Heat Treatment of 

Firewood—Meeting 

the Phytosanitary 

Requirements 

Xiping Wang

Richard Bergman 

Brian K. Brashaw

Scott Meyers

Marc Joyal

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical

Report

FPL–GTR–200
July 2011

Wang, Xiping; Bergman, Richard; Brashaw, Brian K.; Myers, Scott; Joyal, 

Marc. 2011. Heat treatment of firewood—meeting the phytosanitary 

requirements. General Technical Report FPL-GTR-200. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 34 p.

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726–2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin. 

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

The USDA prohibits discrimination in all its programs and activities on the 

basis of race, color, national origin, age, disability, and where applicable, 

sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or a part 

of an individual’s income is derived from any public assistance program. 

(Not all prohibited bases apply to all programs.) Persons with disabilities 

who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET 

Center at (202) 720–2600 (voice and TDD). To file a complaint of discrimination, write to USDA, Director, Office of Civil Rights, 1400 Independence 

Avenue, S.W., Washington, D.C. 20250–9410, or call (800) 795–3272 

(voice) or (202) 720–6382 (TDD). USDA is an equal opportunity provider 

and employer.

Abstract 

The movement of firewood within emerald ash borer- 

(EAB-) infested states and into adjoining states has been 

a major contributor to the spread of EAB throughout the 

United States and Canada. In an effort to stop the further 

spread of EAB from infested areas and to facilitate interstate commerce, USDA Animal and Plant Health Inspection 

Service (APHIS) has required and are enforcing a heattreatment process in the firewood industry to heat sterilize 

firewood before it can be shipped out of infested areas. 

States and firewood producers are now faced with challenges implementing heat-treatment processes and meeting 

the heat-treating standard. The purpose of this project was to 

transfer background knowledge and advanced heat-treating 

technology to field application through field heat-treatment 

demonstrations, on-site workshops and web-based training 

seminars (webinars). We evaluated a series of temperature 

sensors/probes and data loggers for their applicability in 

heat-treating process and constructed easy-to-install temperature monitoring systems suitable for field heat-treatment 

operations of different scales. Successful on-site heat-treatment demonstrations were conducted at four firewood heattreating facilities. Two training workshops were developed 

and presented to regulatory field staff and firewood producers. The content of training included certification of treatment facilities, recommended heat-treating strategies, and 

temperature monitoring and thermal verification. 

Keywords: Demonstration, emerald ash borer (EAB), firewood, heat treatment, kiln, temperature monitoring

English unit Conversion factor SI unit 

BTU 1.055 056 × 103 Joule (J) 

Inch (in.) 25.4 Millimeter (mm) 

Temperature (°F) (T °F –32)/1.8 Temperature (°C) 

Temperature 

 increment (°F) 

0.556 Temperature 

increment (°C) 

Cubic feet (ft3

) 0.0283 Cubic meter (m3)

Pound 453.6 Gram (g) 

Cord 3.6224 Cubic meter (m3

)

Board feet 2.35849 × 10–3 Cubic meter (m3

)

Acknowledgment

This project was funded in part through a grant awarded 

by the Wood Education and Resource Center, Northeastern 

Area State and Private Forestry, Forest Service, U.S. Department of Agriculture (08-DG-11420004-100). We thank 

JoAnn Cruse of USDA APHIS PPQ, Terry Mace of Wisconsin DNR, Jeff Settle of Indiana DNR, Anthony Weatherspoon of Michigan DNR, David Hutton (Indiana) and Brent 

Wachholder (Illinois) of USDA APHIS EAB PHSS, for 

providing assistance to the field demonstration projects.

Contents

Executive Summary.............................................................1

Background..........................................................................1

Objectives ............................................................................2 

APHIS PPQ Enforcement Regulations for Heat 

 Treatment of Firewood.....................................................2

Temperature Monitoring System .........................................2

Heat Treatment Operating Procedure...................................5

On-Site Heat-Treatment Demonstrations.............................5

Training Workshops........................................................... 18

Literature Cited .................................................................. 21

Appendix A–Demonstration Photos—Heat-Treating 

 Facility A........................................................................ 22

Appendix B–Demonstration Photos—Heat-Treating 

 Facility B........................................................................ 25

Appendix C–Demonstration Photos—Heat-Treating 

 Facility C........................................................................ 28

Appendix D–Demonstration Photos—Heat-Treating 

 Facility D ....................................................................... 31

Appendix E–Temperature Conversion Table..................... 34
Heat Treatment of Firewood—Meeting the 

Phytosanitary Requirements

Xiping Wang, Research Forest Products Technologist 

Richard Bergman, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin

Brian K. Brashaw, Program Director

Natural Resources Research Institute, University of Minnesota–Duluth

Scott Myers, Entomologist

USDA APHIS, Center for Plant Health Science and Technology, Buzzards Bay, Massachusetts 

Marc Joyal, Electronics Technician

Forest Products Laboratory, Madison, Wisconsin

Executive Summary

The interstate movement of all hardwood firewood is currently restricted under the Federal Quarantine (68 FR 59088, 

October 8, 2003, as amended at 72 FR 30460, June 1, 2007 

(7CFR301 2011) ) because of the potential risk associated 

with moving emerald ash borer- (EAB-) infested firewood. 

Heat treatment is an approved method to kill EAB in firewood and prevent its transport between regions and states. 

However, states and firewood producers are faced with 

challenges implementing heat-treating processes and safely 

treating firewood for interstate commerce. United States Department of Agriculture (USDA) Animal and Plant Health 

Inspection Service (APHIS) Plant Protection and Quarantine 

(PPQ) officers and regulatory field staff have had little training to bring their knowledge of heat-treatment operations to 

the level desired for program integrity. The purpose of this 

demonstration project was to transfer background knowledge and advanced heat-treating technology to field application through field heat-treatment demonstrations, onsite 

workshops and web-based training seminars (webinars).

A multidisciplinary team that included university and federal laboratory researchers, USDA APHIS PPQ officers, state 

wood utilization and marketing specialists, and regulatory 

field staff from several states participated in this demonstration project. We evaluated a series of temperature sensors/

probes and data loggers for their applicability in the heattreating process and constructed easy-to-install temperature 

monitoring systems suitable for field heat-treatment operations of different scales. Successful onsite heat-treatment 

demonstrations were conducted at four firewood heat-treating facilities: two in Wisconsin, one in Illinois, and one in 

Indiana. Two training workshops were developed and presented to regulatory field staff and firewood producers. One 

was an onsite workshop and hands-on field demonstration 

(February 25, 2009) and one was presented as a webbased seminar (December 17, 2009). The content of 

training included 1) certification of treatment facilities, 

2) recommended heattreating strategies, and 

3) temperature monitoring and thermal 

verification.

Background

Emerald ash borer 

(Agrilus planipennis) 

(EAB) has emerged as 

a devastating killer of 

ash trees in the United 

States and Canada. As 

of May, 2011, 

EAB-infested areas 

include 15 U.S. states 

and two Canadian 

provinces (Michigan State University 

2011). Extensive survey programs have 

been established to 

detect emerging populations in other areas. 

USDA estimates that 

if EAB is not contained or eradicated, it 

will cost local government and homeowners approximately 

$7 billion over the next 25 years to remove and replace dead 

and dying ash trees (FS 2008). This scenario would also 

result in extensive environmental damage and long-term 

changes in the North American forest structure.

The movement of firewood within EAB-infested states and 

into adjoining states has been a major contributor to the 

spread of EAB throughout the United States and Canada. 

In an effort to stop the further spread of EAB from infested 

areas and to facilitate interstate commerce, USDA Animal 

The current heat-treatment 

schedule for EAB in firewood requires the core temperature to reach a minimum 

of 60 °C (140 °F) for 60 min 

(USDA APHIS PPQ 2011). 

Prior to January 2011, a 

more stringent schedule 

(71 °C (160 °F) for 75 min) 

was used (USDA APHIS 

PPQ 2010). The heat-treatment standard for EAB exceeds the ISPM-15 standards 

because of the higher thermal 

tolerance 

of EAB.

This project was conducted 

between July 2008 and November 2010. The heat-treatment demonstrations documented in this report were 

primarily based on the previous heat treatment schedule 

for EAB (71 °C (160 °F) for 

75 min).
General Technical Report FPL–GTR–200

2

and Plant Health Inspection Service (APHIS) has required 

and are enforcing a heat-treatment process in the firewood 

industry to heat sterilize firewood before it can be shipped 

out of infested areas. 

