Environmental, Energy Market, and 

Health Characterization of Wood-Fired 

Hydronic Heater Technologies

Executive Summary 

June 2012

No. 12-15

New York State Energy Research and Development Authority
NYSERDA’s Promise to New Yorkers:

New Yorkers can count on NYSERDA for 

objective, reliable, energy-related solutions 

delivered by accessible, dedicated professionals.

	 Our Mission: Advance innovative energy solutions in ways that improve New York’s 

economy and environment.

	 Our Vision: Serve as a catalyst—advancing energy innovation and technology, 

transforming New York’s economy, and empowering people to choose 

clean and efficient energy as part of their everyday lives.

	Our Core Values: Objectivity, integrity, public service, and innovation.

Our Portfolios

NYSERDA programs are organized into five portfolios, each representing a complementary group of offerings with 

common areas of energy-related focus and objectives.

Energy Efficiency & Renewable Programs

Helping New York to achieve its aggressive clean energy goals – 

including programs for consumers (commercial, municipal, institutional, 

industrial, residential, and transportation), renewable power suppliers, 

and programs designed to support market transformation.

Energy Technology Innovation & Business Development

Helping to stimulate a vibrant innovation ecosystem and a clean 

energy economy in New York – including programs to support product 

research, development, and demonstrations, clean-energy business 

development, and the knowledge-based community at the Saratoga 

Technology + Energy Park®. 

Energy Education and Workforce Development

Helping to build a generation of New Yorkers ready to lead and work 

in a clean energy economy – including consumer behavior, K-12 

energy education programs, and workforce development and training 

programs for existing and emerging technologies.

Energy and the Environment

Helping to assess and mitigate the environmental impacts of 

energy production and use – including environmental research and 

development, regional initiatives to improve environmental sustainability, 

and West Valley Site Management.

Energy Data, Planning and Policy

Helping to ensure that policy-makers and consumers have objective 

and reliable information to make informed energy decisions – including 

State Energy Planning, policy analysis to support the Low-Carbon 

Fuel Standard and Regional Greenhouse Gas Initiative, nuclear policy 

coordination, and a range of energy data reporting including Patterns 

and Trends.
Environmental, Energy Market, and Health Characterization of

Wood-Fired Hydronic Heater Technologies

Executive Summary

Prepared for the

New York State Energy Research and

Development Authority

Albany, NY

nyserda.ny.gov

Ellen Burkhard, Ph.D.

Senior Project Manager 

and

Nathan Russell

Assistant Project Manager

Prepared by:

U.S. Environmental Protection Agency, Office of Research and Development

Research Triangle Park, NC

Brian Gullett, Ph.D.; Rebecca Dodder, Ph.D.; M Ian Gilmour, Ph.D.; Michael Hays, Ph.D.; Mr. John

Kinsey; William Linak, Ph.D.; Dan Loughlin, Ph.D.; Lukas Oudejans, Ph.D.; Tiffany Yelverton, Ph.D.

and

U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards

Research Triangle Park, NC

Gil Wood, 

Mike Toney

ARCADIS U.S., Inc., 

Durham, NC

Abderrahmane Touati, Ph.D.

Post-Doctoral Fellows to the U.S. EPA

Johanna Aurell, Ph.D. (National Research Council)

Seung-Hyun Cho, Ph.D. (Oak Ridge Institute for Science Education)

University of Dayton Research Institute,

Dayton, OH

Sukh Sidhu, Ph.D.; K.A. Moshan S.P. Kahandawala, Ph.D.

NYSERDA NYSERDA 10665 June 2012

Report 12-15 ISBN: 978-1-936842-03-2
NOTICE

This report was prepared in the course of performing work sponsored by the New York State Energy 

Research and Development Authority and the U.S. Environmental Protection Agency’s Office of Research 

and Development. The opinions expressed in this report do not necessarily reflect those of NYSERDA or 

the State of New York, and reference to any specific product, service, process, or method does not 

constitute an implied or expressed recommendation or endorsement of it. Further, NYSERDA and the State 

of New York make no warranties or representations, expressed or implied, as to the fitness for particular 

purpose or merchantability of any product, apparatus, or service, or the usefulness, completeness, or 

accuracy of any processes, methods, or other information contained, described, disclosed, or referred to in 

this report. NYSERDA and the State of New York make no representation that the use of any product, 

apparatus, process, method, or other information will not infringe privately owned rights and will assume 

no liability for any loss, injury, or damage resulting from, or occurring in connection with, the use of 

information contained, described, disclosed, or referred to in this report.
iii

ABSTRACT

This report describes a comprehensive emission, lifetime cost, energy market, and health characterization 

program on four wood-fired hydronic heaters (HHs) that span common to advanced technologies. The HHs 

were variously tested with two species of split logs, hardwood with refuse, and hardwood pellets for their 

performance in meeting the daily heat load requirements of a typical winter day in upstate New York. An 

extensive array of pollutants was sampled in batch and real time, including particulate matter (PM), carbon 

monoxide (CO), volatile organics, semivolatile organics, and greenhouse gases for determination of 

emission factors. Emissions were expressed in terms of energy input, energy output, and on a temporal 

basis as available. Significant differences were observed in energy and emission performance from the four 

units. Tests using a cone calorimeter showed that its emissions were predictive of the full scale units under 

fully ventilated and air starved conditions. Modeling regional residential space heating scenarios showed 

that the wood heat market share determined the total PM emissions for the residential sector, and that 

relatively modest changes in the wood heat market can have substantial impacts on residential and total PM 

emissions. The rate of turnover and retirement of older, highly emitting units to more efficient, loweremitting units is critical to avoiding what could be substantial increases in emissions related to residential 

wood heat over the next 5-10 years. In an assessment of lifetime costs of HHs, fuel costs were shown to 

have the potential to dominate purchase and installation costs; as a result, market competitiveness is driven 

by efficiency and access to low cost wood fuel. Emissions toxicity results from animal exposure 

experiments were inconclusive, as extreme dilution of the combustion gas was necessary to avoid 

immediate acute toxic effects from the CO that at times exceeded 10,000 parts per million (ppm).

KEY WORDS

Outdoor wood-fired HHs, outdoor wood boilers, pellet burners, heat storage, gasification burners, 

emissions, particulate matter, energy, levoglucosan, methoxyphenols, polycyclic aromatic hydrocarbons, 

cone calorimeter, biomass
iv

ACKNOWLEDGMENTS

This research was funded by the New York State Energy Research and Development Authority 

(NYSERDA) with additional support provided by the U.S. Environmental Protection Agency (EPA), 

Office of Research and Development, through a Cooperative Agreement, CR05058. ARCADIS U.S., Inc. 

was funded by EPA through Contract No. EP-C-09-027. Dr. Aurell was supported by a grant from EPA 

through the National Research Council. Dr. Cho was supported by a grant from EPA through the Oak 

Ridge Institute for Science Education.

