LOW EMISSION AND HIGH EFFICIENCY RESIDENTIAL PELLET-FIRED

HEATERS

James E. Houck and Andrew T. Scott

OMNI Environmental Services, Inc.

5465 SW Western Ave., Beaverton, OR 97005

Carol R. Purvis

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

MD-63, Research Triangle Park, NC 27711

Peter H. Kariher

ARCADIS Geraghty & Miller

4915 Prospectus Dr., Ste. F, Durham, NC 27713

 John Crouch and Michael J. Van Buren

Hearth Products Association

1601 N. Kent St., Ste. 1001, Arlington, VA 22209

ABSTRACT

There are an estimated 350,000 residential pellet-fired heaters currently in use in the

United States. In recent years about 30,000 to 40,000 units have been sold annually. 

There are two fundamental technology types: under-feed and top-feed. Pellets originating

from both hardwood and softwood residue are available. During the 1995-1996 heating

season 654,000 tons of pellets were sold nationwide.

Nearly all pellet-fired heaters have been sold since 1989. Even with this recent

introduction, there has been significant improvement in reliability, efficiency, and air

pollutant emissions in current models as compared to the earliest models. Electronic and

microprocessor control of combustion air, fuel feed, and convection fans is primarily

responsible for the improvement. Unfortunately, air emissions and efficiency data in the

open literature and in government reports available to air quality and energy planners and

regulators are still based on the performance of the earliest models introduced ca. 1989-

1990.

Even the old-technology pellet-fired heaters are more efficient than traditional cordwood
stoves. They have lower greenhouse gas and acid precipitation impacts than home heating

options based on fossil fuels, and their particulate and carbon monoxide emissions are

lower than cordwood stoves. 

Air emissions testing and efficiency testing on new under-feed and top-feed commercially

available heaters burning hardwood- and softwood-based pellets were conducted. The

results were compared with data from earlier models. Reductions in air emissions were

documented. The data from both the old- and new-technology stoves confirm that pelletfired heaters offer an environmentally sound option for the utilization of wood waste for

home heating.

Keywords: air emissions, pellet heaters

INTRODUCTION

The use of wood for home heating represents about 9% of the Nation’s space heating

needs (Houck et al., 1998). As with all energy options, there are environmental benefits

and drawbacks associated with residential space heating with wood. Clear environmental

benefits are low greenhouse gas emissions and low acid precipitation impacts as well as

the renewable nature of wood as a fuel. The major environmental concerns have been

particulate matter (PM) and carbon monoxide (CO) emissions.

In the late 1980's pellet stoves and low emission cordwood stoves were developed. The

PM and CO emissions from the pellet stoves were documented as being dramatically

lower than from traditional cordwood stoves (Barnett and Roholt, 1990, Barnett and

Fields, 1991, Barnett et al., 1991 and U.S. EPA 1996). Since the introduction of the first

pellet stove models, considerable improvements have been made in their design with

commensurate decreases in PM and CO emissions.

There have been four studies which have evaluated the air emissions from pellet stoves. 

These are: (1) An in-home study of early-technology U.S. Environmental Protection

Agency (EPA)-certified pellet stoves conducted for the U.S. Department of Energy

(DOE) during the 1989/1990 heating season (Barnett and Roholt, 1990 and Barnett et al.,

1991) – six stoves (two models) and 23 one-week long test periods make up the data base

for the study; (2) An in-home study of early-technology pellet stoves exempt from EPA

certification conducted for the DOE during the 1990/1991 heating season (Barnett and

Fields, 1991 and Barnett et al., 1991) – six stoves (four models) and 24 one-week long

test periods make up the data base for the study; (3) Recent laboratory testing of an early

model (ca. 1990) pellet stove for the EPA under four burn rates; and (4) Laboratory

testing of new under-feed and top-feed pellet stove models using both hardwood and

softwood pellets for the Pellet Fuels Institute (PFI) and Hearth Products Association

(HPA). The results of the first two studies have been published. They are the basis for

the PM and CO emissions factors compiled by the EPA in the AP-42 emissions factor

document (U.S.EPA, 1996) and have been generally used to represent PM with

aerodynamic diameters <10 :m (PM10) and CO emissions characteristic of pellet stoves. 

The results of the latter two studies are presented here for the first time.
Burn rates, PM emissions, and CO emissions were measured in all four studies. Particle

sizing was done as part of the two new studies. The elemental, organic, and carbonate

fractions of particles were also quantified in the PFI/HPA study.