The keys to the success of this heat treatment of firewood 

are to 1) increase the kiln/chamber temperature high enough 

to meet the EAB heat-treatment standard where the firewood core temperature reaches the kill temperature for 

an extended period of time, and 2) monitor the core temperatures of the largest firewood pieces to ensure that the 

temperature-time requirement is met before the heat-treating 

cycle is completed. However, many firewood producers 

have difficulty meeting these key requirements because of 

lack of knowledge of heat-treating operations, insufficient 

heating facilities, and inadequate temperature monitoring 

equipment.

In a previous project funded through the USDA Forest Service Wood Education and Resources Center (WERC), we 

addressed technical issues related to heat-treatment options 

and heating times for ash firewood (Wang and others 2009). 

The project resulted in practical heat-treating strategies for 

various firewood operations. The heating time tables developed benefit the firewood producers in planning and executing effective firewood heat treating as required by the new 

USDA phytosanitary regulations.

The other concern with heat treating firewood is the practical challenge to implement federal regulations and meet 

the new heating standards. Standard procedures in heattreatment operations are lacking. In federal and state management, APHIS Plant Protection and Quarantine (PPQ) 

officers and regulatory field staff have had little training and 

few available internal resources to enhance their knowledge 

of heat-treating operations to provide the safeguarding required in the agency mission. In field operations, managers 

and operators of heat-treatment facilities lack the necessary 

knowledge and expertise to implement temperature measuring systems, conduct heat-treatment operations with appropriate heat-treating schedules, and monitor heat-treatment 

processes to ensure that requirements are met.

Objectives

The goal of this project was to transfer information and 

knowledge of heat-treating technology to field operations 

through onsite demonstration projects and training workshops. The specific objectives were to accomplish the following:

1. Evaluate commercially available temperature sensors/

probes and data logging systems and use this information to design and build temperature measurement 

systems suitable for monitoring chamber/kiln conditions and the core temperatures of firewood for heattreatment operations.

2. Conduct heat-treatment demonstration projects at four 

selected heat-treating facilities in Wisconsin, Illinois, 

and Indiana and train kiln operators and regulatory staff 

on fundamentals of the heat-treatment process and the 

proper procedures for monitoring firewood core temperatures.

3. Develop a generic operating manual for firewood companies incorporating the knowledge gained through the 

demonstration projects to be used by field operators to 

successfully treat firewood materials.

4. Develop training workshops for regulatory field staff 

and firewood producers on certifying treatment facilities and conducting and monitoring heat treatment.

APHIS PPQ Enforcement 

Regulations for Heat Treatment 

of Firewood

The heat treatment of firewood must be performed at an 

approved facility that maintains a current compliance agreement. APHIS PPQ enforcement regulations stipulate that 

a heat-treating facility be inspected and certified by a PPQ 

official for initial qualification. The official certification test 

has three main components: 1) calibrating the temperature 

sensors, 2) thermal mapping (cold spot mapping), and 

3) conducting an actual test treatment.

Certified heat-treatment facilities are required to monitor 

the core temperatures of several firewood pieces during the 

heating process and provide a temperature history record of 

each heat treatment run to verify that the conditions of the 

schedule have been met. The firewood samples monitored 

are required to be placed in the coldest areas of the kiln/

chamber. The internal wood temperature should be collected at least once every 5 min and stored in a data file. The 

sensors used to monitor firewood temperatures need to be 

calibrated annually and read within ± 0.5 °C (0.9 °F) of the 

treatment temperature.

Temperature Monitoring System

Monitoring Air Temperature Inside a 

Kiln/Chamber

Typically, commercial dry kilns and heating chambers 

designed for heat treatment are equipped with one or two 

temperature sensors or temperature gauges that display the 

dry-bulb temperature of the heating medium. Most kilns/

chambers used to heat treat or dry firewood do not have a 

wet-bulb temperature sensor installed (Note: wet-bulb temperature usage allows for greater control of kiln conditions 

necessary for drying lumber). The dry-bulb temperature of 

the heating medium is normally called kiln temperature or 

chamber temperature and is used for real-time checks of 

kiln condition and as guidance for kiln control. In facilities 

without computer monitoring or a control program, kiln 

temperature information is often not recorded. To meet the 

heat treatment monitoring requirement, a firewood producer 

may need to install a temperature recording device to obtain 

a record of temperature history of the kiln/chamber.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

3

Figure 1 shows temperature sensors used to measure the 

dry-bulb temperature of air inside a heat-treatment kiln. 

Figure 2 shows a typical temperature gauge installed on the 

exterior wall for real-time monitoring of the kiln temperature.

Monitoring Core Temperatures of Firewood

Monitoring core temperatures of firewood requires having 

temperature sensors properly inserted into the largest firewood pieces during a treatment run. The sensor should reach 

the center of the cross section if inserted from a side face 

or reach more than 4 in. deep if inserted from the end-grain 

of the piece. Two types of temperature sensors that can be 

used for this application are resistance temperature detectors 

(RTD) and thermocouples (TC).

Resistance Temperature Detector 

Resistance temperature detectors operate on the principle of 

changes in the electrical resistance of pure metals and are 

characterized by a linear positive change in resistance with 

temperature. Most RTD elements consist of a length of fine 

coiled wire wrapped around a ceramic or glass core. The 

element is usually quite fragile, so it is often placed inside a 

sheathed probe to protect it. RTD is one of the most accurate 

temperature sensors in industrial applications, but it is generally more expensive than alternatives because of the careful construction and use of platinum. As an example, Table 1 

illustrates the accuracy of Omega standard RTDs.

Figure 3 shows two examples of RTD probes in different 

lengths. To measure the core temperature of firewood, the 

probe should be inserted into the center of the firewood 

piece from an end through a pre-drilled hole. Using RTD 

probes to measure internal wood temperatures poses some 

challenges:

1. The sheath that houses the RTD element requires drilling a relatively big hole (1/4 in.) either from the end or 

at the midsection of the firewood. Any gap between the 

probe and hole is difficult to seal, thus causing heated 

air to enter into the hole and affect the RTD’s readings 

during the treatment.

2. APHIS PPQ Heat-Treatment Manual requires that the 

pre-drilled hole needs to be a minimum of 4 in. deep if 

the sensor is inserted into the firewood end. Therefore, a 

RTD probe should be at least 4 in. long.

3. The typical RTD probes used in commercial kilns are 

somewhat fragile and can be damaged during the firewood handling process.

Thermocouple 

A thermocouple is a junction between two different metals 

that produces a voltage related to a temperature difference 

(Fig. 4). Thermocouples are widely used temperature sensors suitable for measuring over a large temperature range. 

They are inexpensive, interchangeable, and come fitted with 

standard connectors. A thermocouple is available in different 

Figure 1. Temperature sensors installed inside the kiln for 

measuring dry-bulb temperature of air inside the kiln.

Table 1—Accuracy of standard 

Omega RTDsa

Temperature Ohms

(Ω) 

Deviation 

(degrees) 

(°C) (°F) (°C) (°F) 

–200 –328 ± 0.56 ± 1.3 ± 2.34

–100 –148 ± 0.32 ± 0.8 ± 1.44

0 32 ± 0.12 ± 0.3 ± 0.54

100 212 ± 0.30 ± 0.8 ± 1.44

200 392 ± 0.48 ± 1.3 ± 2.34

300 572 ± 0.64 ± 1.8 ± 3.24

400 752 ± 0.79 ± 2.3 ± 4.14

500 932 ± 0.93 ± 2.8 ± 5.04

600 1,112 ± 1.06 ± 3.3 ± 5.94

700 1,292 ± 1.17 ± 3.8 ± 6.84

800 1,472 ± 1.28 ± 4.3 ± 7.74

900 1,562 ± 1.34 ± 4.6 ± 8.28

a

Omega Engineering, Inc., Stamford, 

Connecticut (2011a).

Figure 2. Temperature gauge for real-time monitoring of kiln 

temperature.
General Technical Report FPL–GTR–200

4

combinations of metals or calibrations. The four most 

common calibrations are J, K, T, and E. Table 2 shows the 

temperature range and standard limits of error of these 

types of thermocouples. Type T (copper-constantan) thermocouples operate in the −200 to 350 °C (–328 to 662 °F) 

temperature range (Omega Engineering, Inc. 2011b) and are 

thus best suited for heat-treatment applications.

Temperature Data Logger

Temperature data loggers are stand-alone data collecting 

devices that can read and store temperature data in internal memory for later download to a computer. Some also 

provide an option for real-time monitoring. The advantage 

of data loggers is that they can operate independently of a 

computer, unlike many other types of data acquisition devices. Data loggers are also cheaper than chart recorders and 

are available in various shapes and sizes. The range includes 

simple economical single channel fixed function loggers to 

more powerful programmable devices capable of handling 

hundreds of inputs. When choosing a temperature data logger, the following parameters should be considered:

1. Number of inputs

2. Size

3. Speed/memory

4. Real-time operation (option)

A variety of thermocouple data loggers are available for 

monitoring and recording temperature data during a firewood heat-treatment operation. Following are five types of 

data loggers we evaluated and used in field demonstration 

projects.