NYSERDA appreciates the guidance of the Project Advisory Committee: Thomas Butcher, Ph.D., 

Brookhaven National Laboratory; Michael Cronin P.E., New York State Department of Environmental 

Conservation; Richard Gibbs, Ph.D., P.E.; Daniel Luttinger, Ph.D., New York State Department of Health; 

Lisa Rector, Northeast States for Coordinated Air Use Management; Richard Schlesinger, Ph.D., Pace 

University; and Judith Schreiber, Ph.D., New York State Office of the Attorney General.

The authors acknowledge the testing assistance of Steve Terll, Bill Preston, Donnie Gillis, Charly King, 

John Nash, and Daniel Janek of ARCADIS U.S., Inc. EPA’s Office of Air Quality Planning and Standards 

(OAQPS) provided two of the four units tested. Dr. Lukas Oudejans of EPA’s National Homeland Security 

Research Center conducted the resonance enhanced multiphoton ionization/ time-of-flight mass 

spectrometry (REMPI-TOFMS) sampling. Representatives from all of the companies that supplied units 

assisted with the unit tie-ins and operation and their contributions are gratefully acknowledged.

We thank Elizabeth Boykin, Debora Andrews, Judy Richards, Jim Lehmann and Rick Jaskot for their 

technical assistance. The emissions, economic, and MARKet Allocation (MARKAL) chapters have been 

reviewed by the Quality Assurance (QA) officers of the National Risk Management Research Laboratory 

and approved for distribution. The planning documents, raw data and health chapter have been reviewed by 

QA officers of the National Health and Environmental Effects Research Laboratory, EPA, and approved for 

distribution. Approval does not signify that the contents necessarily reflect the views and policies of the 

Agency, nor does the mention of trade names or commercial products constitute endorsement or 

recommendation for use. 
S-1

EXECUTIVE SUMMARY

Wood-fired hydronic heaters (HHs) have proliferated in Northern states during the last decade as oil prices have 

increased. Some of these units are inefficient and have resulted in numerous complaints to state air quality and 

health departments because of exceptionally high levels of smoke. Fine particles in wood smoke are primarily 

composed of organic carbon (OC) and contain numerous toxic compounds, including polycyclic aromatic 

hydrocarbons (PAHs). Recent reviews of the health literature indicate that wood smoke exposure likely leads to 

a range of adverse health effects including increases in respiratory symptoms, lung function decreases, increases 

in asthma symptoms, visits to emergency rooms, and hospitalizations (Naeher et al., 2007; Schreiber and 

Chinery, 2008). High-efficiency HH units are relatively common in Europe and now are being manufactured in 

the U.S. by a few companies. The combustion efficiency improvements are due in part to a two-stage 

combustion chamber design that results in gasification of the fuel and more complete combustion in the second 

chamber. Despite the high level of environmental concern due to emissions from the older units and the more 

promising performance of the newer units, little data has been collected to understand emissions and potential 

human health risks associated with HHs. 

A joint project between the U.S. Environmental Protection Agency (EPA) Office for Research and Development 

(ORD) and the New York State Energy Research and Development Authority (NYSERDA) addressed this data 

gap by testing four current and emerging technology HHs, which are also referred to as Outdoor HHs, or HHs, 

and Outdoor Wood-fired Boilers (OWBs). The emissions and energy-efficiency performance of four types of 

residential wood boiler technologies ranging from the common HH to a high-efficiency pellet heater to a unit 

with thermal storage were characterized. Measurements included emissions of particulate matter (PM), 

elemental carbon (EC), carbon monoxide (CO), PAHs, volatile organic compounds (VOCs), semi-volatile 

organic compounds (SVOCs), and polychlorinated dibenzodioxins/dibenzofurans (PCDDs/Fs). This work was 

complemented by an energy and market impacts analysis of HHs for the State of New York. Lastly, the health 

effects of HH emissions were evaluated with an exposure study for pulmonary and systemic biomarkers of 

injury and inflammation. The results of this study are anticipated to be of value to the State of New York in its 

efforts to develop a high-efficiency biomass heating market of technologies with acceptable emissions 

performance. It is also anticipated that these results will be of value to EPA as it sets New Source Performance 

Standards for biomass-fired HHs.

Wood Hydronic Heater Technologies Tested

This project provides a thorough scientific evaluation of the performance of a range of wood boiler 

technologies. The units tested included a commonly-used Conventional Single Stage HH, a newer Three Stage 

HH model, a European Two Stage Pellet Burner, and a U.S. Two Stage Downdraft Burner (see Table 1). Each 

unit was evaluated and tested on the same 24-hour wintertime daily “call for heat” load determined for a typical 

home (2500 ft2

) in Syracuse, New York.
S-2 

Table 1. Outdoor Wood-Fired Hydronic Heaters (HHs) Used in this Study.

Unit Model

Conventional, Single 

Stage HH, Single 

Stage HH

Three Stage 

HH

European Two 

Stage Pellet Burner 

U.S. Two Stage 

Downdraft 

Burner

Unit # 1 2 3 4 

Technology Combustion Three-stage 

Combustion

Staged Combustion Two-stage: 

Combustion and 

Gasification with 

Heat Storage

Fuel Wood logs Wood logs Wood pellets Wood logs

Heat Capacity, 

output Btu/hour 

(kW)

NA 160,000 (46.9)

2 137,000 (40)3 150,000 (44)4 

Water Capacity 

gal (liters)

196 (740) 450 (1700) 43 (160) 32 (120)

1

Not available from the manufacturer

2

Eight hour stick wood test

3

Partial load output, based on manufacturer’s specifications

4

Heat rate based on manufacturer claim

The conventional, Single Stage HH uses a natural draft, updraft combustion single-stage combustion process 

that occurs in a rectangular firebox surrounded by a high capacity water jacket (Figure 1). The hot flue gases are 

vented through a stainless steel, insulated chimney connected to a rear exhaust outlet. Flue gas movement is by 

natural convection, assisted with a fan. Heat flow is regulated by the opening and closing of a combustion 

damper.

Figure 1. The Conventional, Single Stage HH and Illustration of an Up-Draft Combustion Unit.
S-3

The Three Stage HH (46.9 kW, 160,000 BTU/hour, Figure 2) uses a three-stage combustion process in which 

wood is gasified in the primary combustion firebox, the hot gases are forced downward and mixed with superheated air starting the secondary combustion. Final combustion occurs in a third, high temperature reaction 

chamber. Like the conventional, Single Stage HH, the Three Stage HH is regulated by the opening and closing 

of an air damper.

Figure 2. The Three Stage HH Unit and Illustration of a Down-Draft Combustion Unit.

