7.3 Domestic Solid Fuel (Wood)

Categories 288, 289

7.3.1 Introduction

The categories in this chapter estimate the criteria pollutant emissions from residential wood combustion, namely from fireplace (category 288) and woodstove (category 289). Fireplaces are the most commonly utilized wood burning device in a home. They are used primarily for supplemental heating and for aesthetic appeal. Fireplace combustion is characterized by high air-to-fuel ratios and burn rates. Traditional masonry fireplaces typically contain large open fireboxes without combustion air controls and are not highly efficient heating devices. A net heat loss may occur in a residence if colder, outside air is drawn in to replace the inside air used for combustion and lost through the chimney draft. There are also prefabricated (metal) fireplaces which are slightly higher in energy efficiency than masonry fireplaces. Fireplace inserts that fit into the fireplace can increase the heating efficiency by either radiating the heat into the house or venting heated air into the house by circulating air around the insert with the help of a fan.

Woodstoves are used primarily as domestic space heaters and have enclosed fireboxes and dampers to reduce air-to-fuel ratios and burn rates. Since they are stand-alone heating devices, the greater surface area radiates more heat than a fireplace.

The emissions from residential fireplaces and woodstoves are highly variable, depending on the amount of wood burned and the types of woodstoves and fireplaces being used. Many assumptions were made with the realization that any variations in one or more of these variables would substantially change the calculations.

7.3.2 Methodology

The emission estimates for woodstoves and fireplaces were derived from wood burning activity data collected through the District’s annual wood burning telephone survey starting from winter of 2005 and spring of 2006. This survey was performed by True North Research. Data collected from this survey includes wood burning activities via frequency, fuel type, and quantity for the Bay Area nine counties.

A compilation and analysis of this data was performed by Dr. David Fairley, BAAQMD’s statistician, in a District’s internal draft report281. Statistical estimation methods were used in this report to derive the annual and seasonal amount of wood burned by county.

(a) Activity Data / Throughput

Throughputs for these categories, which is the amount of wood burned in tons, were derived based on data obtained from the District’s annual wood burning surveys. A density of five lbs per unit of log was assumed in the calculation.

(b) County Distribution / Fractions

The throughput varies significantly among nine counties in Bay Area. For fireplaces, the more urbanized counties like Alameda, San Francisco and San Mateo tend to have lower amounts burned per household than counties with larger proportion of rural areas such as Sonoma and Napa counties. Alameda, Contra Costa and Santa Clara were estimated to have the highest fireplace wood burning throughput because of high population density. For woodstoves, Sonoma, Marin and Napa have the largest number of logs burned per household. Contra Costa and Santa Clara counties were estimated to have the highest woodstove wood burning throughputs because of population density as well.

The estimated county-specific throughputs were used to derive the county distribution for the two categories, respectively.

(c) Emission Factors

Woodstove and fire place emission factors were based on data obtained from California Air Resource Board (CARB)282 and EIIP volume IV283. Composite emission factors were calculated for woodstove as an weighted average of conventional and EPA phase II woodstove emission factors. The number of conventional versus EPA phase II woodstoves were derived based on a statistical analysis from the 05-06’ winter survey.

(d) Control Factors

Control of residential wood combustion currently falls under EPA’s Standards of Performance for New Stationary Sources (NSPS) for residential wood heater (stoves)284. This is adopted by reference in the District Regulation 10285. The NSPS required that new residential heaters manufactured on or after July 1, 1988, or sold on or after July 1, 1990 must be certified to meet particulate emission standards of 5.5 grams per hour for catalytic wood heaters and 8.8 grams per hour for non-catalytic wood heaters. These are known as Phase I stoves. More restrictive particulate emission standards were set for stoves manufactured on or after July 1, 1990 or sold after July 1, 1992. The standards were set to 4.1 grams per hour for catalytic wood heaters and 7.5 grams per hour for non-catalytic wood heater which are known as Phase II stoves. Although EPA has only set emission limits for particulate matter, emission of reactive organic compounds and carbon monoxide are expected to be limited as well.