1. USB TC-08 Thermocouple data logger (Pico Technology, Ltd., St Neots, Cambridgeshire, UK)

2. OM-SP1700-500 4-channel compact portable data logger (Omega Engineering, Inc., Stamford, 

Connecticut)

3. OM-CP-OCTTEMP 8-channel temperature data logger 

(Omega Engineering, Inc., Stamford, Connecticut)

4. USB-502 RH/temperature data logger (Measurement 

Computing, Inc., Norton, Massachusetts)

5. HOBO U12 Stainless steel temperature data 

logger (Onset Computer Corporation, Pocasset, 

Massachusetts)

Development of a Heat-Treatment 

Monitoring System

The temperature monitoring system for a heat-treatment operation can vary depending on the configuration and capacity of the heating chamber or kiln and the availability of the 

monitoring equipment. In general, a monitoring system for a 

heat-treating operation should include multiple temperature 

sensors (thermocouples or RTD probes), a data acquisition 

and recording device, and a personal computer. In this study, 

we custom-built one temperature monitoring system for 

each participating company based on the type and needs of 

the facility. The goal was to select appropriate temperature 

equipment and build reliable and cost-effective temperature 

measurement systems that typical firewood producers 

can afford and are easy to use and capable of providing 

Figure 3. Two typical resistance temperature detectors 

(RTD) probes in different lengths (Omega Engineering, 

Inc., Stamford, Connecticut).

Figure 4. Typical insulated thermocouple (Omega Engineering, Inc., Stamford, Connecticut).
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

5

satisfactory temperature information (heating condition and 

firewood core temperature) required by federal and state 

regulations. Detailed information of the monitoring systems 

built and evaluated in this study is given in the “Onsite 

Heat-Treatment Demonstrations” section. The typical cost 

for a basic monitoring system that includes thermocouple 

sensors and a data logger ranges from $1,000 to $2,000 depending on the number of data inputs. A desktop computer 

or a laptop computer is essential for initiating the data logger, downloading temperature data, and for real-time monitoring.

Heat Treatment Operating 

Procedure

Based on previous experience gained during laboratory heat 

treatment and field kiln certification processes, we developed a step-by-step procedure for conducting heat-treatment 

runs and monitoring the temperatures of both kiln and firewood samples during the heating process. This operating 

procedure has been demonstrated and improved through 

field demonstration projects.

Basic Operating Procedure

1. Initiate temperature monitoring system.

2. Select monitoring samples (largest firewood pieces).

3. Determine the center of the firewood.

4. Drill a small-diameter hole into the center of the firewood to accommodate the temperature sensor and ensure a minimal gap between wood and sensor.

5. Insert a temperature sensor into the hole and ensure that 

the tip of the sensor reaches the center of a firewood 

sample.

6. Use silicon sealant and a round toothpick to seal the 

hole and secure the sensor in position.

7. Place the firewood samples into the firewood bins or 

baskets, ensuring that all firewood monitoring samples 

are buried deep within each bin, about halfway down. 

Place the bins containing monitoring samples in the 

cold spot areas that were determined by APHIS PPQ 

staff through kiln certification.

8. Complete loading and close the kiln.

9. Check the temperature monitoring system and start 

heating. We recommend that the kiln operator record 

in a kiln operation journal the initial kiln temperature, 

initial firewood core temperatures, and time that the 

heating starts.

10. Periodically monitor the kiln temperatures and the core 

temperatures of the firewood samples.

11. Determine the completion of the heat-treatment cycle 

once the requirements are met.

Onsite Heat-Treatment 

Demonstrations

Onsite heat-treatment demonstrations were conducted at 

four firewood heat-treating facilities, including two in Wisconsin, one in Illinois, and one in Indiana. Michigan was 

originally included in the field demonstration plan; however, 

we learned after the project started that the Michigan cooperator was no longer heat-treating firewood. We also learned 

from USDA APHIS PPQ staff that no firewood producers 

in Michigan have been certified to conduct heat treatment 

because ash trees have been heavily infested in Michigan, 

and the felled ash trees are primarily disposed of and utilized locally. Also, the entire lower peninsula of Michigan is 

now considered a quarantine zone, removing requirements 

for heat treating. Therefore, a second heat-treating facility in 

Wisconsin was identified to conduct the field demonstration.

The selected heat-treating facilities varied in size and type 

of energy source. Different heat-treating strategies were employed in these facilities to meet the particular needs of each 

facility. During the onsite demonstration phase, we customdesigned and built a temperature monitoring system for the 

participating facility and permanently installed the system 

in the kilns. USDA APHIS PPQ officers and state field 

regulatory staff participated in the demonstration projects. 

Each demonstration project included the following technical 

aspects:

•	 Evaluate accuracy and reliability of the temperature 

sensors and data loggers selected for the heat-treatment 

applications;

•	 Evaluate compatibility of the temperature monitoring 

system in firewood heat-treatment operations;

•	 Demonstrate the monitoring process through heattreatment runs and show how to generate a record of the 

temperature history;

Table 2—Common thermocouple temperature rangesa

Thermocouple

(TC) type 

Temperature 

range

Standard 

limits of error 

Specific 

limits of error 

(°C) (°F) (°C) (°F) (%) (°C) (°F) (%) 

J 0 to 750 32 to 1,382 Greater of 2.2 (3.96) or 0.75 Greater of 1.1 (1.98) or 0.4 

K –200 to 1,250 –328 to 2,282 Greater of 2.2 (3.96) or 0.75 Greater of 1.1 (1.98) or 0.4 

E –200 to 900 –328 to 1,652 Greater of 1.7 (3.06) or 0.75 Greater of 1.0 (1.80) or 0.4 

T –200 to 350 –328 to 662 Greater of 1.0 (1.80) or 0.75 Greater of 0.5 (0.90) or 0.4 a

Omega Engineering, Inc. (Stamford, Connecticut) (2011b).
General Technical Report FPL–GTR–200

6

•	 Train facility managers and kiln operators on the entire 

heat-treating process by demonstrating heat-treatment 

procedures.

Demonstration 1—Heat-Treating Facility A 

(Wisconsin)

This heat-treating facility included a commercial dry kiln 

(Koetter Dry Kiln, Inc., Borden, Indiana) that measures 

25- by 19.5- by 12-ft, and a hot water boiler (Mahoning 

Outdoor Furnace, Mahaffey, Pennsylvania) with a heating 

capacity of 550,000 British thermal units (BTU) per hour 

(Appendix A). The kiln holds 27 baskets (4- by 4- by 8-ft) 

of firewood in a full load (approximately 14 cords). The 

boiler is fueled manually with the facility’s waste wood during the kiln drying and heat-treatment operation. The facility 

previously had difficulty raising the kiln temperature sufficiently in winter to levels required to meet the EAB heattreatment standard. Through participation in a previous field 

demonstration project (Wang and others 2009), the owner 

made the following improvements to the kiln:

•	 Added extra fin pipes to the heat exchanger inside the 

kiln to increase heating area;

•	 Added baffles to improve air circulation inside the kiln;

•	 Insulated the exposed hot water pipes between the hot 

water boiler and the kiln.

After the kiln improvements, the kiln was able to reach 

76 °C (170 °F) during the winter months and 82 °C (180 °F) 

in summer, which was proved sufficient to meet the EAB 

heat-treatment standard (Wang and others 2009).

Temperature Monitoring System

Figure 5 shows the layout of the kiln, temperature monitoring system installed, and the locations of temperature 

sensors. The temperature monitoring system consists of four 

Type T thermocouple wires, a 4-channel temperature data 

logger (OM-SP1700-500 Compact Portable Data Logger, 

Omega Engineering, Inc., Stamford, Connecticut), and a 

laptop computer. One thermocouple (TC-4) was mounted 

on the interior rear wall of the kiln (next to the RTD probe 

originally installed) to measure the temperature of return air 

(after circulating through firewood). Three thermocouples 

(TC-1, TC-2, and TC-3) were used to measure the core temperatures of the firewood samples that were placed in each 

of three baskets located in the bottom layer of the back row. 

Based on the thermal mapping through kiln certification 

conducted by AHPHIS PPQ staff, these locations were identified as the cold spots within the dry kiln. At the time of the 

demonstration project, the heat-treating facility did not have 

a control room on site to house a computer. Therefore, realtime monitoring was not available. The temperature data 

stored in the data logger was downloaded and viewed after 

the completion of each heat treatment run by bringing a 

laptop to the site or taking the data logger back to the office. 