The European Pellet unit (Figure 3) is a commercially available, pellet burning HH rated at 40 kW (137,000 

Btu/hour). Combustion occurs on a round burner plate where primary air is supplied. Secondary air is 

introduced through a ring above the burner plate. Fuel is automatically screw-conveyed from the bottom. 

Operation of the screw feeder was regulated by a thermostat. During normal operation, the fan modulates based 

on the measured oxygen level in the exhaust gas, maintaining 8-10% oxygen

The U.S. Two Stage Downdraft Burner (44 kW, 150,000 BTU/hour, Figure 4) is a two-stage heater with both 

gasification and combustion chambers. Air is added to the firebox continuously while the damper is open and is 

blown downwards through the wood logs. The gases are forced into a combustion chamber where additional 

super-heated air is added, resulting in a final combustion of the gases at temperatures higher than 980 °C 

(1800 °F).
S-4

Figure 3. The European Two Stage Pellet Burner and Illustration of a Bottom-Fed Pellet Combustion 

Unit.

Figure 4. The U.S. Two-Stage, Down-draft Combustion and Gasification Unit Schematic.

Secondary 

super-heated 

air supply

Primary 

air supply

Combustion and gasification

Secondary 

combustion zone

Combustion 

zone
S-5

FUEL LOADING AND CHARACTERIZATION

The fuel loading protocol was derived from the simulated heat-load demand profile and the type of unit and its 

capacity. The Conventional, Single Stage HH unit was used to compare emissions for three fuel types including 

seasoned red oak, unseasoned white pine, and red oak with 4.5% by weight supplementary refuse. The Three 

Stage HH was tested solely with seasoned red oak. A European Two Stage Pellet Burner and a split-log wood 

heater (U.S. Two Stage Downdraft Burner) with a simulated heat storage tank were tested under the same heatload demand profile to characterize and compare their emission signatures. A common fuel type (red oak) was 

used across all units (hardwood pellets for the European unit) for comparability. The pellets are made out of 

sawdust from different wood processing industries and consisted of a blend of hardwood (no bark), mostly oak, 

with a diameter of 6 mm. The ultimate and proximate analyses of the fuels are reported in Table 2. Fuel 

moisture was determined using a wood moisture meter for three to four measurements on each of eight pieces of 

split wood chosen randomly from each charge.

Table 2. Fuel Ultimate/Proximate Analysis.

Properties

Fuel

Red Oak Pine Pellets

Ash 1.46% 0.44% 0.52%

Loss on Drying (LOD) 22.52% 9.68% 7.24%

Volatile Matter 84.23% 88.50% 84.27%

Fixed Carbon 14.31% 11.06% 14.11%

C :Carbon 48.70% 51.72% 50.10%

Cl: Chlorine 38 ppm 36 ppm 44 ppm

H: Hydrogen 5.96% 6.57% 5.86%

N: Nitrogen <0.5% <0.5% <0.5%

S: Sulfur <0.05% <0.05% <0.5%

“<” = below detection limit

HEATING PERFORMANCE

The heat load profile (Figure 5) that was used throughout the testing program is derived from a simulation 

program for heat demand (Energy-10TM, National Renewable Energy Laboratory 

[http://www.nrel.gov/buildings/energy10.html?print]) for a 232 m2 (2500 ft2

) home in Syracuse, New York, 
S-6

using an averaged hour-per-hour heat load for the first two weeks of January averaged over 25 years 

(Brookhaven National Laboratory). The average daily heat load for the first two weeks in January is about 

827 MJ (784,000 BTU) with a maximum heat load of about 40,000 BTU/hr.

Figure 5. Syracuse, New York Area Heat Load Profile for the First Two Weeks of January.

The heat load demand was simulated by extracting the HH outlet heat with a water/water heat exchanger 

coupled to the building chilled water supply (Figure 6). The HH units were operated in a mode where hot water 

was continuously circulated through the water/water heat exchanger and the unit’s water jacket. The preinsulated piping system consists of two 25.4 mm (1 inch) oxygen barrier lines that are insulated with high 

density urethane insulation. The same piping system was used for all four units tested. The inlet and outlet 

temperatures of both the chilled water and recirculated hot water were monitored, as well as the chilled water 

flow rate. The heat load demand control system calculated the change between the chilled water outlet 

temperature and the chilled water inlet of the heat exchanger and controlled the heat removal by adjusting the 

chilled water flow rate through the use of a proportional valve. 
S-7

Figure 6. Test System for Wood-Fired Hydronic Heaters.

The units with cyclical damper operation to modulate their heat release resulted in considerable variation of heat 

transfer and concomitant emissions. When the dampers were closed, combustion became oxygen starved, 

resulting in incomplete combustion of the fuel and formation of pollutants. Upon damper opening and gas flow 

through the system, these pollutants are released, resulting in a cyclical increase in pollutant release. The 

modulating combustion also led to considerable nuisance odor (despite the emissions passing through the 

laboratory facility’s additional air pollution control system (APCS) consisting of an afterburner and scrubber) 

and threatened to terminate the project.

A typical heat release rate for the Conventional, Single Stage HH unit is shown in Figure 7. The oscillating heat 

release reflects the cyclical damper opening and closing. Increased heat release is observed during all open 

damper periods when the fuel combustion rate is enhanced by the air supply. The frequency and duration of the 

damper openings is a function of the degree to which the unit is oversized for the heat load. The heat release rate 

is significantly higher than that required for the Syracuse winter load (about 40,000 BTU/hr). The European 

Pellet unit’s moderate cyclical heat release (Figure 8) more closely matches the heat load demand. The U.S. 

Two Stage Burner unit burns continuously, storing its energy in a thermal storage tank (Figure 9).

Heat exchanger

Hot water recirculation loop Chilled 

water

Hot water 

to building

Internal sampling platform

Building 

wall

8” OD stack

10” Stainless duct

To 

inhalation 

chambers

Indoor sampling duct

CEM

Flow Measurements

Particulate Measurements

CEM

M-23

ELPI/TEOM

PAHS

Volatiles

EC/OC

REMIPI/TOFMS

ATOFMS

Air pollution Control system

QStack

Qinput

Qoutput

External sampling platform

Qother losses

Hot water recirculation loop

Primary 

dilution

Secondary 

dilution

Heat exchanger

Hot water recirculation loop Chilled 

water

Hot water 

to building

Internal sampling platform

Building 

wall

8” OD stack

10” Stainless duct

To 

inhalation 

chambers

Indoor sampling duct

CEM

Flow Measurements

Particulate Measurements

CEM

M-23

ELPI/TEOM

PAHS

Volatiles

EC/OC

REMIPI/TOFMS

ATOFMS

Air pollution Control system

QStack

Qinput

Qoutput

External sampling platform

Qother losses

Hot water recirculation loop

Primary 

dilution

Secondary 

dilution
S-8

Figure 7. Heat Release Rate and System Water Temperatures for the Conventional, Single Stage HH

Unit Firing Red Oak.