Natural gas fireplaces, pellet fueled woodstoves, and EPA-certified woodstoves and fireplaces are increasing in popularity every year. Particulate emissions from these heating devices are insignificant. Some cities and counties in the Bay Area have (or are looking at) enacting new ordinances restricting new woodstove and fireplaces to these low particulate emitting heating devices.

In July 2008, the District enacted the Regulation 6 Rule 3 (Rule 6-3)286 to control emissions from wood-burning devices. This rule limits emissions of particulate matter (PM) and visible emissions (VE) from wood-burning devices, including any wood-burning device, pellet-fueled wood heater or any indoor permanently-installed device burning any solid fuel for aesthetic or space-heating purposes which includes fireplaces. The rule will help to reduce PM emissions from the burning of wood in woodstoves and fireplaces. In October 2015, the District amended the rule to prohibit the installation of wood burning devices in new building construction starting from November 1, 2016.

(e) Speciation

CARB’s PM speciation profile # 424 designated for fireplaces and woodstoves was used to estimate PM10 and PM2.5 emissions287. According to the profile, PM10 is 93.5% of PM while PM2.5 is 90.01% of PM.

(f) Monthly Variation

The monthly variation for wood burning in woodstoves and fireplaces was also derived from Bay Area’s telephone survey on wood burning. Monthly variation was estimated based on sample weights assigned from the data analysis. The estimates were appropriately adjusted for county population.

7.3.3 Changes in Methodology

There is no change in methodology compared to previous base year inventory.

7.3.4 Emissions

Residential wood combustion are the key sources of PM emissions especially winter PM2.5. A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.

7.3.6 Uncertainties

There are unknown magnitude of uncertainties for the emissions estimated for residential wood burning. Most uncertainty come from the District’s wood burning surveys (e.g., sample size, response rate) and the statistical model developed to estimate throughput, which in turn impacts the calculation results of composite emission factors, and the monthly variation as the survey mainly focus on winter wood burning activity. In the recognition of those uncertainties, the District is leading an effort to improve the emissions estimate for the two categories by working with a private contractor to develop a more robust and up-to-date methodology and supporting data based on the historical survey data. By the time of this document, the preliminary results provided by the contractor suggested that the overall emissions is comparable to the current base year inventory but the differences varies among counties. The final work product will be used to update the current base year and future emissions inventory.

7.3.7 Contact

Author: Yuan Du

Reviewer: Abhinav Guha, Tan Dinh

Last Update: November 06, 2023

7.3.8 References & Footnotes

  1. David Fairley. Revised Estimates of Wood Burning in the San Francisco Bay Area Based on Telephone Survey Data (District Internal Draft Report).↩︎

  2. CARB. October 2015. Section 7.1 Residential Wood Combustion. https://ww3.arb.ca.gov/ei/areasrc/fullpdf/full7-1_2011.pdf↩︎

  3. E.H. Pechan & Associates, Inc. April 2004. Estimating Ammonia Emissions From Anthropogenic Nonagricultural Sources - Draft Final Report Section III.D, Table III-8; Prepared for USEPA Emission Inventory Improvement Program (EIIP). https://www.epa.gov/sites/default/files/2015-08/documents/eiip_areasourcesnh3.pdf↩︎

  4. USEPA. September 21, 2022. New Source Performance Standards. https://www.epa.gov/stationary-sources-air-pollution/new-source-performance-standards↩︎

  5. BAAQMD. Feburary 16, 2000. Regulation 10. Standards of Performance for New Stationary Sources. https://www.baaqmd.gov/rules-and-compliance/rules/reg-10-standards-of-performance-for-new-stationary-sources↩︎

  6. BAAQMD. November 20, 2019. Regulation 6, Rule 3: Wood-Burning Devices. https://www.baaqmd.gov/~/media/dotgov/files/rules/regulation-6-rule-3/documents/20191120_r0603_final-pdf.pdf?la=en&rev=cbb545815c15468cb98f8c1b23c083d2↩︎

  7. CARB. Speciation Profiles Used in CARB Modeling. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