To allow the kiln operator to monitor the core temperatures 

of the firewood samples, we provided four additional thermocouple wires and a digital thermometer as a secondary 

temperature monitoring system. Each of the three firewood 

samples had an additional TC wire installed so that the core 

temperatures of the firewood could be checked real-time 

periodically using a digital thermometer during the heattreating process.

7.6 m (25 ft)

Hot water boiler

Data logger Digital temperature reader

TC1 TC2 TC3

Wood #1 Wood #2 Wood #3

(Front)

TC4(Back)

3.7 m (12 ft)

5.9 m (19 ft, 6 in)

Water pipe

Heating

coil

Figure 5. Layout of the kiln and temperature monitoring system at Facility A.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

7

Heat-Treatment Run

The facility has been kiln drying firewood for interstate 

commerce for several years. Firewood loads were typically kiln dried weekly throughout the year. In this case, 

a dry-heat schedule would best fit the production needs of 

the facility by integrating a heat-treatment procedure with a 

kiln drying process. The heat-treatment demonstration runs 

were set to meet the heating standard for EAB. The kiln first 

heated the firewood pieces to the target core temperature of 

71 °C (160 °F) and held for at least 75 min. After meeting 

the heating standard, the firewood loads were continuously 

heated and kiln dried until the moisture content of the firewood reached 20% or below.

Three field heat-treatment runs were conducted in the winter 

of 2009 (February 12–16, February 19–25, and February 

25–March 2) using the established step-by-step operating 

procedure. In each treatment run, the kiln was fully loaded 

with 27 baskets of fresh split firewood. Baskets were arranged in three levels (bottom, middle, and upper), with nine 

baskets on each level. The monitoring firewood samples 

were placed in three baskets in the back row of the lower 

level, one in the middle of each basket. The data logger was 

programmed to start temperature measurement before the 

heat treating started.

This kiln has limited heating capacity because the heat energy comes from a wood-fueled hot water boiler. To raise 

kiln temperatures as high as possible, the vents on the back 

wall of the kiln were closed during the heat-treatment phase. 

The vents were then opened to release moisture after the 

heating standard was met to start drying the firewood. This 

procedure proved effective in all three heat-treatment runs. 

Figures 6, 7, and 8 show the temperature data recorded during each treatment run, demonstrating that the cycle met the 

heating standard for EAB.

In the demonstration runs, the kiln temperature reached 

78–81 °C (174–178 °F). The monitoring firewood samples 

reached the core temperature of 71 °C (160 °F) in 17, 27, 

and 31 h respectively. This large variation of heating time 

was associated with differences in initial wood temperature 

and ambient air temperature. But most importantly, heating time was found to be affected by how frequently the 

water boiler was fueled. This result indicates that both the 

kiln temperature and the firewood core temperature should 

be closely monitored and used as a verification of the heattreatment process.

Table 3 shows a portion of the temperature data recorded 

by the data logger. The dates, recording time (time stamp), 

and temperature of each channel are shown. Channel 0 

shows the ambient temperature outside the kiln. Channels 

1 to 3 correspond to the core temperatures of the firewood 

samples. Channel 4 corresponds to the kiln temerature (temperature of return air). A complete record of temperature 

history should be provided to APHIS PPQ staff as a thermal 

verification of the heat-treatment process.

Demonstration 2—Heat-Treating Facility B 

(Wisconsin)

Facility B is a family-owned welding repair and fabrication business. It produces kiln-dried firewood and sells to 

parks and local businesses within the state. This facility has 

0

20

40

60

80

100

120

140

160

180

200

12 24 36 48 60 72 84 96 108

Temperature (°F)

Heating time (h)

TC 1 (Core)

TC 2 (Core)

TC 3 (Core)

TC 4 (Kiln)

TC 0 (Ambient)

EAB standard

Figure 6. Temperature record of heat-treatment run No. 1 at Facility A.
General Technical Report FPL–GTR–200

8

two custom-modified kilns for kiln drying and heat treating 

firewood (Appendix B). At the time of this demonstration 

project, the facility had only Kiln No. 1 fully operational 

and had no temperature monitoring capability for either 

kiln. Because it was located outside of the EAB quarantine 

area, the facility was only required to heat treat firewood for 

gypsy moth standard, which requires the core temperature 

to reach a minimum of 56 °C (133 °F) for 30 min (USDA 

APHIS PPQ 2010).

Kiln No. 1 is a modified Northland dry kiln which measures 

31- by 13- by 11-ft. The kiln holds 44 bins (3- by 3- by 3-ft) 

of firewood in a full load (approximately 5 cords). During 

kiln drying and heat-treatment runs, a manually fed Model 

CL7260 Central Wood boiler (Central Boiler, Greenbush, 

0

20

40

60

80

100

120

140

160

180

200

12 24 36 48 60 72 84 96 108 120 132 144

Temperature (°F)

Heating time (h)

TC 1 (Core)

TC 2 (Core)

TC 3 (Core)

TC 4 (Kiln)

TC 0 (Ambient)

EAB standard

Figure 7. Temperature record of heat-treatment run No. 2 at Facility A.

0

20

40

60

80

100

120

140

160

180

200

12 24 36 48 60 72 84 96 108 120 132

Heating time (h)

EAB standard

Temperature (°F)

TC 1 (Core)

TC 2 (Core)

TC 3 (Core)

TC 4 (Kiln)

TC 0 (Ambient)

Figure 8. Temperature record of heat-treatment run No. 3 at Facility A.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

9

Minnesota) provided 750,000 BTU per hour to the kiln. The 

wood boiler was also capable of burning oil as an alternative. The heating coils and fans are located just beneath 

the ceiling along the central line of the kiln. A tarp baffle is 

used to cover the top bins after the loading is completed and 

guide the hot air circulating through the firewood bins.

Kiln No. 2 was modified from a used freight container that 

measures 8- by 9- by 48-ft. It holds 48 bins (3- by 3- by 3-ft) 

of firewood in a full load (approximately 5.5 cords). The 

heating coils and fans were installed at the upper 

corner through the length of the kiln. Initially, a custombuilt wood furnace system provided hot air through ductwork to the kiln for heating. In fall 2009, the wood furnace 

was replaced with a new manually fed Model CL 40 Central 

Wood Boiler, which provides 500,000 BTU per hour. This 

newly upgraded kiln had not been certified for heat treating 

Table 3—Portion of the temperature data recorded during heattreatment run No. 1 at Facility A

Date

(mm/dd/yy) 

Time 

hh:mm:ss

(a.m.) 

CH-0

ambient

(°F)

CH-1

sample 1

(°F)

CH-2

sample 2

(°F)

CH-3

sample 3

(°F)

CH-4

kiln temp 

(°F)