0 1 2 3 4 5 6

0

20,000

40,000

60,000

80,000

100,000

120,000

140,000

160,000

180,000

200,000

220,000

240,000

0

20

40

60

80

100

120

140

160

180

200

 Heat Release Rate

Heat Release rate (BTU/hr)

Run Time (hr)

 Outlet Water Temperature

 Inlet Water Temperature

 High Heater Temperature Set Point

 Low Heater Temperature Set Point

 Heater Outlet Water Temperature (οF)

Figure 8. Heat Release Rate and System Water Temperatures for the European Two Stage Pellet Burner

Unit.

0 4 8 12 16 20 24

0

200000

400000

600000

800000

1000000

0

20

40

60

80

100

120

140

160

180

200

 Heat Release Rate

Heat Release rate (BTU/hr)

Run Time (Hours)

 Outlet Water Temperature

 Inlet Water Temperature

 Heater Inlet/Outlet Temperature (oF)
S-9

Figure 9. Heat Release Rate from the U.S. Two Stage Downdraft Burner Unit with Thermal Storage.

The performance of HH systems can be evaluated based on their ability to burn the fuel completely (combustion 

efficiency), the effectiveness of the heat exchanger to transfer the heat generated from the combustion process to 

the water (boiler efficiency), and the overall generation of useful heat through its transfer to meet the load 

demand (thermal efficiency). Table 3 summarizes all these efficiencies for all six unit/fuel combinations (boiler 

efficiency is not presented for cyclical units due to the difficulties inherent in quantifying dynamic 

measurements). No thermal efficiency can be calculated for the U.S. Two Stage Downdraft Burner unit because 

measurements of the thermal flows through the water/air heat exchanger were not recorded. The cyclical units 

had lower efficiencies than the pellet unit and the non-cyclical unit with heat storage. Efficiency improvements 

can be achieved by reducing the time spent at idle (closed damper) which can be accomplished by proper unit 

sizing and the use of thermal storage. As the HH’s nominal output increases above that of the building’s heat 

load, the amount of time spent at idle is increased (the damper remains closed for a longer time). The work 

reported here shows that in these closed damper periods energy and emissions performance decreases greatly. In 

the presence of an external thermal storage system, the low mass/volume ratio of the Two Stage Downdraft 

Boiler HH system allows it to run at maximum output under relatively steady-state conditions, improving

performance. The thermal efficiencies, ranging from 22% to 44% for the conventional, three stage, and pellet 

systems, compare poorly with oil and natural gas fired residential systems with thermal efficiencies ranging 

from 86% to 92% and 79% to 90%, respectively (McDonald, 2009).

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

100000

200000

300000

400000

500000

600000

0

20

40

60

80

100

120

140

160

180

200

220

 Heat Release Rate

Heat Release Rate (BTU/hr)

Run Time (Hours)

 Outlet Water Temperature

 Set Point Temperature

 Water temperature (oF)
S-10

Table 3. Hydronic Heater Efficiencies.

Units Thermal Efficiency (%) Boiler Combustion 

Conventional HH RO Average 22 NC 74

STDV 5 3.0

Conventional HH RO + Ref Average 31 NC 87

STDV 2.2 3.4

Conventional HH WP Average 29 NC 82

STDV 1.8 3.2

Three Stage HH/RO Average 30 NC 86

STDV 3.2 1.8

European Pellet/pellets Average 44 86 98

STDV 4.1 3.5 0.16

U.S. Downdraft RO

Average IM 83 90

STDV 0.71 0.79

NC = Not calculated. IM = Insufficient measurements taken for this calculation

The unit efficiencies can also be viewed through the amount of fuel required to satisfy a given heat load. Figure 

10 shows that amount of fuel mass required to supply the 24 hour Syracuse heat load. The European Pellet unit 

requires significantly less wood mass to meet this demand (the U.S. Two Stage Downdraft unit’s wood mass 

could not be calculated because measurements of the thermal flows through the water/air heat exchanger were 

not recorded.

EMISSIONS

Carbon Monoxide

A full emissions characterization for each heater unit consisted of, at a minimum, PM (time integrated and real 

time), total hydrocarbons (THC), PAHs, organic marker compounds, organic carbon/elemental carbon (OC/EC),

CO, CO2, CH4, N2O, and PCDD/F. The results of this study are compared with those of EPA’s Office of Air 

Quality Planning and Standards (OAQPS) ongoing validation tests of EPA Method 28 for HH PM and energy 

efficiency (http://www.vtwoodsmoke.org/pdf/Method28.pdf ), particularly for the seasoned red oak fuel since 

this is the fuel specified in Method 23 OWHH.
S-11

Figure 10. Mass of Fuel Needed for a 24 Hour Syracuse Heat Load. Data are missing for U.S. Downdraft 

RO.

Temporal emission profiles were more a function of the elapsed time from the last fuel charging than that of the 

heat load on the unit (Figure 11). The emissions of CH4, THC, and CO (Figure 12) are consistent with the cyclic 

nature of the damper openings. These emissions are associated with the damper cycle creating alternately poor 

and good combustion conditions. Units that cycle the damper opening to regulate the heat production have much 

higher emissions than the pellet burner and the non-cycling U.S. Downdraft Unit unit. Predictably, lower CO 

emission factors result from those units that minimize pollutant formation. 

Figure 11. CO Stack Concentration as a Function of Damper Opening and Time of Fuel Charging, 

Conventional, Single Stage HH unit.

Conventional HH RO

Conventional HH WP

Three Stage HH RO

European Pellet

US Downdraft RO

0

50

100

150

200

250

300

350

400

450

 Mass of Fuel Needed for the 24-h Syracuse Heat Load (lbs)

Hydronic Heater Unit and Fuel Type

0 3 6 9 12 15 18 21 24

0

1x104

2x104

3x104

4x104

5x104

6x104

7x104

8x104

Run Time (hr)

 Damper Open

2nd charge

 CO Emissions at the Stack (ppmv)
S-12

Figure 12. Typical CO Concentration Traces from the Dilution Tunnel for the Conventional, Single Stage 

HH Unit.

CO emission factors (Figure 13) are complementary to CO2 emission factors (not shown). The European Pellet 

Boiler unit has the lowest value at 0.60 g/MJ (1.39 lb/MMBtu). A value of 7.2 g/MJ (16.6 lb/MMBtu) was 

obtained for the U.S. Downdraft Unit heater while the Conventional, Single Stage HH (average of the three 

fuels) had the highest value at about 8.9 g/MJ (21 lb/MMBtu) input. The European Pellet Burner unit is 

predictably lower in CO emissions as combustion is comparatively steady throughout its 6-hour burn, whereas 

the other units have variation in their combustion rate. These CO emission factors are orders of magnitude 

higher than are typically observed in conventional energy sources such as residential oil-fired heaters (< 0.1 lbs 

CO/MMBtu input, Krajewski et al., 1990).

