5.3 Residential Combustion - Liquid Fuel

Category ID	Description	EIC
286	Domestic - LPgas	61099501200000
287	Domestic - Liquid Fuel	61060612200000

Introduction

This section describes the methodology used to estimate greenhouse gas (GHG) emissions from the domestic combustion of liquefied petroleum gas (LPG) and distillate fuel oil within the Bay Area Air Quality Management District (BAAQMD or Air District). These sources are categorized under categories 286 (LPG combustion) and 287 (distillate fuel combustion) in the emissions inventory framework. The analysis includes emissions of the three primary greenhouse gases associated with fuel combustion: carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). CO₂ is produced in direct proportion to the carbon content of the fuel, CH₄ is formed due to incomplete combustion, and N₂O is emitted in trace amounts depending on combustion conditions such as temperature and oxygen availability.

Liquefied petroleum gas (LPG) is a flammable hydrocarbon fuel that exists in gas form at standard temperature and pressure but is liquefied for storage and transport through compression in pressurized containers. LPG is composed primarily of propane (C₃H₈) and butane (C₄H₁₀), although it may also contain smaller amounts of other hydrocarbons like propylene and butylene. The exact composition of LPG can vary by region and supplier, but propane is the most commonly used form in residential applications in California. LPG is widely used in areas not connected to natural gas pipelines and serves as fuel for space heating, water heating, and cooking in homes and residential buildings.

Distillate fuel oil is a liquid fossil fuel derived from the fractional distillation of crude oil. This process separates crude oil into various components, or "fractions," based on boiling point. Distillate fuels consist of the middle-range fractions—heavier than gasoline but lighter than residual oils—and include several grades commonly used for heating purposes. These fuels are primarily used in space heating systems in homes and residential buildings, especially in rural or off-grid areas.

No. 1 fuel oil: A light distillate similar to kerosene.
No. 2 fuel oil: The most commonly used heating oil in residential boilers and furnaces.
No. 4 fuel oil: A heavier blend is sometimes used in commercial or institutional heating systems.

Methodology

These categories are considered area sources as they account for emissions from combustion devices that are not permitted by the Air District and hence are not systematically or annually reported. The data used to estimate emissions for these sources must be extracted from either the California Air Resources Board (CARB) or California Energy Commission (CEC). The methodology used to calculate emissions for the base year(s) (i.e., years in which CEC data on natural gas usage is available) of these area source categories is as follows:

Base Year(s) Emissions_{county,pollutant}=

Activity Data × Emission Factor_pollutant × Control Factor_pollutant × Fraction_county× Fraction_{in District} × GWP_pollutant

Where:

Activity Data: is the throughput or activity data for applicable reported base years. This data may be determined in one or two ways:
- Apportioning Larger-Scale Data: Throughput data from a larger domain, such as state or national level, is scaled using the proportion of a representative metric in the regional domain relative to the larger domain. For example, the ratio of a county’s population to the state population can be used as a scaling factor to determine the county throughput from state-level throughput.
- Using Local Sources: Alternatively, data from a locally published and verifiable source may be used, such as the county-level natural gas usage data provided by the CEC.
Emission Factor_pollutant: is a factor that allocates an amount of emissions, in mass, of a particular pollutant by unit of activity data. For example, tons of CO₂ per gallon of gasoline burned or pounds of N₂O per million standard cubic feet (MMSCF) of natural gas combusted. This factor generally comes from a published literature source such as USEPA AP-42 (USEPA, 1998) or CARB’s Mandatory Reporting Requirement (MRR) for Greenhouse Gases (CARB, 2019).
Control Factor_pollutant : is a fractional ratio (between 0 and 1) that captures the estimated reduction in emissions as a result of Air District rules and regulations.
Fraction_county: is the fraction of total regional emissions (between 0 and 1) estimated to be allocated to a particular county. It is typically derived from regional socioeconomic metrics and/or actual county-level throughput data.
Fraction_{in District} : The Air District jurisdiction covers only a portion of Solano and Sonoma County. For this reason, additional allocation must be made for these counties to determine the proportion of the county’s emissions occurring within the Air District’s jurisdiction.
GWP_pollutant is the Global Warming Potential of a particular greenhouse gas (GHG) pollutant. The current version of the GHG emissions inventory incorporates the global warming potential (GWP) reported in the Fifth Assessment report of the Intergovernmental Panel for Climate Change (IPCC, 2014). The GWPs for the three principal GHGs are 1 for carbon dioxide (CO₂), 34 for methane (CH₄), and 298 for nitrous oxide (N₂O), when calculated on a 100-year basis with climate-carbon feedback included.

Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are estimated using growth profiles as follows:

Current Year Emissions_county = Base Year(s) Emissions_county X Growth Factor

Where:

Growth Factor: is a scaling factor that is used to derive historical emissions estimates for years for which activity data and/or emissions are not available, and to forecast emissions for future years, using surrogates that are assumed to be representative of activity and/or emissions trends.

More details on throughput, county distribution, emission factors and controls are provided in the following subsections:

Activity Data

Estimating emissions from domestic fuel combustion requires reliable estimates of fuel throughput, which refers to the total volume of fuel consumed during a given year. In this methodology, throughput for LPG and distillate fuel oil is estimated indirectly, based on available data for natural gas consumption and the relative prevalence of each fuel type in residential households.

The analysis covers calendar years 2010 through 2022. For the years 2010 to 2021, actual annual county-level natural gas usage data are available from CEC. Because data for 2022 were not available at the time of analysis, 2021 throughput values were used as a proxy for 2022, under the assumption that residential fuel use patterns remained relatively stable year over year. This assumption is reasonable in the absence of significant changes in population, fuel availability, or energy policy. All throughput values are reported in thousands of gallons.

Throughput for LPG and distillate is not reported directly in statewide datasets. Instead, it is estimated using a fuel-substitution ratio method, which scales known natural gas usage in each county by the relative number of households using alternative fuels (LPG or distillate) compared to those using natural gas for space heating.

The method proceeds in four main steps:

Obtain county-level residential natural gas usage in British thermal units (Btu) from the CEC (CEC, 2023).
Calculate the ratio of households using LPG or distillate to those using natural gas, using data from the American Community Survey (ACS, 2023).
Apply the ratio to natural gas usage to estimate the equivalent energy demand served by LPG or distillate.
Convert Btu to gallons, using the higher heating value (HHV) of each fuel type. HHV are obtained from EPA Title 40 Code of Federal Regulations (CFR), Subpart C, Part 98, Subpart C, Table C-1 (EPA, 2016).

In summary, the equations used to estimate throughput (in thousands of gallons) are:

Throughput_Distillate = (H_Distillate / H_NG) × (U_NG / HHV_Distillate) ÷ 1000

Throughput_LPG = (H_LPG / H_NG) × (U_NG / HHV_LPG) ÷ 1000

Where:

Throughput_Distillate = Estimated annual distillate fuel consumption (thousand gallons)
Throughput_LPG = Estimated annual LPG consumption (thousand gallons)
H_Distillate = Number of households using distillate fuel for space heating
H_LPG = Number of households using LPG for space heating
H_NG = Number of households using natural gas for space heating
U_NG = Annual residential natural gas consumption in Btu (obtained from CEC)
HHV_Distillate = Higher heating value of distillate fuel
HHV_LPG = Higher heating value of LPG

County Distribution / Fractions

County fractions are based on the calculated throughputs of distillate and LPG.