2/13/2009 4:00:00 30.70 159.30 158.50 159.70 166.80 

2/13/2009 4:05:00 30.70 159.30 158.50 159.70 166.80 

2/13/2009 4:10:00 30.79 159.30 159.20 159.80 167.50 

2/13/2009 4:15:00 30.79 159.30 159.20 159.80 167.50 

2/13/2009 4:20:00 30.88 159.40 159.30 160.50 167.60 

2/13/2009 4:25:00 30.88 160.10 159.30 160.50 168.30 

2/13/2009 4:30:00 30.88 160.10 160.00 160.50 168.30 

2/13/2009 4:35:00 30.88 160.10 160.00 161.20 167.60 

2/13/2009 4:40:00 30.88 160.70 160.00 161.20 168.30 

2/13/2009 4:45:00 30.88 160.70 160.00 161.20 168.30 

2/13/2009 4:50:00 30.88 160.70 160.60 161.90 168.30 

2/13/2009 4:55:00 30.88 161.30 160.60 161.90 168.30 

2/13/2009 5:00:00 30.88 161.30 161.20 161.90 168.90 

2/13/2009 5:05:00 30.88 161.30 161.20 161.90 168.90 

2/13/2009 5:10:00 30.88 161.30 161.20 161.90 168.90 

2/13/2009 5:15:00 30.99 162.10 161.30 162.60 169.60 

2/13/2009 5:20:00 30.99 162.10 161.30 162.60 169.60 

2/13/2009 5:25:00 30.99 162.10 162.00 162.60 169.60 

2/13/2009 5:30:00 30.99 162.70 162.00 163.20 169.60 

2/13/2009 5:35:00 30.99 162.70 162.00 163.20 169.60 

2/13/2009 5:40:00 30.88 163.20 162.50 163.10 170.20 

2/13/2009 5:45:00 30.88 163.20 162.50 163.80 170.20 

2/13/2009 5:50:00 30.74 163.10 162.40 163.70 170.10 

2/13/2009 5:55:00 30.74 163.10 163.00 163.70 170.10 

2/13/2009 6:00:00 30.74 163.80 163.00 164.30 170.10 

2/13/2009 6:05:00 30.74 163.80 163.00 164.30 170.70 

2/13/2009 6:10:00 30.74 163.80 163.70 164.30 170.70 

2/13/2009 6:15:00 30.74 163.80 163.70 164.30 170.70 

2/13/2009 6:20:00 30.74 163.80 163.70 164.30 170.10 

2/13/2009 6:25:00 30.74 164.40 163.70 164.30 170.70 

2/13/2009 6:30:00 30.85 164.50 163.80 165.00 170.80 

2/13/2009 6:35:00 30.99 164.60 163.90 164.50 170.90 

2/13/2009 6:40:00 31.14 164.80 163.90 165.20 171.10 

2/13/2009 6:45:00 31.23 164.80 164.00 165.30 171.10 

2/13/2009 6:50:00 31.23 164.80 164.00 165.30 170.40 

2/13/2009 6:55:00 31.32 164.80 164.10 165.40 171.10 

2/13/2009 7:00:00 31.32 164.80 164.80 165.40 171.10 

2/13/2009 7:05:00 31.46 165.00 164.30 165.60 171.30 

2/13/2009 7:10:00 31.46 165.00 164.90 165.60 171.30 

2/13/2009 7:15:00 31.55 165.70 165.00 165.60 171.40 

2/13/2009 7:20:00 31.55 165.70 165.00 166.30 171.40 

2/13/2009 7:25:00 31.55 165.70 165.00 166.30 171.40 

2/13/2009 7:30:00 31.55 165.70 165.70 166.30 172.00 

2/13/2009 7:35:00 31.55 165.70 165.70 166.30 172.00 

2/13/2009 7:40:00 31.55 165.70 165.70 166.30 171.40 

2/13/2009 7:45:00 31.55 166.40 165.70 166.30 171.40 

2/13/2009 7:50:00 31.55 166.40 165.70 166.30 172.00 

2/13/2009 7:55:00 31.55 166.40 166.30 166.90 172.00 

2/13/2009 8:00:00 31.55 166.40 166.30 166.90 172.70 

Note: Data within the box indicates that the treatment has met temperaturetime requirement for EAB. 
General Technical Report FPL–GTR–200

10

and the air circulation condition was unknown at the start of 

this project.

Our demonstration project at this facility was primarily focused on installing a temperature monitoring system at Kiln 

No. 1 and demonstrating the proper heat treating and temperature monitoring processes. At the owner’s request, we 

expanded the monitoring system to include Kiln No. 2 also. 

Working with the state field regulatory staff, we conducted 

thermal mapping and kiln certification on Kiln No. 2 and 

installed six TC wires at appropriate locations for monitoring purposes.

Temperature Monitoring System

Figure 9 shows layout of the temperature monitoring system 

installed at this facility. The monitoring system was originally designed just for heat-treatment process of Kiln 

No. 1 and included 8 type-T TC wires, an 8-pair TC extension cable, and an 8-channel data logger (USB TC-08 Thermocouple Data Logger, Pico Tech, Cambridgeshire, United 

Kingdom). Later, when the second kiln was added next to 

Kiln No. 1, we expanded the system by adding six TC wires 

to Kiln No. 2. The heat-treatment operation of Kiln No. 2 

can be monitored by connecting these six sensors into the 

monitoring system. Because the data logger has only eight 

channels, only one kiln can be fully monitored at a time.

A certification test had been completed on Kiln No. 1 before 

we installed the temperature monitoring system. The thermal mapping results indicated that the cold spots in Kiln 

No. 1 were located in the right side of the kiln near the two 

vents on the kiln wall (Fig. 10). Although the vents were 

completely closed during the heat-treating period, some heat 

loss seems to be occurring through the vents, causing the air 

temperature to drop in that area.

The computer and the data logger were both housed in a 

heated computer control room that was about 100 ft away 

from Kiln No. 1. The TC wires were connected to the data 

logger through an 8-pair TC extension cable. The data logger was connected to the USB port of a desktop computer. 

The USB connection allows the data logger to be powered 

directly by the USB bus, eliminating the need for an external power supply.

The location of the TC sensors in Kiln No. 1 is shown in 

Figure 10. TC wires 1 to 4 were placed at four different 

locations along the interior walls, with two on each side. 

These TC sensors are intended for measuring air temperatures or as a backup sensor. In the demonstration runs, TC-1 

to TC-4 were all used to measure air temperatures in order 

to check the heat distribution. In a normal operation, two TC 

sensors should be sufficient for monitoring the kiln temperatures; the other two can be used as a replacement if any other TC sensors were broken. TC wires 5 to 8 were distributed 

in the cold areas (right side of the kiln) to measure the core 

temperatures of firewood samples in four different bins.

Heat-Treatment Runs (Kiln No. 1)

The facility produces split firewood and places firewood 

pieces into 3- by 3- by 3-ft steel-wired bins before loading 

the kiln. For a full load operation, firewood bins were arranged in 3 rows with 9 bins on each side row and 4 bins 

on the center row. The rows were stacked 2 bins high, for a 

total of 44 bins in each kiln run. The kiln operation of this 

facility included both heat treating and kiln drying. The firewood load was first heated to meet the heat-treatment standard (heat-treatment stage), then kiln dried to 20% or below 

(kiln-drying stage).

Two field heat-treatment runs were conducted in Kiln No. 1 

on May 13–18 and May 27–June 1, 2009. Figures 11 and 12 

show the temperature history of the two demonstration runs. 

The whole process of heat treatment and kiln drying took 

5 days for both runs, which was a typical duration for drying 

green firewood at this facility.

In the first heat-treatment run, the kiln temperature was 

raised to 82 °C (180 °F) for the entering hot air and 76 °C 

(170 °F) for the return air (after circulating through firewood). The operation passed the heat-treatment standard 

for gypsy moth in 72 h and achieved the heat-treatment 

standard for EAB in 96 h. Although the facility was only 

required to meet the gypsy moth standard, this demonstration run indicated that Kiln No. 1 also had the capability to 

meet the EAB heat-treatment standard.

In the second heat-treatment run, the process was set only 

to meet the heat-treatment standard for gypsy moth and kiln 

dry the firewood as their normal operation. The highest kiln 

temperature during the treatment reached 76 °C (170 °F) for 

the entering air and 73 °C (165 °F) for the return air. The 

operation passed the gypsy moth standard in 16 h.

Thermal mapping of Kiln No. 2

Wisconsin state regulatory personnel conducted a certification test on Kiln No. 2 on December 3–11, 2009, for intrastate movement of firewood. The purpose was to certify the 

kiln for heat treating firewood per gypsy moth standard, 

which stipulates that the core temperature of the firewood 

be heated to 56 °C (133 °F) for 30 min. HOBO U12 

Figure 9. Layout of the temperature monitoring system at 

Facility B.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

11

stainless steel temperature loggers (Onset Computer Corporation, Pocasset, Massachusetts) were used to separately 

measure the air temperature and core temperature of firewood. A total of 26 probes were placed into the kiln. Thirteen probes were inserted into firewood pieces located in the 

bottom layer of firewood bins to measure the core temperatures of the firewood, and 13 adjacent to them to measure air 

temperatures. Temperature data were recorded every 5 min 

and downloaded from the loggers following the heat treatment.

Figure 13 shows the recorded temperatures from 13 temperature probes that were used to map the air temperature 

distribution within the kiln. Figure 14 shows the recorded 

temperatures from 13 data loggers that were monitoring 

core temperatures of the firewood samples. All the probes 

measured temperatures above the gypsy moth standard.

Thermal mapping results indicate that the air was not well 

circulated in this modified kiln. The temperature difference 

between the hot spots and cold spots was about 7 to 8 °C 

(12–15 °F). The bottom bins No. 1, 2, 5, and 11 were identified as cold spots that had the lowest air temperatures during 

the heating cycle (Fig. 15a). We recommended that 

Facility B place the TC sensors in these cold areas to 

monitor both kiln temperatures and firewood temperatures 

in future heat-treatment operations (Fig. 15b).

Demonstration 3—Heat-Treating Facility C 

(Illinois)

Facility C produces several different wood products including heat-treated firewood and pallets. In 2007, USDA 

APHIS certified its dry kiln for heat treatment to allow the 

company to move firewood outside the EAB quarantine 

area.

Figure 10. Arrangement of firewood bins and location of TC 

sensors in Kiln No. 1.
General Technical Report FPL–GTR–200

12

0

20

40

60

80

100

120

140

160

180

200

12 24 36 48 60 72 84 96 108 120 132

Heating Time (h)

TC 1 (Air)

TC 2 (Air)

TC 3 (Air)

TC 4 (Air)

TC 5 (Core)

TC 6 (Core)

TC 7 (Core)

TC 8 (Core)

EAB standard

Temperature (°F)

Figure 11. Temperature record of heat-treatment run No. 1 at Facility B.

