EPA’s Methodology for Estimating Emissions from 

Residential Wood Combustion

Briefing for OMB

August 21, 2013
Purpose of Briefing

► Explain how EPA estimates emissions from 

residential wood combustion (RWC) for the 

National Emission Inventory (NEI)

2
National Emission Inventory (NEI)

► Published on a triennial cycle (e.g., 2005, 2008, 2011)

► Criteria pollutants and their precursors, and hazardous air 

pollutants

► NEI is a mix of State/Local/Tribal (SLT) data, and EPA data

► The EPA data are used for SLTs that do not submit

► For 2011, agencies that submitted their own RWC estimates are:

CA, DE, IL, MD, ME, MN, OR, WA, WV, Clark county (NV), & 8 tribes

► EPA & SLTs estimate emissions using the Residential Wood 

Combustion Tool

3
Residential Wood Combustion Tool

► Tool first created for 2005 NEI

► Updated for 2008 and 2011

► Includes previously ignored appliance types, like outdoor hydronic 

heaters (aka outdoor wood boilers (OWB)), wax logs, indoor 

wood furnaces, and other outdoor burning (firepits, chimeneas)

► As well as fireplaces, fireplace inserts, and woodstoves

► Other outdoor burning added in 2011 

► Includes criteria pollutants and HAPs

► Key pollutants include PM2.5

► Microsoft Access-based tool

► Transparent and flexible

► Allows SLTs to view and update parameters at the county level to provide localspecific emissions estimates

► Includes queries to recalculate emissions

► Determines EPA-certified woodstoves and inserts separately

4
How Does the Tool Estimate Emissions?

Emissions = n × b × d × EF

n = number of appliances

b = burn rate of appliance

Cords of wood burned/yr

d = wood density (lbs/ft3)

Converts cords of wood burned to tons of 

wood burned

Must be oven dried basis

EF = emission factor 

lbs pollutant / ton of oven dry wood burned

5
Appliance Counts

► Fireplaces, Inserts, and Woodstoves 

► Uses the American Housing Survey (AHS) from the US Census

► Derives county-specific number of households that use these for 

heating by using percentages from AHS and number of occupied 

households from Dept of Census

► Separately counted for primary heating or supplemental heating

► Other data sources are state surveys 

► Outdoor Hydronic Heaters

► We have population numbers by state

• Sales data by state from NESCAUM for hydronic heaters from 1990 to 

2005

• Population grown to 2008 and 2011 using data from MN, VT and a Frost 

and Sullivan market research report

• Allocated to county in various ways (too much detail for this briefing)

6
Appliance Counts (con’t)

► Pellet Stoves

► Population determined by sales data

• Sales data from Pellet Fuel Institute by state thru 2008

• Population in tool held constant from 2008 to 2011 due to limited 

resources

– PM emissions are 1/10 that of a conventional woodstove on a per 

ton burned basis 

► Indoor Wood-Fired Furnaces

► MANE-VU states:

• Have estimated number of centralized cordwood heating systems1

• Determine indoor wood-fired furnaces by subtracting the outdoor hydronic heater 

count

• Data are based on 2002

• We assume no growth since no reliable growth rate data are available2

7

1 “Control Analysis and Documentation for Residential Wood Combustion in the MANE-VU Region,” Prepared for: Mid-Atlantic Regional Air 

Management Association, Inc., James E. Houck and Brian N. Eagle. OMNI Environmental Services, Inc. December 19, 2006. 

2 Personal communication from John Crouch of Hearth, Patio, and Barbeque Assoc. saying that sales have been very slow since the NSPS in 1988.
Appliance Counts (con’t)

► Indoor Wood-fired Furnaces (con’t)

► LADCO states:

• Use Minnesota data to estimate population

“Residential Fuelwood Assessment, State of Minnesota, 2002-2003 heating 

season,” Minnesota Department of Natural Resources

• Report has number of units in state

– Allocate to county level same technique as OWBs

► For all other states:

• Populate states with furnaces proportionately to 0.7% of occupied units

– This percent is from 2001 survey in WA, OR, and ID

• Allocate to county level same technique as OWBs

• Zero out counties in climate zones 4 and 5 (southern US)

8
Burn Rates

► Forest Service Reports from 8 states 

► MN, IN, WI, KS, ND, SD, NE, MI

► Years varying from 1992 to 2002

► Surveys of residential fuel wood consumption

► Problem: How to adjust burn rates in states with warmer 

climates?

► Use ratio of the average BTU consumption to heat a house in climate 

zone 5 to the average BTU consumption in climate zone 1 to adjust 

burn rates in warmer climates

• Based on US Energy Information Administration (EIA) 2005 data

► (Climate zones map on next slide)

9
Climate Zones for Burn Rate Adjustment

10

CCD = Cooling degree days 

= days above 75 °F

HHD = Heating degree days 

= days below 65 °F
Wood Density

► Burn rate data in cords (volume unit) and emission factors in tons 

of oven-dried wood (mass unit)

► Wood density is needed to make equations work

► Use the Timber Products Output database, US Forest Service

► County level database

► Survey results of sawmill operators that provides volume of wood by 

species for several different categories of use, one of the uses being 

fuel wood

► Used averages where no county sawmill data available

► Assumed 80 ft3/cord to account for airspaces

11
Wood Density Result

12
Emission Factors

► Fireplaces

► AP42, section 1.9

► Woodstoves, Inserts, & Pellet Stoves

► AP42, section 1.10

► Indoor Furnaces

► 2006 MARAMA report referenced on Slide 7

► Wax Firelogs

► Content and emission characteristics of Artificial Wax Firelogs, 

Environment Canada

13
Emission Factors (con’t)

►Outdoor Wood Boilers:

► EPA tested OWBs in 2009

► Test covered a conventional unit, a unit with cleaner 

burning design, and a unit that burns wood pellets

► Results released in 2012 show: 

• Conventional units are very dirty (emissions ~ twice that of a 

conventional woodstove on a per ton wood burned basis);

• New design emits significantly less (~10% of conventional units); 

and

• Pellet unit emits lowest emissions (~1% of conventional units).

14
QUESTIONS?

15