ID	Description	ALA	CC	MAR	NAP	SF	SM	SNC	SOL	SON
286	Domestic - LPgas	0.16	0.13	0.09	0.04	0.11	0.08	0.21	0.04	0.13
287	Domestic - Liquid Fuel	0.10	0.15	0.03	0.01	0.17	0.13	0.24	0.04	0.13

BAAQMD Jurisdiction Fraction

The Air District’s jurisdiction only accounts for a part of Solano and Sonoma counties. The remaining area is covered by other Air Districts. The percentage of Solano and Sonoma County households within the Air District boundary (or jurisdiction) are estimated using the Association of Bay Area Government’s (ABAG) Plan Bay Area 2050 dataset (ABAG, 2021). The dataset summarizes population by Travel Analysis Zones (TAZs). The number of households for each TAZ within the Air District’s jurisdictional boundaries is summed and divided by the total county-wide household count to estimate the percentage of population within the Bay Area for Solano and Sonoma counties. These proportions are shown below:

County	% of Population within the District’s jurisdiction
Solano	0.70
Sonoma	0.85

Emission Factors

GHG emission factors are sourced from the EPA Title 40 Code of Federal Regulations (CFR), Subpart C, Part 98, Subpart C (EPA, 2016), specifically:

Table C-1, which provides CO₂ emission factors and HHV for fuels, expressed in kilograms of CO₂ per million British thermal units (kg CO₂/MMBtu).
Table C-2, which provides CH₄ and N₂O emission factors in kg/MMBtu.

To convert these emission factors for use with throughput in gallons, the following steps were applied:

Multiply by the higher heating value (HHV) of the fuel to convert from per MMBtu to per gallon.
Convert kilograms to pounds, using the factor: 1 kg = 2.2 lb.
Scale values to represent emissions per 1,000 gallons for consistency with throughput data.

The final emission factors used in the inventory are summarized below:

Greenhouse Gas	Emission Factors (lb/thousand gallons of fuel)
Greenhouse Gas	LPG	Distillate
CH₄	0.61	0.92
N₂O	0.12	0.18
CO₂	12516	22870

Control Factors

Regulation 9, Rule 4 (Nitrogen Oxides from Residential Central Furnaces; BAAQMD, 2023) will require complete replacement of natural gas furnaces with all-electric equivalents starting in 2028, with full transition across Bay Area homes expected by 2046. This regulation does not specifically target furnaces powered by LPG and distillate oil, so no separate control factors are applied to sources covered in this document. However, it is expected that Regulation 9, Rule 4 will indirectly but strongly reinforce the continuous and long-standing reduction in demand for LPG and distillate fuels for home-heating purposes and, eventually, eliminate all emissions from this subsector over time. A very minor fraction of SFBA homes (0.3-0.4% use LPG, and 0.05% use distillate) use liquid fuels for home heating, so the overall reduction relative to the entire regional total will be minimal.

Historical Emissions

For category 286 (LPG), historical throughput estimates from 1990 to 2010 were developed using household projections from the Association of Bay Area Governments (ABAG, 2021). The assumption underlying this backcast is that LPG usage is closely correlated with the number of households using this fuel for space heating. As a result, the trend in household growth serves as a proxy for changes in fuel consumption. Analysis of ABAG’s historical household projections suggests a gradual increase in LPG use over this period, consistent with modest regional population growth and stable fuel use patterns in rural and off-grid areas where LPG is more common.

For category 287 (distillate), a different approach was used for backcasting due to a lack of household-specific historical data. Instead, a historical throughput adjustment profile was developed, assuming a 2% annual decrease in distillate fuel use starting in 1992, reflecting a long-term decline in residential distillate use as homes transitioned to natural gas or electric heating. This decline is consistent with broader trends in building electrification and urban fuel-switching observed across California during that time.

Future Projections

For category 286, future emissions from LPG combustion are projected using statewide modeling developed as part of the CARB 2022 Scoping Plan Update (CARB, 2022). Specifically, the PATHWAYS model, developed by Energy + Environmental Economics (E3), provides forecasts of residential LPG use under a business-as-usual (BAU) scenario. The PATHWAYS model simulates the evolution of California’s energy system based on current policies, technological adoption rates, and demographic trends.

