K-2.i

APPENDIX K-2. EMISSIONS FACTORS FOR FUEL COMBUSTION FROM 

NATURAL GAS, LPG, AND OIL-FIRED RESIDENTIAL WATER HEATERS

TABLE OF CONTENTS

K-2.1 OVERVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.1

K-2.2 EMISSIONS FACTORS BY FUEL TYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.2

K-2.2.1 All Fuels: Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.2

K-2.2.2 Natural Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.3

K-2.2.2.1 Oxides of Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.3

K-2.2.2.2 Sulfur Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.3

K-2.2.3 Fuel Oil #2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.3

K-2.2.3.1 Oxides of Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.3

K-2.2.3.2 Sulfur Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.4

K-2.2.4 LPG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.4

K-2.2.4.1 Oxides of Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.4

K-2.2.4.2 Sulfur Dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.5

K-2.3 SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K-2.5

TABLE

Table K-2.1 Estimated National Average Emissions Factors for Household Fuel

Combustion of Natural Gas, Fuel Oil #2, and LPG . . . . . . . . . . . . . . . . . . . . K-2.2
*

0.454 kg = 1 lb; 1.055 GJ = 1 MBtu

K-2.1

APPENDIX K-2. EMISSIONS FACTORS FOR FUEL COMBUSTION FROM 

NATURAL GAS, LPG, AND OIL-FIRED RESIDENTIAL WATER HEATERS

K-2.1 OVERVIEW

The modified version of the National Energy Modeling System (NEMS) used for the water

heater energy-efficiency standards analysis, called NEMS-BRS, comprehensively considers a wide

range of aspects of energy use. However, this model does not consider household emissions of NOx

and SO2 from the combustion of natural gas, residential heating fuel oil #2 (i.e., high-sulfur distillate

fuel oil), and LPG. Because an efficiency standard could result in changes in these emissions, the

Department has performed some elementary research to determine appropriate emissions factors for

carbon, NOx, and SO2 from household appliances. Specifically, this work focuses on emissions rates

from the use of water heaters and is used to calculate emissions savings from proposed efficiency

standards.

This analysis attempts to verify that the emissions factors from the U.S. Environmental

Protection Agency (EPA) for carbon, NOx, and SO2 from natural gas, fuel oil #2, and LPG

combustion are representative of the U.S.1 EPA cautions against the use of these emissions factors

as representative of actual emissions. Rather, they are to be used, and are used here, as general

approximations of national average emissions factors in order to calculate emissions savings that

result from proposed water heater energy-efficiency standards. This analysis compares EPA’s

emissions factors with those from other sources and with regional regulations.

Table K-2.1 summarizes the emissions factor estimates for the three fuels of interest. The

values presented here are based on EPA’s assessment of combustion emissions (with the exception

of SO2 from LPG, see discussion below). These emissions estimates represent an approximate

emissions factor for the U.S. as a whole. They are used, together with estimated energy savings, to

calculate end-use site emissions savings. The emissions factors discussed in this analysis are

represented as mass (g or kg) of the specific emission of interest per GJ of energy input to the water

heater.*
K-2.2

Table K-2.1 Estimated National Average Emissions Factors for Household Fuel

Combustion of Natural Gas, Fuel Oil #2, and LPG

Carbon

(kg/GJ)

NOx

(g/GJ)

SO2

(g/GJ)

Natural Gas 13.8 42 0

Fuel Oil #2 18.7 61 218

LPG 16.0 66 7

There are significant differences in state regulations, making it difficult to generalize about

localized emissions. For residential fuel oil combustion, stricter state standards exist in the

northeastern United States and southern California than elsewhere. State and local agencies like

southern California’s South Coast Air Quality Management District (SCAQMD), the New Jersey

Department of Environmental Protection, and the New York Department of Environmental

Conservation have established regulations for residential fuel combustion emissions. These

regulations, however, are local to individual counties and do not apply to all regions in the Northeast

or even all of California. 

K-2.2 EMISSIONS FACTORS BY FUEL TYPE

K-2.2.1 All Fuels: Carbon

NEMS-BRS tracks emissions of carbon as CO2, but factors are included here for

completeness. The combustion of natural gas, fuel oil #2, and LPG oxidizes virtually all carbon in

the fuel to CO2. This facilitates estimating carbon emissions factors because the carbon content of

the fuel is the only determinant of emissions. The carbon estimates in Table K-2.1 are based on

EPA’s assessment of carbon combustion from natural gas, fuel oil #2, and LPG use. Other sources

of information include the U.S. DOE’s Energy Information Administration (EIA), Oregon State EPA,

and the Gas Research Institute (GRI). All agree to within ± 2.0 % of EPA’s assessed estimate of

carbon emissions. EPA’s carbon emissions factors for natural gas, fuel oil #2, and LPG combustion

are therefore a reasonable evaluation of nationally averaged carbon emissions rates.

