3 DEVELOPMENT OF ENERGY USE ESTIMATES FROM FLEOHS 

For each equipment class, DOE estimated the energy usage of a given piece of equipment 

by multiplying the characteristic equipment output capacity by the FLEOHs appropriate to each 

combination of representative building type and climate location. The product is effectively the 

total annual heat output from the boiler. The input energy is then determined by dividing the 

annual heat output by the thermal efficiency of the equipment at each efficiency level. The 

thermal efficiency is used here for all equipment classes because it defines the relationship 

between energy input and useful output of a commercial packaged boiler. 

4.3.1 Rated Thermal Efficiency for Energy Use Estimates 

For the two equipment classes where a thermal efficiency (Et) metric was not specified 

by ASHRAE Standard 90.1-2007, and subsequently not indicated for higher efficiency levels in 

TSD chapter 3, an estimate of the thermal efficiency of equipment meeting the combustion 

efficiencies (Ec) specified in TSD chapter 3 was developed based on DOE’s market analysis. For 

these two equipment classes, DOE developed estimates of the rated thermal efficiency based on 

an estimated 1.5 average percentage point difference (or delta (∆)) between thermal and 

combustion efficiency for large, gas-fired hot-water boilers (see Table 4.3.1) and an estimated 2 

average percentage point difference between thermal and combustion efficiency for large, oilfired hot-water boilers (see Table 4.3.2). These efficiency deltas correspond to approximately the 

average differences between thermal and combustion efficiencies for these classes of equipment 

in the DOE boiler database discussed in chapter 2 of the TSD. 

In addition, for condensing hot-water boilers, it is recognized that the thermal efficiency 

of a commercial packaged boiler in actual use depends on the return-water conditions. This is 

particularly true in the case of condensing boilers, where most condensing boilers will not 

condense unless the return-water temperature is lower than some threshold or they are operating 

in a low-fire regime, or both. In turn, the return-water conditions are dependent upon the 

hydronic system design and control. For DOE’s analysis, the rated thermal efficiencies for fully 

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Table 4.3.1 Large Gas-Fired Hot-Water Commercial Packaged Boiler Efficiency Levels 

Efficiency Level 

Combustion Efficiency 

(EC) Levels (%) 

Estimated Rated Thermal 

Efficiency (Et) Levels (%) 

Baseline Efficiency Level 0 82 80.5 

Level 1 83 81.5 

Level 2 84 82.5 

Level 3 85 83.5 

Level 4 (Condensing) 95 93.5 

Table 4.3.2 Large Oil-Fired Hot-Water Commercial Packaged Boiler Efficiency Levels 

Efficiency Level 

Combustion Efficiency 

(EC) Levels (%) 

Estimated Rated Thermal 

Efficiency (Et) Levels (%) 

Baseline Efficiency Level 0 84 82 

Level 1 86 84 

Level 2 87 85 

Level 3 88 86 

condensing equipment were further adjusted to reflect return-water conditions based on 

installation in existing buildings with conventional hydronic heating coils. In this analysis, DOE 

assumed that most boilers will go into typical hydronic or steam system applications serving 

heating coils in air handlers (either centrally or via fan coils or unit ventilators). Furthermore, 

DOE assumed that most of the installations at the time the energy conservation standards would 

go into place are retrofit (approximately two-thirds of the market). DOE assumed that most of 

the existing systems using hot-water boilers have been designed for typical hydronic system 

design conditions of 180 °F supply water and 160 °F return water (20 °F ∆T) to meet peak 

heating loads, and that most systems have some type of hot-water reset control already or the 

reset control can be easily added. 

DOE also assumed that new condensing hot-water boilers going into place would at a 

minimum be fitted with existing or new hot-water reset capability, which would allow the 

supply-water temperature to be reset lower than that used for design conditions to provide for 

higher-efficiency boiler operation when building loads permit. 