For category 287, future emissions assume the same approach as the backcast profile, where residential distillate use will continue to decline over time at 2% per year, eventually approaching zero as the fuel is phased out in favor of cleaner alternatives such as electricity or renewable gas. This assumption reflects ongoing trends in residential electrification, increased adoption of heat pumps, and policy-driven fuel switching in California.

Sample Calculations

An example of calculation of emissions from category 286 for base year 2022 for the portion of Sonoma County within the Air District jurisdiction is shown below. Emissions are calculated in metric tons of CO₂ equivalents (MTCO₂eq).

Step 1	Collect total number of households using Liquefied Petroleum gas (LPG) in Sonoma County (ACS, 2023)	11,255
Step 2	Determine fraction of households in Sonoma County within Air District (ABAG, 2021)	0.848
Step 3	Determine average natural gas consumption in Sonoma County (MMBtu/household; CEC, 2023)	59.21
Step 4	Gather LPG HHV (thousand gallons/MMBtu; USEPA, 2016)	94.986
Step 5	Calculate total LPG used in Sonoma County households (thousand gallons)	11,255 households × 0.848 × 59.21 MMBtu/household ÷ 94.986 kGal/MMBtu = 5949 kGal
		CO₂	CH₄	N₂O
Step 6	Emission Factors (lb/thousand gallons; USEPA, 2016)	12,516	0.6085	0.1217
Step 7	Global Warming Potential	1	34	298
Step 8	Calculate GHG emissions (MTCO₂eq)	5949 kGal × 12,516 lb/kGal × 1 ÷ 2000 lb/ton × 0.907185 ton/MT = 33,773 MTCO₂eq	5949 kGal × 0.6085 lb/kGal × 34 ÷ 2000 lb/ton × 0.907185 ton/MT = 56 MTCO₂eq	5949 kGal × 0.1217 lb/kGal × 298 ÷ 2000 lb/ton × 0.907185 ton/MT = 98 MTCO₂eq

Assessment of Methodology

The general approach remains the same between the current and previous base years, but with several updated data sources in the current methodology.

Base Year	Revision	Reference
2022	Updated with 2021 activity data and county distributions based on calculated county-level throughputs for distillate and LPG Updated GHG emission factors and HHV values with Title 40 CFR values Updated Sonoma/Solano County splits using fraction of households within Air District jurisdiction Updated natural gas usage using California Energy Commission data by county through 2021 Backcast profile updated using number of households in each county	ACS, 2023 USEPA, 2016 ABAG, 2021 CEC, 2023 ABAG, 2021
2015	Throughput activity data was determined from the number of housing units heated by LPG and distillate oil provided by the 2010 Department of Finance’s Census of Population and Housing Bay Area natural gas fuel usage was provided by Pacific Gas and Electric Company Natural gas usage was converted to average BTUs per household using fuel-specific HHV County distribution of emissions was based on the Department of Commerce’s 2010 Census of Population and Housing for California, using the number of homes using LPG and fuel oil for heating in each county Emission factors were taken from EPA, AP-42 oil and LPG fuel data Historical emissions for LPG combustion follow household population data while those for distillate oil combustion was estimated to decrease over the past years at 2% per year Future projections were determined for LPG using the CARB Scoping Plan projection for LPG BAU (business as usual)	CADOF, 2010 PG&E BAAQMD USDOC, 2012 USEPA, 2023 ABAG, 2023 CARB, 2023

Emissions

The table below summarizes greenhouse gas emissions for the base year 2022 in MTCO₂eq.

ID	Description	CH4	CO2	N2O	Total
286	Domestic - LPgas	391.2	236659.1	685.7	237736.0
287	Domestic - Liquid Fuel	20.6	15091.1	36.3	15148.0

Summary of Base Year 2022 Emissions

The contribution of liquid fuel combustion categories to the Commercial & Residential sector and to the overall regional total are shown in the table below.