12 24 36 48 60 72 84 96 108 120 132

Heating time (h)

TC 1 (Air)

TC 2 (Air)

TC 3 (Air)

TC 4 (Air)

TC 5 (Core)

TC 6 (Core)

TC 7 (Core)

TC 8 (Core)

Gypsy moth standard

0

20

40

60

80

100

120

140

160

180

200

Temperature (°F)

Figure 12. Temperature record of heat-treatment run No. 2 at Facility B.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

13

The heat-treatment facility consists of a direct-fired dry 

kiln (Kiln-Direct, Burgaw, North Carolina), a gas burner, 

and a small computer control room next to the kiln (Appendix C). The kiln was originally designed to heat treat pallets 

per the International Standards for Phytosanitary Measures

(ISPM) 15 standard. The facility burns natural gas as its heat 

source for heat treating both pallets and firewood. The kiln 

measures 48- by 15.5- by 11.5-ft, which holds 54 bins (4- by 

4- by 4-ft) of firewood (approximately 11.5 cords). Heattreated firewood is stored inside the main building and then 

packaged into 0.75 ft3 (21 L) aerated plastic bags for their 

distributors. The dry kiln produces 24 pallets of packaged 

firewood per treatment run.

Temperature Monitoring System

The heat-treatment kiln at this facility has an existing temperature monitoring system that was included with the dry 

kiln. This existing system employed eight 2-in. long RTD 

probes for temperature measurements: two for air temperatures located at the rear wall, about 4 ft above ground 

and six for core temperatures of firewood samples that are 

placed in the center of bins at the rear, middle, and front of 

the kiln. A computer is used to monitor the sensors in real 

time. The kiln was programmed to complete the heat 

treatment after the lowest of these six probes had reached 

71 °C (160 °F) for 75 min (EAB standard). In the past, the 

kiln operator had trouble with data collection under severe 

weather conditions. The owner agreed to install a second 

temperature monitoring system in the kiln as a backup system if the primary system was not operational. This situation provided us with an opportunity to test our custom-built 

system and compare the use of two different temperature 

sensors (RTD versus TC).

120

125

130

135

140

145

150

155

0 20 40 60 80 100 120 140 160

Air 1

Air 2

Air 3

Air 4

Air 7

Air 11

Air 12

Air 14

Air 15

Air 18

Air 19

Air 20

Air temperature (

Air 21

°F)

Heating time (h)

Figure 13. Air temperature recorded for thermal mapping in Kiln No. 2 at Facility B.

120

125

130

135

140

145

150

0 20 40 60 80 100 120 140 160

Wood 1

Wood 2

Wood 3

Wood 4

Wood 11

Wood 12

Wood 13

Wood 14

Wood 15

Wood 17

Wood 18

Wood 19

Wood 20

Wood temperature (°F)

Heating time (h)

Gypsy moth standard

Figure 14. Internal wood temperature recorded during thermal mapping in Kiln No. 2 

at Facility B.
General Technical Report FPL–GTR–200

14

Figure 16 shows the layout of the heat-treatment kiln, the 

new temperature monitoring system, and the location of 

the TC temperature sensors. The new monitoring system 

consists of eight type-T thermocouple wires, an 8-channel 

temperature data logger and a desktop computer. The computer and the data logger are both housed in a small control 

room next to the kiln (Appendix C, demonstration photo C5 

and C6). The 8-channel Omega data logger measures and 

records up to 14,563 temperature measurements per channel 

and includes a real-time clock that records the temperature 

data with a time stamp. The TC wires run from the data 

logger to the rear wall of the dry kiln, and then are 

distributed to the following locations as shown in Figure 16.

•	 TC-1, 2, and 3 are located in the first bin of each row in 

the bottom layer at the rear of the kiln;

•	 TC-4, 5, and 6 are located in the second bin of each row 

in the bottom layer;

•	 TC-7 is located at the rear of the kiln and next to the 

burner assembly (hot air);

•	 TC-8 is located along the rear wall of the kiln (return 

air).

TC No. 1 through No. 6 are used to measure the core temperatures of the firewood samples in the bottom six bins. 

These bins are located in the cold spot areas as identified 

through thermal mapping and kiln certification (previously 

conducted by APHIS PPQ staff). TC No. 7 is used to measure the temperature of hot air coming out of the burner 

assembly. TC No. 8 is used to measure the return air 

temperature.

Heat-Treatment Runs

Facility C heat treats split hardwood firewood that has been 

air dried in the wood yard. Kiln drying is not required in the 

heat-treating process. The firewood heat-treatment operation 

uses the dry heat produced through the gas burner. The facility normally conducts 16 to 18 firewood heat-treatment runs 

annually to produce certified firewood. To demonstrate the 

heat-treating process and test the new temperature monitoring system, we conducted three heat-treatment runs between 

September 2009 and April 2010. Split firewood was conFigure 15. Thermal mapping and recommended layout of TC wires for Kiln No. 2 of Facility B.

Figure 16. Layout of the kiln and temperature monitoring system at Facility C.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

15

tained in 4- by 4- by 4-ft metal-tube bins. These bins were 

loaded into the kiln by a forklift and arranged in three rows. 

Each row was nine bins long and stacked two bins high for 

a total of 54 bins. Firewood bins were staggered to force 

heated air to make better contact with the firewood.

During the demonstration runs, both RTD probes (previous 

monitoring system) and TC wires (new monitoring system) 

were inserted into selected firewood samples. TC wires (TC 

No. 1 through No. 6) were inserted into the center of each 

firewood sample at the midsection according to the established procedure. The 2-in.-long RTD probes (RTD No. 1 

through No. 6) were inserted into firewood samples 

from the end through a pre-drilled hole (1/4 in. diameter 

and 2 in. deep), and then plugged with a putty patch 

(Appendix C, demonstration photo C7). We noticed during 

installation that the putty patch attachment was not as secure 

as the TC wires. The installed RTD probes occasionally 

pulled out of the firewood during normal handling due to 

their loose attachment. The other disadvantage of using this 

short RTD probe is that it can only go into the firewood 2 

in. deep from the end. Therefore, the temperature measured 

may not reflect the true core temperature for that piece of 

firewood.

In this case, the temperatures measured by RTD probes 

and TC wires were all monitored in real time and recorded 

through the desktop computer located in the control room. 

During a normal kiln operation, the Kiln-Direct program 

controlled the heating process. The kiln was programmed 

to stop the heat treatment after the lowest of the six RTD 

probes had reached 71 °C (160 °F) for 75 min.

Figures 17–19 are the temperature plots of both TC wires 

and RTD probes for three kiln runs conducted on September 

1, 2009 (run No. 1), October 28, 2009 (run No. 2), and April 

28, 2010 (run No. 3). Data from the RTD probes indicated 

that the firewood samples reached 71 °C (160 °F) in 5 h 

20 min in run No. 1; 15 h and 35 min in run No. 2; and 5 h 

and10 min in run No. 3. Data from the TC wires show that 

the firewood samples reached 71 °C (160 °F) in 7 h for run 

No. 1 and 16 h 20 min for run No. 2. The firewood samples 

of run No. 3 did not reach 71 °C (160 °F) based on the TC 

readings. There were significant differences between the TC 

readings and the RTD readings. Considering that both RTD 

probes and TC wires have been calibrated, we projected that 

the possible causes of this difference were 1) poor installation of RTD probes from the end of firewood; 2) possible 

falling out of the RTD probes from some firewood samples; 

and 3) insufficient depth of RTD probes placed into the 

firewood samples.

TC wires No. 1 to No. 6 (that measured firewood core temperatures) worked well during all three runs. No wire damage or wire falling out of firewood was observed. However, 

the plastic conduit used to guide, protect, and anchor the TC 

wires on the kiln walls was not high-temperature resistant 

and consequently became deformed and fell off the wall 

during the first kiln run. This did not affect the readings of 

TC No. 1 to No. 6, as they were still securely inserted into 

the firewood. But TC No. 7 and No. 8, which were originally installed up near the burner assembly (No. 7) and on the 

rear wall (No. 8) through the plastic conduits, fell off from 

the anchor and dropped onto the ground because of conduit 

deformation. This explained why temperature readings of 

TC No. 7 and No. 8 in run No. 1 were much lower than the 

RTD readings 2 h into the treating process. All the TC wires 

were reinstalled through steel conduits prior to the second 

kiln run.