Additional particulate emission data beyond those available from the four studies are also

obtainable from EPA certification records of pellet stoves (U.S.EPA, 1998). The EPA

certification records provide PM emission rates at the method-prescribed weighted burn

rate for 23 different models.

EXPERIMENTAL

Because much of the PM from residential wood combustion is composed of organic

compounds which are semivolatile (i.e., they are partitioned between the gas and PM

phases), the method of sample collection will affect the mass of PM emissions measured. 

PM emissions from pellet stoves have been primarily measured by three techniques: (1)

Dilution tunnel approaches of which Method 5G is used in the certification process (U.S.

EPA, 1988) (2) A method based on the traditional EPA Method 5 industrial source

sampling train which is referred to as Method 5H when used for woodstove certification

(U.S. EPA, 1988), and (3) automated in-home samplers. The emissions factors compiled

in AP-42 for pellet stoves measured in the two in-home studies (Barnett and Roholt, 1990,

Barnett and Fields, 1991 and Barnett et al., 1991) were obtained by using an in-home

sampler referred to as an automated emissions sampler (AES). For the two laboratory

studies, PM emissions were determined in dilution tunnel systems similar to Method 5G. 

Emissions rates listed by the EPA for certified pellet stoves are reported as Method 5H

equivalents. Equations to convert PM emissions data collected with the AES to

equivalent 5G values have been developed by the EPA (1988). Similarly, equations also

have been developed relating data collected by methods 5G and 5H (U.S. EPA, 1988 and

E.H. Pechan & Associates, 1993). While it is generally accepted that the conversion

equations are not highly accurate, to permit direct comparison of PM emissions, they were

used here to put all data in a 5G-like dilution tunnel format.

CO for pellet stoves in homes with the AES systems was measured using Tedlar bags

which collected a portion of the sampler’s flow. CO concentrations in the bags were

determined with commercial CO analyzers. CO emissions factors for the two recent

laboratory studies were determined from periodic measurements of CO concentrations in

the exhaust gas combined with periodic measurements of stack flows.

Efficiency values were calculated by combining the flue loss method (i.e., sensible and

latent heat loss out the exhaust) and the combustion efficiency. The fraction of unburned

residue was determined gravimetrically. Greenhouse gas and acid precipitation impacts

were estimated by summing the emissions of greenhouse and acid gases in each step of the

energy production process (energy trajectory) leading to the production of space heat

from pellets produced from uncut standing trees (Houck, et al., 1998).
RESULTS AND DISCUSSION

CO and PM emissions from the four studies are the key results presented here. Ancillary

data on stove efficiencies and solid waste issues, along with the results of a review of

greenhouse and acid gas impacts for home space heating, are also included.

CO Emissions

CO emissions factors for new and early model pellet stoves, as well as the average value

for conventional cordwood stoves, are shown in Table 1. The studies of emissions from

early models under in-home use show that, on the average, CO is reduced by more than

75% under actual use as compared to traditional uncertified cordwood stoves. The

laboratory testing of an early-model pellet stove at burn rates near those encountered in

homes reveal similar CO emissions factors. However, at a higher burn rate (for example,

see the data for the 1.6 kg/hr burn rate), CO emissions factors can become significantly

larger for the early-model pellet stoves. This is consistent with acknowledged difficulty in

optimizing combustion conditions with early pellet stoves with manual and/or independent

controls of fuel feed rates, combustion air blowers, and dampers. Newer models generally

have microprocessor control of one or more of these functions which permits better

optimization of combustion conditions over all burn rates. However, the effect of varying

combustion conditions can still be seen in CO emissions from new model stoves, as CO

emissions are not uniform throughout the burn rate range (Figure 1). Although higher CO

emissions are seen at both the low and high burn conditions, the CO emissions factors

from new-technology pellet stoves are still markedly lower than for early pellet stove

models. It should be further noted that, even with these differences between new- and

old-technology pellet stoves, the CO emissions from both new- and early-technology

pellet stoves, at typical in-home burn rates, are much lower than from traditional

uncertified cordwood stoves. For the pellet stove models and pellet fuels tested, there was

no significant difference in CO emissions from new-technology under-feed and top-feed

technology types or between hardwood and softwood pellets used in them.

PM Emissions

Like CO emissions factors, the four studies demonstrate that PM emissions factors for

early-model pellet stoves are much lower than the emissions factors for traditional

uncertified cordwood stoves, and new-technology pellet stoves show considerable

reduction in their PM emissions factors, compared to the early-model pellet stoves (Table

1).

In addition to the results from the four studies, information on particulate emissions can be

obtained from emission certification tests. As with cordwood stoves, emission

certification requirements have been promulgated for pellet stoves (U.S. EPA, 1988). 