In a conventional gas-fired boiler, condensation in the flue will occur with flue gas 

temperatures below approximately 130 °F, and supply-water temperature reset control can at best 

bring the boiler-water delivery temperature down to approximately 135 °F, but below that it 

would cause raining (condensation) to occur in a conventional boiler. So in practice, the 

conventional hydronic system isn’t designed to supply-water temperature below 140 °F and 

supply-water temperature is not reset below 140 °F to avoid problems with condensation. 

At a 140 °F supply-water temperature, the temperature difference between the return air 

at approximately 60 °F is 80 °F. At 180 °F supply-water temperature the temperature difference 

between the supply-water and return air is 120 °F. So under the 140 °F supply-water temperature 

condition, the overall total temperature difference between air stream and supply-water stream is 

reduced to 67 percent of what it is at design conditions, with a corresponding decrease in coil 

capacity. This simple analysis allows one to estimate the outdoor temperature suitable for the 

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reset schedule. In this example, the reset schedule is to deliver design supply-water temperature 

at the outdoor design conditions of 0 °F or below, and to deliver 140 °F water temperature at an 

outdoor air temperature corresponding to 67 percent of the system load or at any higher outdoor 

air temperature. Assuming that the heating load is linear from some building balance point of 

zero load (~50 °F for a typical commercial building application) and 100 percent load (based on 

an outdoor design temperature of 0 °F.), the 67-percent load point occurs at an outdoor air 

temperature of approximately 16 °F. 

A reset schedule in this conventional boiler should provide that the supply-water 

temperature at an outdoor condition of 16 °F or higher would be at least 140 °F. In practice, 

many reset schedules are not this aggressive because peak building loads may depend on 

building pickup or other conditions. Nevertheless, for outdoor air temperatures above a defined 

set point (16 °F) the conventional boiler supply temperature remains fixed at no less than 140 °F. 

For outdoor air temperatures below 16 °F, the load on the system is such that the supplywater temperature must be above 140 °F to allow the hydronic coils to meet the load, and no 

condensing can occur in the system regardless of boiler design (condensing or non-condensing 

boiler). Above an outdoor temperature of 16 °F, the reset controls are fixed in the conventional 

boiler system to prevent condensation from occurring. The return-water condition in this 

example is calculated, based on the 67-percent load on the coil, to have approximately 67 percent 

of the design coil delta T, or to be 13.4 °F below the supply-water temperature, or 123.6 °F. In 

practice, this temperature may be too low for proper boiler operation for the non-condensing 

boiler and the supply-water temperature would be raised to compensate for the low return-water 

temperature. 

It is not necessary for a condensing boiler hold the supply-water temperature artificially 

high (to prevent flue corrosion) when a high supply-water temperature it is not needed to satisfy 

the coil loads. A condensing boiler is designed to allow for condensation to occur in the boiler 

and consequently delivers cooler flue gases to the flue system. Determination of whether 

condensation occurs is based on the return-water temperature. Condensing boilers commonly 

use a counter flow heat exchange design where the flue temperature gases are exposed to the 

lowest water temperature (i.e., the return-water temperature)a

 to maximize heat exchange before 

being exhausted to the flue. The reset schedule for the condensing boiler is modified to allow for 

even lower supply-water temperatures at higher outdoor air temperatures. These lower supplywater temperatures can continue to meet the expected building load, but because condensation is 

allowed to occur in the system, the operating efficiency of the boiler is higher under these low 

load conditions than with the conventional boiler. The seasonal efficiency of the condensing 

boiler is determined by the fraction of the building heating load being served while operating in a 

non-condensing mode (typically with supply-water temperatures above approximately 135 to 

140 °F) and operating in a condensing mode (supply-water temperatures below approximately 

135 °F). 

 

a

 Cooke, J. 2005. “Condensing Boiler Technology.” Powerpoint presentation by Jim Cooke, Mechancial Systems 

N.W. to Puget Sound ASHRAE Chapter. 