Carbon monoxide (CO) can be formed by incomplete carbon oxidization, causing a severe

health hazard. CO emissions only result from poorly functioning appliances, and since NEMS-BRS

assumes that all water heaters are functioning properly, it is unlikely that these emissions will change.
K-2.3

K-2.2.2 Natural Gas

K-2.2.2.1 Oxides of Nitrogen

Emissions of NOx that result from the combustion of natural gas from appliances are also

covered by regulations in some jurisdictions. The EPA’s estimated NOx emissions factor of 42 g/GJ

from gas-fired water heaters is equal to the regulation set by the state of New York. This is

approximately 20% higher than the stricter standard set by SCAQMD for southern California.

SCAQMD provides extensive research into emissions factors 2 and California regulations tend to

influence many other states. New Jersey is the only other state that is known to have NOx emissions

regulations for water heaters, with a limit of 86g of NOx/GJ of heat input. The absence of regulations

in other states suggests that most have emissions rates comparable to EPA’s estimated value. 

K-2.2.2.2 Sulfur Dioxide

SO2 emissions from natural gas combustion are assumed to be zero.

K-2.2.3 Fuel Oil #2

K-2.2.3.1 Oxides of Nitrogen

NOx formation in combustion processes is significantly more complex than SO2 or CO2.

NOx can result from oxidization of N2 in the fuel, but the larger source is N2 in the combustion intake.

This process is sensitive to combustion chemistry, and therefore, quite variable. It also implies that

appliance design can lower NOx emissions. An added difficulty is that the potential hazard of

residential NOx emissions is also hard to quantify. NOx is both an acid precipitation and ozone (O3)

precursor, but because residential appliances are a relatively small and dispersed (i.e., hard to

monitor) source, these emissions tend to play a small role in NOx control strategies. The O3 hazards

of NOx emissions are highly variable both spatially and over time, as weather conditions favor or

disfavor O3 formation. One would, therefore, expect NOx emissions rules to vary significantly across

jurisdictions.

The value estimated by EPA of 61 g of NOx/GJ is for distillate oil-fired boilers with a

combustion rate less than 106 GJ/h 3

 (or 100 MBtu/h) of heat input. This combustion type is

characteristic of residential water heaters. Additional research into specific state regulations reveals

good matches with EPA’s emissions factor: New Hampshire limits NOx emissions to 60 g/GJ for

non-utility boilers and New York and New Jersey regulations cap emissions at 52 g of NOx/GJ. These

state limits confirm that EPA’s estimate can serve as a reasonable national average.
K-2.4

K-2.2.3.2 Sulfur Dioxide

Emissions of SO2 are a direct function of the sulfur content of the fuel. Assuming complete

combustion of sulfur to SO2 makes it easy to assess and compare emissions across data sources.

Individual state and county regulations limiting the amount of sulfur in residential fuel oil, however,

reveal substantial variability in SO2 emissions factors from EPA’s estimate of 218g of SO2/GJ of fuel

energy content. This is roughly equivalent to 0.5% sulfur content by weight using an emissions factor

for fuel oil #2 of 43,950 g/GJ weight per unit energy content. A straight multiplication of the fuel

energy content and the percentage of sulfur content in the fuel provide the emissions factor. 

In the state of New York, SO2 emissions limits range from 88-659 g/GJ depending on the

county. In New Hampshire, the emissions rate of SO2 from high-sulfur distillate fuel oil combustion

cannot exceed 176 g/GJ. In Maine and Michigan, the regulations are higher than EPA’s assessment,

limiting SO2 emissions from fuel oil to 500 g of SO2/GJ of heat input. Such broad ranges and varying

limits in certain regions are a strong argument for using EPA’s averaged national value over a

regionally weighted average incorporating the various state and county regulations.

Comparing the sulfur content of fuel oil from different refineries also demonstrates regional

variability. Most sources were obtained through online websites detailing product specifications.

Mobil Corporation’s residential heating fuel oil (#2 high-sulfur) for most states contains a maximum

0.5% sulfur by mass (219 g of SO2/GJ of heat input), which is very close to EPA’s emissions factor.

Mobil also markets fuel with lower sulfur content to some regions in the Northeast. For example,

Mobil’s lowest sulfur content residential heating fuel oil contains 0.2% sulfur by weight (88 g/GJ),

as required in New York City, Philadelphia, parts of Delaware, and most of New Jersey. Other

regional regulations adhered to by Mobil are 132 g/GJ for most of Pennsylvania and parts of New

Jersey, a limit of 127 g of SO2/GJ of heat input for the state of Illinois, and an SO2 emissions factor

of 145 g/GJ in Massachusetts. The sulfur contents of distillate fuel oil from Chevron, Phillips 66, and

the American Society for Testing and Materials (ASTM) exactly match EPA’s estimate,

substantiating the use of the latter for the analysis. Considerable variation does exist among counties

and states, especially in the Northeast. Nonetheless, EPA provides a reasonable estimate of emissions

from residential oil-fired water heaters that is representative of the national average. 