There have been 23 pellet stove models certified since 1988 (U.S. EPA, 1998). However

only one model is currently (as of October 21, 1999) listed as certified for sale (U.S. EPA,

1999). Most pellet stoves are exempt from certification requirements because they have a
greater than 35:1 air-to-fuel ratio at one or more burn settings. The results of the

certification test do confirm low PM emissions for pellet stoves. The average 5-G

adjusted emissions factor for all 23 certified stove models, based on the 5-H rate data and

a weighted burn rate of 1.16 dry kg/hr, is 0.70 g/dry kg (Table 1).

PM emissions factors compiled in AP-42 assume that all PM emitted from pellet stoves is

smaller than 10 :m in aerodynamic diameter (PM10), and the emissions factors developed

from the total PM data generated in the two field studies are represented as PM10 data. 

Measurements with the 1990 model pellet stove for the EPA and on the new under-feed

and top-feed models for the PFI/HPA study show that not all, but on the average about

84%, of the total PM emissions are PM10. Interestingly, the same data base also shows

that about 81% of the PM emissions are smaller than 2.5 :m (PM2.5). These results are

consistent with the general understanding of the source of PM from biomass combustion. 

Most are submicron size particles formed from the chemically incomplete combustion of

fuels, some are large particles of entrained ash or unburned char, and very little PM falls

between the extremes. The ramification of the PM size distribution is that emissions

factors based on total PM should be reduced to 84 and 81% for PM10 and PM2.5 emissions

factors, respectively.

The elemental, organic, and carbonate carbon contents of the PM emitted from the new

under-feed and top-feed models studied for the PFI/HPA were quantified. From the

organic carbon data, the organic compound content can be estimated using a multiplier of

2.0 to account for the mass of oxygen and hydrogen associated with carbon in organic

compounds. Similarly, the fraction of the PM emissions composed of entrained ash can be

estimated from the carbonate carbon content, using a multiplier of 3.0 since wood

combustion ash is about one-third by weight carbonates. The carbon analysis revealed a

dichotomy between PM emissions from under-feed and top-feed pellet stove models

(Figure 2) and between pellet stoves in general and cordwood stoves. A large fraction of

the PM emitted from the top-feed model was elemental carbon. Entrained ash, however,

as indicated by the carbonate carbon, was not detectable. The elemental carbon fraction

increased with increasing burn rate, reaching 88% of the total PM emissions at the highest

burn rate. In contrast, virtually no elemental carbon was detected in the PM emitted from

the under-feed model, but entrained ash was estimated as comprising 26 and 8% of the

PM emissions at a medium burn rate for softwood and hardwood pellets, respectively. 

Visual observation of filters used to collect the PM samples confirmed the difference in

chemical makeup of the PM. The filters used to collect PM from both the top-feed model

used for the PFI/HPA study and the 1990 top-feed model used for the EPA study were

black, characteristic of elemental carbon (also called graphitic carbon or soot). However,

the filters used to collect PM from the under-feed model were light tan. The chemical

makeup of PM emitted from a cordwood stove is unlike that from either top- or underfeed pellet stoves. Typically, particles from a cordwood stove are composed of 10 to 20%

elemental carbon and less than 1% inorganic ash (Watson et al., 1988 and Houck et atl.,

1989). In addition, cordwood stove PM filters are generally dark brown to black.

Total PM emissions have often been used as a surrogate for air emissions of specific toxic
compounds such as polycyclic organic matter (POM). The gross chemical differences

among PM emitted from top-feed pellet stoves, under-feed pellet stoves, and traditional

cordwood stoves demonstrated that total PM emissions cannot reliably be used as a

surrogate for individual or groups of specific organic species when comparing emissions

among these different technology types.

Efficiency

The efficiencies of pellet stoves are considerably higher than those of cordwood stoves. 

Efficiencies for new-model pellet stoves have been measured, by OMNI Environmental

Services, to be as high as 87%; whereas, the efficiencies for uncertified cordwood stoves

are estimated as 54% (U.S. EPA, 1996). The efficiencies of new-technology pellet stoves

are much higher than the earliest models which had efficiencies documented from the inhome studies of 56 and 68% for exempt and certified models, respectively (U.S. EPA,

1996). The significance of higher efficiency, beyond its obvious desirability, is that less

fuel is used by a home occupant to produce the same amount of heat, consequently the

effective reduction in air emissions per unit of heat produced is even greater than the

emissions factors, reported in units of mass pollutant per mass of fuel burned, would

imply.