<www.pugetsoundashrae.org/PDF_files/AshraeCondensingtechnology.ppt> (Accessed on September 12, 2008). 

EERE-2008-BT-STD-0013 COMMENT 06
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DOE estimated that in a typical commercial application where a boiler that serves an 

existing air handler is being replaced and where there is no modification of the hydronic coils, 

but allowing for a revised reset strategy, a seasonal average thermal efficiency for a condensing 

boiler would be approximately 88 percent (see TSD Appendix 4-A for calculations). DOE 

recognized that more expansive system retrofits that included coil replacement or applications to 

other types of hydronic systems (e.g., heated slab) could result in significantly higher seasonal 

efficiencies, but at increased cost, and such retrofits were not assumed to occur under DOE’s 

high-efficiency standards scenario. Based on this estimate, the seasonal thermal efficiencies for 

small and large gas-fired hot-water boilers at efficiency Level 4 were modified to be 88 percent 

for the purposes of the annual energy calculations. The seasonal efficiencies for the other boiler 

classes were assumed to be represented by the rated thermal efficiencies. 

4.3.2 Calculation of Seasonal Energy Consumption 

DOE calculated annual energy consumption as follows: 

AEC = 1000 x BOILCAP x FLEOH / EFFseas Eq. 4.1 

where 

 AEC = Annual Energy Consumption (million Btu/yr) 

 BOILCAP = Boiler output capacity (kBtu/hr) 

 FLEOH = Full Load Equivalent Operating Hours for Location and Building Type 

 EFFseas = Seasonal Thermal Efficiency 

DOE calculated annual energy consumption for each market segment defined by the 

FLEOH for each combination of State and building type for each efficiency level. 

4.3.3 Estimated Boiler Usage Weights by State and Building Type 

DOE developed estimated weights by State and building type for commercial boiler sales 

based on data found in the CBECS and data reported in the DOE Energy Information 

Administration’s (EIA’s) National Energy Modeling System (NEMS). First, DOE developed 

estimates for the fraction of building stock by each census region for 2014 based on data from 

the EIA NEMS as developed for the Annual Energy Outlook 20085

, which provides floorspace 

projections for each of the seven building types and for each census division of the country. The 

floorspace for the Mountain and Pacific Census divisions were then disaggregated into the subcensus divisions (mountain-north, mountain-south, pacific-north, pacific-south) using the 

identical fraction of floorspace allocated to each region as determined in the 2000 Screening 

Analysis (64 percent of the Mountain Census division is allocated to mountain north, 36 percent 

to mountain south; 18 percent of Pacific Census floorspace is allocated to pacific north, 

82 percent to pacific south). 

The fraction of commercial floorspace using commercial packaged boilers differs by 

region of the country and by building type. DOE used the equipment shares matrix for 

commercial packaged boilers developed for the 2000 Screening Analysis, which provided the 

estimated fraction of commercial building floorspace using boilers for each of the 11 sub-census 

divisions (see Table 4.3.3). 

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Table 4.3.3 Fraction of Floorspace Using Boilers by Region and Building Type 

Building Type 

Sub-Census 

Division Assembly Education 

Food 

Service Lodging Office Retail Warehouse 

New England 0.36 0.76 0.07 0.41 0.42 0.25 0.14 

Mid-Atlantic 0.36 0.76 0.07 0.41 0.42 0.25 0.14 

East N. Central 0.36 0.76 0.07 0.41 0.42 0.25 0.14 

West N. Central 0.36 0.76 0.07 0.41 0.42 0.25 0.14 

South Atlantic 0.35 0.46 0.06 0.24 0.21 0.05 0.04 

East S. Central 0.35 0.46 0.06 0.24 0.21 0.05 0.04 

West S. Central 0.35 0.46 0.06 0.24 0.21 0.05 0.04 

Mountain-North 0.42 0.62 0.18 0.31 0.29 0.18 0.08 

Mountain-South 0.27 0.40 0.02 0.50 0.31 0.03 0.04 

Pacific-North 0.42 0.62 0.18 0.31 0.29 0.18 0.08 

Pacific-South 0.27 0.40 0.02 0.50 0.31 0.03 0.04 

The product of the floorspace projections by building type and sub-census division and 