0 3 6 9 12

0

1000

2000

3000

4000

5000

6000

7000

8000

0

1000

2000

3000

4000

5000

6000

7000

8000

0

1000

2000

3000

4000

5000

6000

7000

8000

Run Time (hr)

Oak Wood

Pine Wood

 CO Emissions at the Dilution Tunnel (ppmv)

 Damper Open Oak Wood & Refuse
S-13

Figure 13. Carbon Monoxide Emission Factors. RO = red oak, WP = white pine, Ref = refuse.

Fine Particle Emissions

Testing showed a wide range of PM emissions depending on both unit and fuel types. Figure 14 compares 

average daily PM emissions from the four units and different fuels for a typical Syracuse, New York home on a 

January heating day. These data are analogous to the emissions based on thermal output as the different units 

attempt to match their thermal outputs to the Syracuse load demand. The Conventional, Single Stage HH

burning white pine produced the highest total daily PM emissions [6.3 kg (14 lbs)] and the European Pellet 

Burner heater with red oak reported the lowest [0.036 kg (0.08 lb)]. Emissions for the Three Stage HH and U.S.

Downdraft Unit units were comparable at 0.69 and 0.62 kg/day (1.51 and 1.37 lbs/day), respectively. Again, 

white pine combustion in the Conventional, Single Stage HH unit produced daily PM emissions that were 40% 

greater than red oak and 70% greater than red oak plus refuse.

Conventional HH RO

Conventional HH WP 

Conventional HH RO + Ref

Three Stage HH RO

European Pellet

US Downdraft RO

0

5

10

15

20

25

30

Heat Input

Hydronic Heater Unit and Fuel Type

0

20

40

60

80

100

120

Carbon Monoxide Emission Factor (lb/106BTU\(41)

Heat Output

NA
S-14

Conventional HH RO

Conventional HH WP 

Conventional HH RO + Ref

Three Stage HH RO

European Pellet

US Downdraft RO

0

2

4

6

8

10

12

14

16

Total PM Emitted per Daily Syracuse Heat Load demand (lbs)

Hydronic Heater Unit and Fuel Type

Figure 14. PM Generated per Syracuse Day for All Six Unit/Fuel Combinations. RO = red oak, WP = 

white pine, Ref = refuse.

For the Conventional, Single Stage HH, the PM emissions on a thermal input basis (see Figure 15) for the three 

fuels vary between approximately 2.9 and 5.1 lb/MMBTU with the emissions from the red oak and the red oak 

plus refuse being generally similar (2.9-3.0 lb/MMBTU). The PM emissions almost double, however, when 

white pine is burned in the same unit. Average emissions on a thermal energy input basis ranged from 0.54 

lb/MMBTU for the Three Stage HH, 0.39 lb/MMBTU for the U.S. Downdraft Unit gasifier, and 0.037 lb/106

BTU for the European Pellet Burner. Lower PM emissions from these three units reflect the more advanced 

technologies and generally higher combustion efficiencies compared to the older Conventional, Single Stage 

HH unit. The Three Stage HH employs a secondary combustion chamber and larger thermal mass. The 

European Pellet Burner pellet unit uses a consistent uniform fuel and a more steady-state, but still cyclic, fuel 

feeding approach. The lower emissions from the U.S. Downdraft Unit are likely related to both its two-stage 

gasifier/combustor and its thermal storage design, where batches of fuel are burned during short, highly 

intensive, presumably more efficient periods and the extracted heat is stored for future demand. It should be 

noted, however, that due to our inability to properly measure the thermal flows through the heat storage, the 

thermal output for the U.S. Downdraft Unit was estimated using the heat loss method (boiler efficiency). 
S-15

Conventional HH RO

Conventional HH WP 

Conventional HH RO + Ref

Three Stage HH RO

European Pellet

US Downdraft RO

0

1

2

3

4

5

6

Heat Input

Hydronic Heater Unit and Fuel Type

0

4

8

12

16

20

Total PM Emission Factor (lb/106BTU)

Heat Output

NA

Figure 15. PM Emission Factors for all Six Unit/Fuel Combinations. RO = red oak, WP = white pine, Ref 

= refuse.

A comparison of PM emission factors determined from the current work with other published HH test data is 

shown in Figure 16. These data are taken from different studies (OMNI 2009; OMNI 2007, Intertek 2008), and 

were collected using EPA Method 28 OWHH. The percent rated load calculated from this testing is compared to 

the emission factor from the Method 28 OWHH report for the burn category that represents the same load. For 

the Conventional, Single Stage HH and 2300 this was Category II and for the U.S. Downdraft Unit it was 

Category IV. In the latter case, the maximum rated capacity was used. Also, the pellet emission factor is shown 

on the plot but there are no Method 28 OWHH data available for the pellet burner. The Other Conventional and 

Multi-Stage units are included only for comparison purposes. Data are presented in terms of mass of PM emitted 

per mass of wood burned and only the red oak and hardwood pellet data from this study are included. As shown, 

the EPA method tends to somewhat under-predict the emissions compared with the current work. This underprediction is probably due to the differences between the EPA protocol method (e.g., use of cord wood in this 

project versus crib wood in Method 28 OWHH) and the use of a winter season heat load demand approach used 

here to characterize emissions. Finally, the PM emission rate for an oil-fired boiler is given for reference at 

0.08 g/kg of fuel and cannot be shown on Figure 16.
S-16

Figure 16. Comparisons of PM Emission Factors to other HH Test Data. Note that residential fuel oil = 

0.08 g/kg fuel (Brookhaven National Laboratory).

Particle Composition

The ratio OC/EC was within the range of 20-30 for the Conventional and Three-Stage units regardless of fuel 

type (Figure 17). This ratio is typically greater than one for biomass combustion sources and less than one for 

fossil fuel sources. The OC/EC ratio for the European Pellet Burner pellet unit, on the other hand, was much 

lower indicative of higher combustion efficiency and lower emissions. The OC/EC ratio of the U.S. Downdraft 

unit, however, was only slightly lower than the Conventional and Three-Stage models indicating somewhat 

better combustion efficiency. Emission factors for black carbon in the particulate matter less than or equal to 2.5 

micrometers in diameter (PM2.5) were determined; these are believed to be the first such data for these unit 

types.