Greenhouse Gas and Acid Precipitation Impacts

There are two well documented air quality advantages associated with all residential wood

combustion (not just pellets) which are important to consider when evaluating the

environmental implications of home heating options. These advantages are low

greenhouse gas emissions and low acid precipitation impacts. When all the steps involved

in energy production are taken into consideration, home heating with wood produces less

than half the carbon equivalents of greenhouse gases per unit of energy than any other

home heating option (Houck, et al., 1998). The release of methane and carbon dioxide

into the atmosphere from the activities leading to the production of space heat is

responsible for the greenhouse impacts from home heating. In addition to the high energy

return on investment (EROI) associated with wood fuel, harvesting of mature trees for

fuel permits more rapid carbon fixation in younger replacement trees and reduces the

effective greenhouse impacts from wood fuel combustion.

Most acid precipitation impacts are produced by sulfur gases or nitrous oxide gases

released during the extraction, processing, and the higher temperature combustion of fossil

fuels. Little fossil fuel is invested in the production of space heat from wood (including

pellets). A detailed analysis of emissions from each step of the energy production process

shows that residential wood combustion produces the lowest amount of acid equivalents

(a measure of the acid precipitation potential) per unit of heat among all the home space

heating options (Houck, et al., 1998).

Solid Waste Disposal
The residue (unburned wood char and inorganic salts) remaining after combustion of fuel

in a cordwood stove typically ranges from 1 to 5% of the fuel mass. The residue for a 

pellet stove averages less than 0.5% of the fuel mass. Due to the higher efficiencies of

pellet stoves, less fuel mass is required to satisfy the same heat demand with a pellet stove

than with a cordwood stove. The combination of less fuel mass burned and a lower

percent residue production makes solid waste disposal from pellet stoves significantly less

of an issue than for cordwood stoves. In addition, wood ash (derived from either

cordwood or pellets) is relatively benign. In fact, its high calcium carbonate and

potassium content makes it a good agricultural soil amendment.

CONCLUSIONS

The key conclusions that were reached by reviewing the pellet stove emission data are:

C New-technology pellet stoves produce much less CO than uncertified cordwood

stoves.

C New-technology pellet stoves produce much less PM than uncertified cordwood

stoves.

C The emissions factors for both CO and PM are lower for new-model pellet stoves

than for earlier models.

C Not all PM emitted from pellet stoves are PM10 or PM2.5. Approximately 84% of

PM is PM10 and about 81% is PM2.5. PM10 and PM2.5 emissions factors, if based

on total PM measurements, should be adjusted accordingly.

C The chemical makeup of PM emitted from top-feed pellet stoves and under-feed

pellet stoves is different, and the chemical makeup of PM from both technology

types is different from that emitted from cordwood stoves. Consequently, total

PM emissions are not accurate surrogates for emissions of specific organic

compounds such as those identified as “air toxics.”

C Pellet stoves generate less solid waste than cordwood stoves.

C The high efficiencies and low emissions of PM and CO characteristic of newtechnology pellet stoves, combined with low greenhouse gas impacts, low acid

precipitation impacts, and minimal solid waste issues, make pellet stoves an

environmentally sound home space heating option.

ACKNOWLEDGMENTS

The authors would like to thank: Dane Harman of Harman Stove Company for providing

a pellet stove, Jerry Whitfield and Doug Jenks of Pyro Industries for providing a pellet

stove and information on older model stoves, Averill Cook of Catamount Fuel

Corporation for providing pellet fuel, Robert Cary of Sunset Laboratory for conducting

carbon analyses, and Robert McCrillis, now retired from the U.S. EPA, for his

participation in the pellet stove testing program. 

REFERENCES
1. Barnett, S.G. and Fields, P.G., 1991, In-home Performance of Exempt Pellet

Stoves in Medford, Oregon, OMNI Environmental Services, Inc. report to U.S.

Department of Energy, DOE/BP-04123-2, Portland, OR, 44 pp. plus appendices.

2. Barnett, S.G. and Roholt, R.B., 1990, In-home Performance of Certified Pellet

Stoves in Medford and Klamath Falls, Oregon, OMNI Environmental Services Inc.

report to U.S. Department of Energy, DOE/BP-04143-1, Portland, OR, 35 pp.

plus appendices.

3. Barnett, S.G., Houck, J.E. and Roholt, R.B., 1991, In-Home Performance of

Pellet Stoves in Medford and Klamath Falls, Oregon, presented at A&WMA 84th

Annual Meeting, Vancouver, BC, June 16-21, 1991, paper 91-129.3.