From TC temperature charts, we observed that the temperature of hot air entering into the kiln fluctuated dramatically 

during the heat-treating process. This could have been related to the heating cycle of the burner and the strong turbulence of the hot air near the burner assembly. Thermocouple 

readings of the return air also showed slightly larger variation as compared with the RTD readings in run No. 2. This 

may be related to the movement of the TC wire under the 

turbulence of the return air.

Demonstration 4—Heat-Treating Facility D 

(Indiana)

This facility was certified on May 13, 2009, to heat treat 

firewood for movement outside of an EAB quarantine zone. 

The dry kiln (Nova Dry Kiln, formerly Koetter Dry Kiln, 

New Albany, Indiana) measures 18- by 18- by 10-ft, which 

holds approximately 7.5 cords of firewood (Appendix D). 

A wood boiler is manually fed wood scraps to supply hot 

water for heating the dry kiln during heat treatments. This 

facility does not produce firewood but heat treats firewood 

for other producers. The firewood is brought to the facility 

in small bundles stacked on pallets. Before arriving onsite, 

the producer wraps the bundles in a mesh to keep the bundles from moving. The firewood is heat treated in stacked 

and palletized bundles. Pallets are arranged in four rows, 

each five pallets long and stacked two pallets high for a total 

of 40 pallets per treatment run.

Before the installation of our system, the facility inserted 

single temperature probe into firewood placed in the center 

of a pallet at the rear of the kiln to monitor real-time temperature during the heat-treatment run. A circular chart recorder 

(Dickson No. KT803, Addison, Illinois) recorded temperature data from this probe. The run was completed when 

the chart recorder read 71 °C (160 °F) for 75 min. Hard 

copies of the circular charts were sent to the USDA APHIS 

personnel as verification. Typical heat-treatment runs last 24 

to 48 h depending on the season and feeding of the boiler.

New Temperature Monitoring System

To help create a more efficient heat-treating operation, a 

new temperature monitoring system was installed at this 

facility on September 28, 2009, to electronically collect 

the kiln temperature and firewood temperature data. Figure 

20 shows the layout of the kiln and the new temperature 

monitoring system. It consists of three 4-in.-long thermocouple probes (for measuring firewood temperature), one 
General Technical Report FPL–GTR–200

16

thermocouple wire (Type T) (for kiln temperature), and a 

4-channel temperature data logger. The Omega data logger 

was installed on the outside kiln wall in an adjacent storage room. The TC wires run from the data logger to the rear 

interior wall of the kiln. The TC No. 1 (wire) was mounted 

next to the RTD probe originally installed to measure the 

temperature of return air (RTD measurement is only read 

real-time, not recorded, through a temperature meter on the 

control panel attached to the outside kiln wall), and TC No. 

2 through No. 4 were placed in three palletized firewood 

bundles on the back row. The back row was identified as the 

kiln cold spot through thermal mapping and kiln certification. 

Heat-Treatment Runs

Three heat-treatment runs were conducted on September 

28, 2009 (run No. 1), October 30, 2009 (run No. 2), and 

April 29, 2010 (run No. 3) (Appendix D). During the heattreatment runs, three of the five lower pallets closest to the 

rear wall had a sensor located in the center of the bundle to 

Figure 17. Temperature records of heat-treatment run No. 1 at Facility C.

40

60

80

100

120

140

160

180

200

220

0 1 2 3 4 5 6 7 8 9 10 11 12

TC1 (Core)

TC2 (Core)

TC3 (Core)

TC4 (Core)

TC5 (Core)

TC6 (Core)

TC7 (Kiln)

TC8 (Kiln)

RTD (Kiln)

Ambient

Heating time (h)

Temperature (°F) – TC

EAB

Standard

Failed

(a) Temperature records of TC sensors

40

60

80

100

120

140

160

180

200

220

0 1 2 3 4 5 6 7 8 9 10

RTD (Kiln)

RTD (Kiln)

RTD1 (Core)

RTD2 (Core)

RTD3 (Core)

RTD4 (Core)

RTD5 (Core)

RTD6 (Core)

Temperature (°F) – RTD

EAB

Standard

Heating time (h)

(b) Temperature records of RTD probes
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

17

40

60

80

100

120

140

160

180

200

220

0 2 4 6 8 10 12 14 16 18 20

TC 1 (Core)

TC 2 (Core)

TC 3 (Core)

TC 4 (Core)

TC 5 (Core)

TC 6 (Core)

TC 7 (Kiln-hot air)

TC8 (Kiln-return air)

Ambient

Temperature (°F) – TC probe

Heating time (h)

EAB Standard

0 2 4 6 8 10 12 14 16 18 20

RTD 1 (Core)

RTD 2 (Core)

RTD 3 (Core)

RTD 4 (Core)

RTD 5 (Core)

RTD (Kiln-return air)

Heating time (h)

EAB Standard

(b) Temperature records of RTD probes

(a) Temperature records of TC sensors

40

60

80

100

120

140

160

180

200

220

Temperature (°F) – RTD

Ambient

monitor firewood temperature. To install the temperature 

sensors, the mesh on the pallet was partially removed to access the bundles located in the center of the bins. Three TC 

probes were inserted into the firewood samples 4 in. deep 

from the end and the gap was sealed using silicon sealant.

The Omega monitoring system showed that all three runs 

passed the firewood heat treatment requirement for EAB. 

As for the original Dickson monitoring system, the facility 

owner ran it once and decided not to use it again based on 

the ease of the Omega monitoring system.

For run No. 1, only 20 pallets were heat treated (one-half 

of a total load) due to insufficient firewood supply. Both the 

Dickson circular chart recorder and the Omega monitoring 

system measured core temperatures of firewood above the 

current heat-treatment standard for EAB (Figs. 21 and 22). 

The heat treatment lasted roughly 48 h. Insufficient fueling 

of the wood boiler resulted in the system not maintaining 

kiln temperatures at night, prolonging the heat-treatment 

cycle.

For runs No. 2 and No. 3, 40 pallets were heat treated (a full 

load). Results from the Omega monitoring system showed 

that all core wood temperatures passed the 71 °C (160 °F) 

for 75 min mark at about 37 and 30 h after starting the runs 

(Figs. 23 and 24). All three bins met the temperature/time 

Figure 18. Temperature records of heat-treatment run No. 2 at Facility C.
General Technical Report FPL–GTR–200

18

mark almost simultaneously, indicating good air 

movement through the palletized firewood.

For a hot water heated kiln facility, the heat-treatment process greatly depends on the heating capacity of the system 

and how well the hot water boiler is operated during the 

heating process. Kiln certification is necessary for any heattreatment facility to test its heating capacity for meeting the 

heat-treatment standards. If a kiln operation is intended for 

both heat treatment and kiln drying, the heating times for 

the core temperature to meet the heat-treatment standard are 

usually not critical as long as the heating standard is met 

before the kiln drying process is completed.

Training Workshops

Based on the baseline information developed through a previous WERC project and by incorporating what we learned 

through field demonstration projects, we developed two 

technical workshops to educate and train field operators 

and regulatory staff in states affected by EAB infestation 

or where commerce of hardwood firewood is under federal 

quarantine. These workshops outlined the fundamentals 

of heat treatment of firewood, the use of temperature 

monitoring systems, and the certification and verification of 

heat-treatment operations.

Figure 19. Temperature records of heat-treatment run No. 3 at Facility C.

40

60

80

100

120

140

160

180

200

220

0 1 2 3 4 5 6 7 8 9 10

TC 1 (Core)

TC 2 (Core)

TC 3 (Core)

TC 4 (Core)

TC 5 (Core)

TC 6 (Core)

TC 7 (Kilnhot air) 

TC 8 (Kilnreturn air) 

Ambient

Heating time (h)

EAB Standard

Failed

40

60

80

100

120

140

160

180

200

220

0 1 2 3 4 5 6 7 8 9 10

RTD 1 (Core)

RTD 2 (Core)

RTD 3 (Core)

RTD 4 (Core)

RTD 5 (Core)

RTD 6 (Core)

RTD (Kiln-

 return air)

Ambient

Heating time (h)

EAB Standard

Temperature (°F) – TC probe Temperature (°F) – RTD

(b) Temperature records of RTD probes

(a) Temperature records of TC sensors
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

19

Figure 20. Layout of the kiln and new temperature monitoring system for Facility D.

Figure 21. Temperature record of heat-treatment run No. 1 at Facility D produced 

by a circular chart recorder.
General Technical Report FPL–GTR–200

20

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80

Heating time (h)

TC 1 (Kiln)

TC 2 (core)

TC 3 (core)

TC 4 (core)

Ambient

EAB Standard

Temperature (°F) – TC probe

(b) Temperature record of TC proves

Figure 22. Temperature record of TC proves heat-treatment run No. 1 at Facility D. 