0

5

10

15

20

25

30

Conventional Three-Stage European 

Pellet

US 

Downdraft

Other 

Conventional

Multi-Stage

Total PM Emission Factor (g/kg dry fuel)

Comparison of Current Data to EPA Method 28 OWHH

Current Study

Method 28 OWHH
S-17

Conventional HH RO

Conventional HH WP

Conventional HH RO + Ref

Three Stage HH RO

European Pellet

US DownDraft RO

0

10

20

30

40

50

OC, EC and Ash EMission Factors (g/kgFuel dry)

Hydronic Heater Unit and Fuel Type

 Organic Carbon

 Elemental Carbon

 Ash

Figure 17. Average Organic Carbon, Elemental Carbon, and Ash for the Six Unit/Fuel Combinations.

Molecular Composition of the Organic Component of PM

Gas chromatography/mass spectrometry (GC/MS) techniques identified and quantified the PM bound semivolatile organic compounds (SVOCs), which accounted for 9% w/w of the PM emitted from the HH boilers on 

average. The HH PM comprised 1-5% weight percent levoglucosan, an anhydro-sugar and important molecular 

marker of cellulose pyrolysis. The levoglucosan compound accounted for approximately 40% of the quantified 

species. Organic acids and methoxyphenol (lignin pyrolysis products) SVOCs were the compound/functional 

group classes with the highest average concentrations in the HH PM. These compounds are naturally abundant, 

also used as atmospheric tracers, and are important to understanding the global SVOC budget. 

The PAHs explained between 0.1-4% w/w of the PM mass (Figure 18). All 16 of the original EPA priority 

PAHs were detected in the HH PM emissions. The older, Conventional, Single Stage HH unit technology 

emitted PM with higher PAH fractions. In general, the unit/technology type significantly influenced the SVOC 

emissions produced. Combustion of the white pine fuel using the older unit produced notably high SVOC 

emissions per unit energy and per unit mass of wood consumed; particle enrichment of SVOCs was also 

confirmed for this case. Addition of refuse to the seasoned red oak biomass generally resulted in a negligible 

increase in SVOC emissions per unit energy produced with the saturated hydrocarbons noted as an exception. 

Use of the pellet boiler generated the lowest SVOC emissions of the HH tested on a mass of fuel burned basis. 

Nevertheless, the U.S. Downdraft Unit gasifier unit showed the lowest SVOC emissions per unit energy 
S-18

produced. Results show that the phase of the burn cycle can influence the emissions on a compound class basis. 

These and similar differences are highlighted in the main body of the report.

Figure 18. Total PAH Emission Factors.

PCDD/PCDF Emissions

Polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) emissions were sampled and ranged from 0.07 to 

2.1 ng toxic equivalents (TEQ)/kg dry fuel input, with the lowest value from the U.S. Downdraft unit and the 

highest from the Conventional, Single Stage HH with red oak + refuse (see Figure 19). The lowest value, from 

the U.S. Downdraft unit, may be due to the non-cyclical combustion resulting in consistent combustion and 

more complete burnout, but the limited data make this speculative. These values are consistent with biomass 

burn emission factors of 0.91 to 2.26 ng TEQ/kg) (Meyer et al. 2007), woodstove/fireplace values of 0.25 to 2.4 

ng TEQ/kg (Gullett et al., 2003), pellet and wood boilers values of 1.8 to 3.5 ng TEQ/kg, and wood stoves and 

boilers of 0.3 to 45 ng TEQ/kg (Hübner et al, 2005). 

21

13

54

4.6

11

0.49

0

10

20

30

40

50

60

Red Oak Red Oak + 

Refuse

White Pine Red Oak Red Oak Pellets

Conventional HH Three stage HH U.S. DownDraft European

Emission factors

Total PAH mg/Mjinput
S-19

Figure 19. PCDD/PCDF Emissions with Non-Detects = Detection Limit and Zero.

ENERGY AND EMISSIONS IMPACTS OF WOOD HEATING TECHNOLOGIES IN THE HEATING 

MARKET

An energy systems model termed MARKAL (MARKet ALlocation), with the U.S. EPA’s 9-Region database 

(Loughlin et al., 2011; Shay & Loughlin, 2008), was used to examine the broader energy and emissions impact 

of HHs. The goals of this analysis were to: (a) identify possible future scenarios for the penetration of HHs and 

other advanced wood heating systems, (b) place those scenarios in the context of total residential demand for 

space heating and total residential energy demand, and (c) determine the emissions implications of those 

scenarios between 2010 and 2030. Because of the unique nature of the market for wood heating devices and 

wood and pellet fuels, and the non-economic variables that often come into play, modeling this market in a pure 

cost optimization framework presents a challenge. We therefore used the model in a “what if” scenario 

framework, rather than in a predictive framework, asking a number of targeted questions, and running the model 

to assess the impact of certain assumptions regarding total wood heat market size, technology mix, rates of 

turnover, availability (or not) of advanced and high efficiency units, fuel price and availability, and emissions 

rates.

A baseline scenario and four alternative scenarios were examined. The baseline scenario models a modestly 

decreasing market share for wood heat in general, but greater penetration of outdoor HHs over the 2005 through 

2015 time period, along with a changeover from existing wood stoves to cleaner wood stoves. The contribution 

of wood stoves and outdoor HHs to the full market for residential space heating is shown in Figure 20.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Red Oak Red Oak + 

Refuse

White Pine Red Oak Red Oak Pellets

Conventional HH Three stage 

HH

U.S. 

DownDraft

European

Emission factors 

ng TEQ/MJinput

ND = DL

ND = 0
S-20

Figure 20. Market for Residential Space Heating for “Baseline” Scenario (PicoJoules of Usable Energy).

In terms of emissions, this scenario was pessimistic in the assumption that cleaner, more efficient outdoor HHs 

would not be available for the entire modeling horizon. Figure 21 shows the PM emissions trends over time for 

this scenario for all residential energy use (not just space heating). It becomes clear from this comparison that 

even though wood heat is a relatively small contributor to meeting total residential energy demand, it can 

dominate the emissions profile for the residential sector. 

Figure 21. PM Emissions (ktonnes/year) for Total Residential Energy Use for “Baseline” Scenario.

0

100

200

300

400

500

600

700

800

900

1000

2005 2010 2015 2020 2025 2030

PJ useful energy

Conventional HH

Newer Wood Stoves

Existing Wood Stoves

Electricity

Natural Gas

Liquified Petroleum Gas

Kerosene

Heating Oil

0

10

20

30

40

50

60

70

80

90

2005 2010 2015 2020 2025 2030

Emissions (ktonne/yr)

Conventional OWHH

Newer Wood Stoves

Existing Wood Stoves

Electricity

Natural Gas

LPG

Kerosene

Heating Oil
S-21

The “baseline” represents only one possible scenario, and not necessarily the most likely. How the market for 

wood heat, and HH units in particular, will evolve over the next 5-15 years is highly uncertain, and is driven by 

consumer preferences and behavior that are difficult to capture in a quantitative framework. The role that policy 

measures will play in terms of the rate of technology turnover, efficiency of new units, and emissions, adds 

another layer of uncertainty. Figure 22 shows the range of potential emission outcomes for a number of 

scenarios. 