Contribution of Residential Combustion - Liquid Fuel Emissions by Sector

Subsector	Sector	Subsector GHG Emissions (MMTCO2eq)	Sector GHG Emissions (MMTCO2eq)	% of Sector
Residential Combustion - Liquid Fuel	Commercial + Residential	0.25	12.85	1.97%

Contribution of Residential Combustion - Liquid Fuel Emissions to Regional Total

Subsector	Subsector GHG Emissions (MMTCO2eq)	Regional Total GHG Emissions (MMTCO2eq)	% of Regional Total
Residential Combustion - Liquid Fuel	0.25	65.68	0.39%

Trends

The time series chart below shows the emission trends for categories 286 and 287.

Summary of Trends

Emissions from residential combustion of liquid heating fuels are projected to decline significantly in the coming decades as part of a broader transition away from fossil fuel use in buildings.

Historically, LPG consumption has generally increased in line with regional population growth and the expansion of residential development in rural and off-grid areas where natural gas service is limited. This upward trend in household LPG use has driven a gradual increase in associated emissions over time. However, recent throughput data suggest that this growth has begun to stabilize or plateau, with emissions levels flattening in the last several years.

In contrast, distillate fuel oil use has been steadily declining since the 1990s as homeowners have transitioned to cleaner and more efficient heating systems. The backcast methodology reflects this decline, with assumed annual decreases in fuel use consistent with historical fuel-switching trends.

Notably, both LPG and distillate categories experienced a temporary increase in emissions in 2020 and 2021, which may be attributed to the COVID-19 pandemic. Widespread adoption of work-from-home practices likely increased residential energy demand during those years, particularly in colder months when heating loads are higher.

Emissions from both fuel categories are expected to decline steadily through the 2030s and 2040s, driven by:

Statewide climate policies that promote building electrification and reduced reliance on fossil fuels.
Local regulatory action, including BAAQMD Regulation 9, Rule 4, which phases out the use of fossil-fueled space and water heating appliances in new construction and, eventually, existing homes (BAAQMD, 2023).
Technological advancements and market trends, such as the increasing adoption of electric heat pumps and induction appliances.

Emissions from distillate combustion are expected to phase out more quickly, consistent with its already-declining usage and limited remaining application in the residential sector. LPG emissions are expected to decline at a slower rate, particularly in areas without access to natural gas infrastructure, but will also ultimately be eliminated as part of regional decarbonization strategies.

Uncertainties

Several factors contribute to uncertainties in the emissions estimates for residential combustion of liquid fuels:

Activity Data and Household Fuel Use Estimates

Data Availability and Resolution:
Throughput estimates are based on indirect indicators, namely, residential natural gas usage data from the CEC and household heating fuel type data ACS. These sources offer only partial visibility into the actual quantity of LPG and distillate fuel consumed.
Survey Limitations:
ACS fuel-use data are self-reported and may contain classification errors (e.g., misidentifying propane vs. natural gas). In rural or low-density counties, small sample sizes may increase sampling error and year-to-year variability.
Assumption of Uniform Energy Use:
The methodology assumes households using LPG or distillate consume the same amount of energy as those using natural gas. However, energy demand may vary by building size, insulation, appliance efficiency, and climate zone. These assumptions may under- or overestimate throughput at the county level.
Multi-fuel Homes and Off-grid Use:
The model does not account for homes using multiple heating fuels (e.g., wood and LPG), nor does it capture unmetered or off-grid fuel deliveries, which are more common for LPG in rural areas.

Fuel Composition and Appliance Variability

Actual emissions can vary depending on the specific fuel blend (e.g., propane vs. butane content in LPG), combustion technology (e.g., boiler vs. furnace), maintenance, and operating conditions. These factors are not explicitly accounted for.