the fraction of the floorspace using boilers results in an estimate of the total floorspace using 

boilers in 2014 by building type and sub-census division. This value is then normalized to 

provide the fraction of floorspace using boilers to 1,000 ft2

 of total floorspace in the nation. 

Finally, within each sub-census division, the allocated floorspace using boilers was 

further disaggregated between States using relative population data from the 2000 Census. The 

result is an estimate of, for each 1,000 ft2 of floorspace using commercial boilers in the nation, 

the relative fraction of that floorspace that would be allocated to each State and each of the 7 

building types. This estimate was then used as a proxy for the relative shipments of boilers to 

market segment (i.e., each State and building type combination) for which it derived FLEOH 

estimates as discussed previously. 

4.4 ENERGY ANALYSIS SUMMARY RESULTS 

Table 4.4.1 shows summary results for national average energy consumption estimates by 

efficiency level and boiler class. 

Table 4.4.1 National Average Site Energy Consumption Estimates by Boiler Class and 

Efficiency Level 

Equipment Class 

National Average Site Energy Consumption (Million Btu/yr) 

Baseline 

Level 0 Level 1 Level 2 Level 3 Level 4* 

Small Gas-Fired Hot Water 1077 1051 1026 1002 979 

Small Gas-Fired Steam All Except Natural Draft 1091 1077 1064 1051 1038 

Small Gas-Fired Steam Natural Draft 1119 1105 1091 1077 na 

Small Oil-Fired Hot Water 1051 1026 1002 979 na 

Small Oil-Fired Steam 1064 1051 1038 1014 na 

Large Gas-Fired Hot Water 4013 3964 3916 3869 3671 

Large Gas-Fired Steam All Except Natural Draft 4090 4039 3989 3940 3893 

Large Gas-Fired Steam Natural Draft 4196 4142 4090 4039 3989 

Large Oil-Fired Hot Water 3940 3846 3801 3757 na 

Large Oil-Fired Steam 3989 3940 3893 3846 3757 

*

 na means that no efficiency level was designated for analysis. 

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REFERENCES 

1. American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). 

2008. Energy Standard for Buildings Except Low-Rise Residential Buildings. 

ANSI/ASHRAE 90.1-2007, ASHRAE, Atlanta, Georgia. 

2. Hydronics Institute of GAMA. Testing Standard BTS-2000 (Rev06.07) – Method to 

Determine Efficiency of Commercial Space Heating Boilers. 2007.

3. U.S. Department of Energy/Energy Information Administration (DOE/EIA). 2003. 

Commercial Buildings Energy Consumption Survey. Available at: 

http://www.eia.doe.gov/emeu/cbecs/ (Accessed September 2008). 

4 U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy (DOEEERE). 2000. Energy Conservation Program for Consumer Products: Screening Analysis 

for EPACT-Covered Commercial HVAC and Water-Heating Equipment Screening Analysis. 

DOE-EERE, Washington, D.C. 

5. U.S. Department of Energy/Energy Information Administration (DOE/EIA). 2006. Annual 

Energy Outlook 2006. DOE/EIA-0383(2006), DOE/EIA, Washington D.C.

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ABBREVIATIONS AND ACRONYMS 

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning 

BTS Boiler Testing Standard 

DOE U.S. Department of Energy 

EIA (DOE) Energy Information Administration 

FLEOH Full Load Equivalent Operating Hour 

HI Hydronics Institute 

HVAC heating, ventilation, and air-conditioning 

LCC life-cycle cost 

NEMS National Energy Modeling System 

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