Figure 22. Total Residential PM Emissions “Baseline” and Four Alternative Scenarios (ktonnes/yr) 

In contrast to the “baseline” scenario, the “slow phase-out of conventional HH” scenario assumes the same 

wood heat market share, but now allows for some introduction of advanced HHs. However, this scenario forces 

the conventional HH units to maintain part of the total HH market at least out to 2020. For 2015, the market for 

conventional outdoor HH and advanced HH (including higher efficiency outdoor HHs and indoor wood boilers)

is split 50/50, but by 2025 there are no conventional outdoor HHs in the market. Two additional scenarios 

examine what happens under the same wood heat market share, when advanced HHs come into the market more 

rapidly. Under the scenario, “rapid phase-out of conventional HHs,” new HHs start to enter the market in 2010. 

Another scenario “rapid phase-out of conventional HHs with lower emissions rate of advanced HHs” looks at 

the same market split over time, but with lower emissions for the advanced units coming in to the market. This 

is the most optimistic scenario from the PM standpoint. Finally, “shift from oil to wood heat” illustrates a 

different scenario both for wood heat in general and for the mix of technologies within the wood heat market. In 

contrast to the earlier scenarios, this scenario shows a growth in the wood heat market, with a large decline in 
S-22

heating oil, and major shift in the mix of wood heat technologies away from stoves. The key insights from this 

cross-scenario comparison are: (1) the extent to which wood space heating emissions dominate the total 

emissions from total residential energy usage, even out to 2030; and (2) the potential for wide variation in future 

emissions, depending upon the evolution of the technology mix within the market for wood heat, as seen in 

Figure 22. 

Lifetime heating costs of wood boiler technologies in comparison to oil, natural gas, and electricity

Engineering economic techniques were used to compare estimated lifetime costs of alternative technologies, 

including HHs, automated pellet boilers, high efficiency wood boilers with thermal storage, natural gas and fuel 

oil boilers, and electric heat pumps. Assumptions for each technology and for fuel prices are listed in Table 4

and Table 5, respectively. 

Table 4. Assumed Characteristics of Residential Heating Devices. For the wood devices, nameplate 

efficiencies are shown in parentheses alongside the observed operational efficiency.

Technology Tested Efficiency

(Rated Efficiency)

Output

(BTU/hr)

Base

Capital Cost

Scaled

Capital Cost

Natural gas boiler 85% 100k $3,821 $3,821

Fuel oil boiler 85% 100k $3,821 $3,821

Electric heat pump 173% 36k $5,164 $11,285

Conventional HH 22% (55%) 250k $9,800 $9,800

Advanced HH 30% (75%) 160k $12,500 $12,500

High efficiency wood boilerwith 

thermal storage

80% (87%) 150k $12,000* $12,000*

Automated pellet boiler, no thermal 

storage

44% (87%) 100k $9,750 $9,750

* The high-efficiency indoor wood boiler cost is assumed to include a supplemental hot water storage tank at a 

cost of $4,000. 
S-23

Table 5. Assumed Fuel Prices for the State of New York. National Values are Provided in Parentheses.

Fuel Price

Fuel wood $225 / cord

Pellets $280 / ton

Fuel oil #2 $2.83 / gal

($2.80 / gal)

Natural gas

$1.37 / therm

($1.00 / therm)

Electricity

$0.183 / kwh

($0.109 / kwh)

The engineering economic calculations used here are relatively simple, accounting for capital and fuel costs 

over the lifetime of the device, but ignoring other costs. Results of the NPV calculations are shown below in 

Table 6. 

Table 6. Calculated annual fuel costs and net present value lifetime costs of various residential space 

heating technologies.

Technology

Annual 

Fuel Cost

NPV

Automated pellet boiler $3,900 $64,000

High efficiency indoor wood boiler with 

hot water storage

$1,300 $30,000

Conventional HH $4,700 $75,000

Advanced HH $3,400 $62,000

Electric heat pump $3,100 $55,000

Natural gas boiler $1,600 $26,000

Fuel oil boiler $2,400 $37,000

Under baseline assumptions, natural gas boilers were shown to have the lowest net present value of cost of all of 

the home heating options that were examined. Natural gas is not available in all parts of the State of New York, 

however, and many low-density, rural areas do not have access to natural gas distribution systems. It is in these 
S-24

rural areas that HHs are likely to compete with electricity and fuel oil for market share. Of these technologies, 

HHs were cost-competitive only with the pellet boilers under tested efficiencies and market prices for wood. 

These results do not imply that wood heat cannot be cost-effective, however. For example, the high efficiency 

indoor wood boiler with hot water storage had a lifetime cost that was less than all non-natural gas options that 

were examined.

Sensitivity analysis suggested that there may be situations where HHs are cost competitive. Major factors that 

can contribute to this result are wood price, HH efficiency, and the prices of competing fuels. The sensitivity 

analysis is summarized in Figure 23. 

Figure 23. Comparative Technology Costs.

Figure 23 shows the combinations of wood price and thermal efficiency at which an advanced HH becomes cost 

competitive with other devices. A good starting point for interpreting the graph is the rectangular area created by 

the intersection of advanced HH efficiencies in the mid-20s to mid-30s and wood prices between $210 and 

$240, encompassing the baseline assumptions. The rectangle falls below all of the technology-specific lines on 

the graph except for the automated pellet boiler, indicating that the advanced HH is more costly than those 

technologies from a Net Present Value (NPV) perspective. Increasing efficiency or lowering the price of wood 
S-25

can result in the advanced HH becoming competitive, however. For example, increasing efficiency to above 

35% results in the HH having a lower NPV cost than the electric heat pump (at a wood price of $225). 

Similarly, a wood price of below approximately $55 per cord is necessary for the NPV cost of the HH to equal 

that of the natural gas boiler (at an advanced HH efficiency of 30%). It is important to note that decreasing the 

wood price also has the effect of lowering the NPV cost of the high efficiency indoor wood boiler with storage, 

and the HH must achieve even higher efficiencies to be cost competitive. The solid and hashed red lines on the 

graphic indicate that competitiveness with oil is highly dependent on oil price. At a price of $4.50 per gallon, the 

advanced HH needs only achieve an efficiency of approximately 33% to rival the oil boiler. In contrast, at a fuel 

oil price of $2.83 per gallon, the HH unit must achieve a thermal efficiency greater than 60%. 