K-2.2.4 LPG

K-2.2.4.1 Oxides of Nitrogen

Although NES results are presented for LPG, EPA estimates emissions for only propane and

butane, fuels within the LPG family of gases. In the United States, the composition of LPG is

typically 90% propane (C3H8) by liquid volume, 5% propylene (C3H6), and 2.5% butane (C4H10).4

This analysis uses EPA’s propane emissions estimate to verify its comparability with LPG

combustion or determines a more suitable estimate from other sources that best represents LPG

emissions rates.
K-2.5

EPA estimates an emissions factor of 66 g of NOx/GJ of energy input from propane-fired

water heaters, which is slightly higher than the natural gas estimate. The University of New

Hampshire’s Energy Office estimates an emissions factor of 43 g/GJ from LPG combustion for the

state of New Hampshire. In contrast, the state of New Jersey’s EPA establishes an 86 g/GJ emissions

factor. Both sources, therefore, support EPA’s national assessment of NOx emissions from residential

propane combustion.

K-2.2.4.2 Sulfur Dioxide

The sulfur content of LPG is very low, 5

although a few regulations exist in certain parts of the

country. EPA’s SO2 emissions estimate is derived from the sulfur content of the propane fuel. With

90% of LPG comprised of propane, the EPA’s propane emissions factor is a reasonable value for LPG

emissions rates. Under the New Hampshire Department of Environmental Services (DES) regulation,

the maximum sulfur content allowable is 13.3 g/GJ of SO2 from LPG combustion. The San Diego

County Air Pollution Control District enforces a stricter limit of only 8.8 g of SO2/GJ of energy input

using EPA’s recommended value. Both the ASTM and Santa Barbara County Air Pollution Control

District specify a sulfur standard of 0.0185% sulfur by weight, which approximates an emissions

factor of 4.0 g of SO2/GJ of energy input. This calculation is based on an LPG weight per unit energy

of 21,756 g/GJ. Even lower, Nett Technologies Inc. estimates a 0.012% sulfur content by mass LPG

average for the United States as well as Canada, which is approximately 2.6 g of SO2/GJ of heat input.

 

The wide range of emissions factors for SO2 makes it difficult to determine a representative

estimate for the emissions factor and the very small net effect of any energy-efficiency standard argue

against making a significant research effort. The obtained estimates range in magnitude from

approximately 4 to 13 g/GJ of energy input. The statistical average or arithmetic mean of these four

estimates is 7.2 g/GJ, the value provided in Table K-2.1. The averaged emissions factor estimate of

7.2 g/GJ energy input is reasonable to explain three of the five sources presented and suitably

accounts for the average among the two extreme estimates. Table K-2.1 therefore adequately

represents national SO2 emissions from residential LPG combustion.

K-2.3 SUMMARY

This investigation of emissions factors from natural gas-, fuel oil-, and LPG-fired residential

water heaters indicates that using the values estimated by EPA’s AP-42 is a robust and credible basis

for analysis with NES output. This analysis indicates that regulations at the state and county level,

especially in the northeastern region of the United States, are generally not good estimators of the

national average set forth by the EPA AP-42 report. This is especially true for SO2 emissions factors

from fuel oil combustion where individual state and county regulations are highly variable. Thus,

although it is worthwhile examining regional legislation to support the national average, this analysis

demonstrates the complexity that would be required to establish a weighted regional calculation for

each emission. EPA’s own lack of data on the sulfur content of LPG made it difficult to derive a

nationally representative emissions factor for the country as a whole. External sources were therefore

incorporated in the Table K-2.1 estimate for SO2 emissions from LPG combustion.
K-2.6

REFERENCES

1. U.S. Environmental Protection Agency, Compilation of Air Pollutant Emission Factors, AP42, Fifth Edition, Volume I: Stationary Point and Area Sources,1998. October. 

< http://www.epa.gov/ttn/chief/ap42.html>

2. South Coast Air Quality Management District, March, 1995. < http://www.

aqmd.gov/rules/r1121.html>

3. New Jersey State Department of Environmental Protection, September 1993.

<http://www.state.nj.us/dep/aqm/2719968.htm#Boilers>

4. United States Hemispheric Clean Cities Program, 1994. <http://www.hemisccities.doe.gov/altfuels/propane.html>

5. U.S. Environmental Protection Agency, Compilation of Air Pollutant Emission Factors, AP42, Fifth Edition, Volume I: Stationary Point and Area Sources, Chapter 1, External

Combustion Sources,1998. October. < http://www.epa.gov/ttn/chief/ap42c1.html>