Regulatory Implementation and Future Projections

The adoption of Regulation 9, Rule 4 (BAAQMD, 2023), is expected to phase out natural gas-powered furnaces over time. It does not specifically target LPG and distillate oil powered furnaces but is expected to reduce demand for LPG and distillate fuels for home-heating purposes. Since the impact of Regulation 9, Rule 4 on LPG and distillate powered home furnaces is uncertain, the methodology uses conservative or “business as usual” assumptions to project activity and emissions for future years.

Contact

Author: Sally Newman

Reviewer: Abhinav Guha

Last Update: 08/27/2025

References

ABAG, 2021. Plan Bay Area 2050, Association of Bay Area Governments. https://planbayarea.org/finalplan2050

ABAG. 2022. Historical Growth Profiles from Archived Internal Database, Association of Bay Area Governments. Accessed October 3, 2022.

ABAG. 2023. Open Data Catalog, Association of Bay Area Governments. [accessed 2023 Jan 30]; https://abag.ca.gov/tools-resources/data-tools/open-data-catalog

ACS. 2023. American Community Survey, 2023. (https://data.census.gov/table?q=DP04&g=040XX00US06_050XX00US06001,06013,06041,06055,06075,06081,06085,06095,06097&y=2020, for year 2020. Accessed 2023.

BAAQMD. 2023. Regulation 9- Inorganic Gaseous Pollutants, Rule 6 – Nitrogen Oxides Emissions from Fan-type Residential central Furnaces, Bay Area Air Quality Management District. Available at: https://www.baaqmd.gov/~/media/dotgov/files/rules/reg-9-rule-4-nitrogen-oxides-from-fan-type-residential-central-furnaces/2021-amendments/documents/20230315_rg0904-pdf.pdf?rev=fd4b39bc0c834bcbba8b76646526afc6&sc_lang=en

CARB. 2019. Regulation for the Mandatory Reporting of Greenhouse Gas Emissions, California Air Resources Board - https://ww2.arb.ca.gov/sites/default/files/classic/cc/reporting/ghg-rep/regulation/mrr-2018-unofficial-2019-4-3.pdf

CARB. 2022. AB 32 Inventory Modeling Data Spreadsheet, California Air Resources Board. https://ww2.arb.ca.gov/sites/default/files/2022-11/2022-sp-PATHWAYS-data-E3.xlsx. Accessed 2024.

CEC. 2023. Gas Consumption by County, California Energy Commission. https://ecdms.energy.ca.gov/gasbycounty.aspx. Accessed 2023.

IPCC. 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II, and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyers (eds.)]. IPCC, Geneva, Switzerland, 151 pp. Available here: https://www.ipcc.ch/site/assets/uploads/2018/02/SYR_AR5_FINAL_full.pdf

USDOC. 2012. 2010 Census of Population and Housing, United States Department of Commerce. 2012. [accessed 2023 March 13]. https://www2.census.gov/library/publications/decennial/2010/cph-2/cph-2-6.pdf

CADOF. 2010. Census 2010, California Department of Finance. [accessed 2023 March 13]. https://dof.ca.gov/reports/demographic-reports/census-2010/

USEPA. 1998. AP-42, Fifth Edition, Volume I, Compilation of Air Emissions Factors from Stationary Source, Chapter 1 External Combustion Sources, Subchapter 4 Natural Gas Combustion, United States Environmental Protection Agency. https://www.epa.gov/sites/default/files/2020-09/documents/1.4_natural_gas_combustion.pdf

USEPA. 2016. Title 40 Code of Federal Regulations (CFR), Subpart C, Part 98, Subpart C – General Stationary Fuel Combustion Sources, United States Environmental Protection Agency. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-98/subpart-C?toc=1. Table C-1 (link for emission factors for CO₂ and HHV values for all fuels) and Table C-2 (link for emission factors for CH₄ and N₂O). Accessed 2023.

USEPA. 2023. AP-42, United States Environmental Protection Agency. [accessed 2023 March 13]. https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emissions-factors