As indicated by the figure, a major factor in the engineering economic assessment of HHs is the price for wood 

fuel. Many rural households have their own wood supply, which they may perceive to be low cost or free, even 

if the labor costs associated with carrying and splitting the wood are factored in, these homeowners may still 

perceive HHs as the most cost-effective option. This hints at the importance of difficult-to-quantify factors. 

Most homeowners may not undertake the analysis carried out here. They also may not go through an explicit 

process to evaluate the value of their time. They may not be aware of the correlation between wood and oil 

prices in many markets. Instead, it is likely that those who have chosen to install HHs have been motivated by 

qualitative perceptions of the technology’s cost, perceived environmental benefits, and ability to hedge against 

increases in fuel prices. Tax credits may also be a highly motivating factor, even if they are far less important 

than device efficiency and fuel cost in determining lifetime heating costs. These factors cannot easily be 

quantified within an engineering economic assessment and yet may be the dominant factors in decision-making.

There are additional unmodeled factors that both work for and against the competitiveness of HHs. For example, 

it is likely that the thermal efficiencies used in this analysis are higher than would be experienced in practice 

since the units would likely be used during the fall and spring months when loads and efficiencies would be 

lower. Further, the high emission rates associated with HHs have resulted in some counties and communities to 

pass ordinances that ban or limit HH use. Space considerations also come into play. Households must have 

room to store delivered wood fuel, and many residents may find it inconvenient to have to go outside to load 

wood into the boiler. The high efficiency indoor wood boiler also requires firewood storage. It does, however, 

address efficiency concerns by storing heat in a large water tank, allowing the unit to operate without cycling. 

The increased efficiency associated with this configuration is dramatic, and the unit is able to compete well in 

NPV cost with even the natural gas boiler. Combining hot-water storage with an HH is also an option that may 

improve thermal efficiency. The high BTU output of many HH units would require a very large storage tank, 

however, and this option was not examined in our study. 
S-26

HEALTH CHARACTERIZATION

A health assessment of emissions from three different HHs was conducted to determine if one unit or operating 

condition was better or worse than another. Adult CD-1 mice were exposed to filtered air, filtered wood smoke 

or unfiltered wood smoke for four hours per day for one or three consecutive days, then pulmonary and systemic 

biomarkers of injury and inflammation were assessed. Three days of exposure to either the filtered or whole 

wood smoke caused statistically significant increases in tumor necrosis factor in lung fluid and creatine kinase 

in serum. In the second study the only notable change was increased ferritin in the lung after a three-day 

exposure to whole or filtered wood smoke and smaller increases in creatine kinase in the filtered only group. 

The third study utilizing the pellet heater resulted in higher numbers of macrophages in the lung 24 hours after a 

one- and three-day exposure. The results show that none of the exposures caused acute lung injury but were 

associated with inconsistent increases in inflammatory signaling pathways. Still, the overall emission toxicity 

results from animal exposure experiments were inconclusive, as extreme dilution of the combustion gas was 

necessary to avoid immediate acute toxic effects from the carbon monoxide that at times exceeded 10,000 ppm.

CONCLUSIONS:

Comparison testing of four HH units, ranging from common to newer technologies, with different fuel types 

showed large differences in energy and emission performance. HH units that operated with cyclical damper 

openings and closings to regulate the supply of heat generally resulted in poorer efficiencies and higher levels of 

pollutants. The Pellet-fired unit and Two Stage Downdraft unit with heat storage showed greater combustion 

performance and lower emissions. Use of thermal storage allowed the Two Stage Downdraft HH to run at 

maximum output under relatively steady-state conditions, improving efficiency performance. For cyclical units, 

efficiency improvements can likely be achieved by reducing the time spent at idle (closed damper) through 

proper unit sizing. The thermal efficiencies, ranged from 22% to 44% for the conventional Single Stage HH, 

Three Stage HH, and European Pellet Burner. These values compare poorly with oil and natural gas fired 

residential systems with thermal efficiencies ranging from 86% to 92% and 79% to 90%, respectively 

(McDonald, 2009).

Testing showed a wide range of emissions depending on both unit and fuel types. The Conventional, Single

Stage HH burning white pine produced the highest total daily PM emissions [6.3 kg (14 lbs)] and the European 

Pellet Burner with red oak reported the lowest [0.036 kg (0.08 lb)]. Emissions for the Three Stage HH and U.S.

Downdraft Unit units were comparable at 0.69 and 0.62 kg/day (1.51 and 1.37 lbs/day), respectively. CO 

emissions showed a similar unit to unit trend, with the lowest value from the European Pellet Burner at 0.60 

g/MJ (1.39 lb/MMBtu). This value was about 15 times lower than that of the Conventional, Single Stage HH 

(average of the three fuels). These CO emission factors are orders of magnitude higher than are typically 
S-27

observed in conventional energy sources such as residential oil-fired heaters (< 0.1 lbs CO/MMBtu input, 

Krajewski et al., 1990).

Market and energy modeling show that while wood heat is a relatively small contributor to meeting total 

residential energy demand, it is the largest contributor to emissions from the residential energy sector. While 

different regulatory and technology scenarios for the future can have a significant impact on emissions, 

pollution from residential wood space heating is likely to dominate the total emissions from total residential 

energy usage, even out to 2030. Economic calculations for residential heating options, accounting for capital 

and fuel costs over the lifetime of the device, show that natural gas systems have the lowest net present value 

cost of all examined home heating options, including HHs. However, natural gas is not available in all parts of 

the State of New York. In the predominantly rural areas where it is unavailable, HHs are likely to compete with 

electricity and fuel oil for market share, especially when thermal storage is incorporated. The rate of turnover 

and retirement of older, highly emitting units to more efficient, lower emitting units is critical to avoid what 

could be substantial increases in emissions related to residential wood heat over the next 5-10 years.

NYSERDA, a public benefit corporation, offers objective 

information and analysis, innovative programs, technical 

expertise and funding to help New Yorkers increase 

energy efficiency, save money, use renewable energy, 

and reduce their reliance on fossil fuels. NYSERDA 

professionals work to protect our environment and 

create clean-energy jobs. NYSERDA has been 

developing partnerships to advance innovative energy 

solutions in New York since 1975.

To learn more about NYSERDA programs and funding 

opportunities visit nyserda.ny.gov

New York State

Energy Research and

Development Authority

17 Columbia Circle

Albany, New York 12203-6399

toll free: 1 (866) NYSERDA

local: (518) 862-1090

fax: (518) 862-1091

info@nyserda.ny.gov

nyserda.ny.gov
Environmental, Energy Market, 

and Health Characterization of 

Wood-Fired Hydronic Heater 

Technologies

Executive Summary

June 2012

ISBN: 978-1-936842-03-2 

New York State Energy Research and Development Authority

Francis J. Murray, Jr., President and CEO 

State of New York

Andrew M. Cuomo, Governor 

QR Code 

Placement 

for Print 

version only