4. E.H. Pechan & Associates, 1993, Emission Factor Documentation for AP-42

Section 1.10, Residential Wood Stoves, report for U.S. Environmental Protection

Agency’s Office of Air Quality Planning and Standards, EPA Contract No 68-D0-

0120.

5. Houck, J.E., Chow, J.C., Watson, J.G., Simons, C.A., Pritchett, L.C., Goulet, J.M.

and Frazier, C.A., 1989, Determination of Particle Size Distribution and Chemical

Composition of Particulate Matter from Selected Sources in California, OMNI

Environmental Services, Inc. report to California Air Resources Board, NTIS

PB89 232805., Sacramento, CA.

6. Houck, J.E., Tiegs, P.E., McCrillis, R.C., Keithley, C. and Crouch, J., 1998, Air

Emissions from Residential Heating: The Wood Heating Option Put into

Environmental Perspective, in: Proceedings of the A&WMA and EPA Specialty

Conference: Emissions Inventory: Living in a Global Environment, vol. 1, pp.

373-384.

7. U.S. Environmental Protection Agency, 1988, 40 CFR Part 60, Standards of

Performance for New Stationary Sources; New Residential Wood Heaters, Federal

Register, Vol. 53, No. 38, Friday, February 26, 1988, pp. 5860-5926.

8. U.S. Environmental Protection Agency, 1996, Compilation of Air Pollutant

Emission Factors, AP-42, Fifth Edition, Volume I, Stationary Point and Area

Sources, Chapter 1, External Combustion Sources, Section 1.10, Residential

Wood Stoves (Supplement B), October 1996, 

http://www.epa.gov/ttn/chief/ap42c1.html

9. U.S. Environmental Protection Agency, September 21, 1998, Master

Manufacturer List, Woodheater Program, Telephone 202-564-7021,

Manufacturing, Energy and Transportation Division, Washington, D.C., 59 pp. 

10. U.S. Environmental Protection Agency, October 21, 1999, List of Certified

Woodstoves, Wood Heater Program, Telephone 202-564-7021, Manufacturing,

Energy and Transportation Division, Washington, D.C., 24 pp.

11. Watson, J.G., Chow, J.C., Richards, L.W., Neff, W.D., Andersen, S.R., Dietrich,

D.L., Houck, J.E. and Olmez, I., 1988, The 1987-88 Metro Denver Brown Cloud

Study, Volume III: Desert Research Institute report to 1987-88 Metro Denver

Brown Cloud Study, Inc., Denver, CO.
Table 1 Carbon Monoxide and Particulate Emissions

Description Pellet burn rate

(dry kg/hr)

CO emission factor

(g CO/dry kg fuel)

PM emission factor

(g PM/dry kg fuel)a

Conventional

cordwood stove

NAb115.4 12.0

90/91 exempt pellet

stoves, avg. 24 oneweek runs, softwood

pellets 

0.58 26.1 2.77

89/90 certified pellet

stoves, avg. 23 oneweek runs, softwood

pellets 

0.70 22.4 1.29

Certification test

results, avg. 23

models 

1.16 NDc0.70

Lab tests 1990

model, hardwood

pellets 

0.7 23.2 3.5

0.8 27.8 2.0

0.9 29.7 3.0

1.6 155 7.6

New top-feed,

softwd. pellets, test 1 

0.72 7.19 0.44

New top-feed,

softwd. pellets, test 2

1.46 2.34 0.60

New top-feed,

softwd. pellets, test 3

2.45 8.17 1.0

New bottom-feed,

softwd. pellets, test 4

1.55 1.8 0.26

New bottom-feed,

hrdwd. pellets, test 5

1.55 2.7 0.40

a

 All PM data adjusted to U.S. EPA Method 5G (40CFR, Part 60, App. A) equivalent to

permit comparisons. Data for conventional cordwood stove from AP-42 (reference 8).

b

 Not applicable. 

c

 Not detected. 
0

50

100

150

200

250

300

350

400

450

1.58 2.40 3.22 4.06 4.90

Burn Rate (kg/hr)

Exhaust CO (ppm)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1 2 3 4 5

Test #

Particulate Emissions (g/kg)

Organic Compounds

Elemental Carbon

Ash

Figure 1. Carbon Monoxide Concentration in Exhaust Gas versus Burn Rate with a New

Top-Feed Pellet Stove Model Burning Softwood Pellets.

Figure 2. Composition of PM emissions. (Tests 1-3 are for a top-feed model burning

softwood pellets at 0.72, 1.46, and 2.45 kg/hr. Tests 4 & 5 are for an under-feed model.

Softwood pellets at 1.55 kg/hr were burned in test 4, hardwood at 1.55 kg/hr in test 5.)