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70 80

TC 1 (Kiln)

TC 2 (core)

TC 3 (core)

TC 4 (core)

Ambient

Heating time (h)

EAB Standard

Temperature (°F) – TC probe

Figure 23. Temperature record of heat-treatment run No. 2 at Facility D. 
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

21

The first workshop was held on February 25, 2009, in 

Madison, Wisconsin, in conjunction with one of the 

demonstration projects. Fifty-four people representing 

both private and public organizations attended. The second 

workshop was conducted on December 17, 2009, through 

an interactive webinar, with over 60 attendees representing 

at least 15 states and federal agencies. The webinar sessions 

have been recorded and archived for viewing in the future. 

The workshop covered the following content:

1. Federal and state regulations on EAB-infested firewood;

2. Current heat-treatment standards for firewood and treating facility certification processes;

3. Fundamentals of heat-treating processes; and

4. Heat-treating options, temperature monitoring, and thermal verification.

The archived webinar containing the PowerPoint slides and 

presenter audio can be accessed at the following web location: https://umconnect.umn.edu/p89465540/.

Literature Cited

CFR. 2011. Title 7, Volume 5, Revised as of January 1, 

2011, Washington, D.C.: U.S. Code of Federal Regulations, 

U.S. Government Printing Office. pp. 62–67.

FS. 2008. Emerald ash borer: Control may be on the horizon. Newtown Square, PA: U.S. Department of Agriculture, 

Forest Service, Northern Research Station. Research Review Vol. 2, Winter 2008. 5 p.

Michigan State University. 2011. Emerald ash borer, a 

multinational effort in Michigan, Illinois, Indiana, Iowa, 

Kentucky, Maryland, Minnesota, Missouri, New York, Ohio, 

Pennsylvania, Tennessee, Virginia, West Virginia, Wisconsin, Ontario, and Quebec. http://www.emeraldashborer.info/. 

Accessed on July 13, 2011.

Omega Engineering, Inc. 2011a. Technical reference—

RTDs. Stamford, CT: http://www.omega.com/rtd.html. 

Accessed on April 18, 2011. 

Omega Engineering, Inc. 2011b. Technical reference—Thermocouples. Stamford, CT: http://www.omega.com/thermocouples.html. Accessed on April 18, 2011.

U.S. Department of Agriculture, APHIS. 2011. USDA revises heat-treatment schedule for Emerald Ash Borer. [News 

release]. January 19, 2011. Washington, DC: Animal and 

Plant Health Inspection Service.

U.S. Department of Agriculture, APHIS PPQ. 2010. Treatment schedules T300—Schedules for miscellaneous plant 

products. T314—Logs and Firewood P.5-4–5-38. 

Washington, DC: Animal and Plant Health Inspection 

Service, Plant Protection and Quarantine.

U.S. Department of Agriculture, APHIS PPQ. 2011. Treatment schedules T300—Schedules for miscellaneous plant 

products. T314—Logs and Firewood P.5-4-38. Washington, 

DC: Animal and Plant Health Inspection Service, Plant Protection and Quarantine.

Wang, X.; Bergman, R.; Simpson, W.T.; Verrill, S.; Mace, T. 

2009. Heat-treatment options and heating times for ash firewood. Gen. Tech. Rep. FPL–GTR–187. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 29 p.

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80 90 100 110

TC 1 (kiln)

TC 2 (core)

TC 3 (core)

TC 4 (core)

Ambient

Heating time (h)

EAB standard

Temperature (°F) – TC probe

Figure 24. Temperature record of heat-treatment run No. 3 at Facility D. 
General Technical Report FPL–GTR–200

22

Appendix A Demonstration Photos—Heat-Treating Facility A

A1. The dry kiln of Facility A measures 25 ft by 

19.5 ft by 12 ft and holds 

approximately 14 cords 

of firewood (42 face 

cords, 1,792 ft3) in a full 

load.

A2. The hot water boiler 

is fueled with the facility’s waste wood to 

provide heat for heat 

treating and kiln drying 

firewood.

A3. Freshly split firewood 

pieces are loaded into 

4-ft by 4-ft by 4-ft steel 

baskets prior to loading 

into the kiln.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

23

A4. Firewood baskets are 

loaded into the kiln and 

arranged in three levels 

(bottom, middle, and upper), with nine baskets in 

each level. The monitoring firewood samples 

were placed in the three 

baskets of the back row 

in the lower level. One 

instrumented piece was 

placed into each basket.

A5. Thermocouple wires 

were inserted into the 

center of firewood samples at the midsection.

A6. Firewood samples 

were placed in the center 

of a basket.
General Technical Report FPL–GTR–200

24

A7. A 4-channel temperature 

data logger was installed on 

the back wall outside of the 

kiln and housed in a steel 

case.

A8. The kiln and firewood 

temperature data were downloaded using computer software after a heat-treatment 

cycle was completed.

A9. During heat treatment, a 

digital thermometer is used 

to periodically check the core 

temperature of the firewood.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

25

Appendix B Demonstration Photos—Heat-Treating Facility B

B1. Heat-treating facility with 

two custom-modified kilns for 

kiln drying and heat treating 

firewood.

B2. Kiln No. 1, a modified 

Northland dry kiln (Northland 

Kilns, Inc., Bagley, Minnesota), measures 31 ft by 13 

ft by 11 ft with a capacity of 

8,000 board feet.

B3. Kiln No. 2 was modified 

from a used freight container 

and measures 8 ft by 9 ft by 

48 ft with a capacity of 10,000 

board feet.
General Technical Report FPL–GTR–200

26

B4. The temperature monitoring system was installed at 

Facility B.

B5. Thermocouple wire layout in Kiln No. 1.

B6. Real-time temperature 

monitoring was conducted 

using a desktop computer 

and a data logger housed in 

a control room.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

27

B9. Thermal mapping 

test is conducted at Kiln 

No. 2.

B7. Kiln No. 1 is fully 

loaded with 44 bins of 

firewood.

B8. Firewood is loaded 

into Kiln No. 2
General Technical Report FPL–GTR–200

28

Appendix C Demonstration Photos—Heat-Treating Facility C

C1. Direct-fired dry kiln 

(Kiln-Direct, Burgaw, North 

Carolina) measures 48 ft by 

15.5 ft by 11.5 ft with a capacity of about 10,000 board 

feet.

C2. Interior of the heat-treatment kiln.

C3. A gas burner provides 

heat into the kiln for heat 

treating pallets and firewood.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

29

C4. Thermocouple wires are 

installed inside the kiln.

C5. An 8-channel temperature 

data logger (OM-CP-OCTTEMP, Omega Engineering, 

Inc., Stamford, Connecticut) 

was installed in the computer 

control room. All the TC wires 

ran from the data logger to 

the rear wall of the dry kiln 

and were then distributed to 

the designated location.

C6. The computer and the 

data logger are both housed 

in a small control room next 

to the kiln.
General Technical Report FPL–GTR–200

30

C7. A thermocouple wire 

was inserted into the center 

of the firewood at the midsection. A RTD probe from 

the old monitoring system 

was also inserted into the 

firewood sample about 1-1/2 

in. deep from the end.

C8. Firewood bins were 

loaded into the kiln by a 

forklift and arranged in 

three rows; each was nine 

bins long and stacked two 

bins high for a total of 54 

bins. Bins were staggered 

to force heated air to make 

better contact with the firewood.

C9. The heat-treated firewood is packaged into 

0.75 ft3 aerated plastic 

bags now ready for 

shipment.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

31

Appendix D Demonstration Photos—Heat-Treating Facility D

D1. Dry kiln facility certified to 

heat treat firewood for movement outside of EAB quarantine zone. The kiln measures 

18 ft by 18 ft by 10 ft with a 

capacity of 10,000 board feet. 

The firewood to be treated is 

brought to the facility in bundles wrapped in mesh 

and stacked on pallets.

D2. A wood boiler is manually 

fed wood scraps to supply 

hot water for heating the dry 

kiln during heat-treatment 

operation.

D3. The heating coil is mounted between the rear wall and a 

separation frame. The thermocouple wires and temperature 

probes are installed through 

the kiln wall and put in designated locations.
General Technical Report FPL–GTR–200

32

D4. Facility D’s old temperature monitoring system used 

a circular chart recorder 

(Dickson No. KT803, Addison, Illinois) to record 

temperature data. The hard 

copies of the circular charts 

are used as verification.

D5. During the heat-treatment 

runs, three of the five lower 

pallets closest to the rear wall 

had a sensor located in the 

center of the bundle to monitor firewood temperature.

D6. A 4-channel temperature 

data logger (Omega OMSP1700-500, Omega Engineering, Inc., Stamford, Connecticut) was installed on the 

outside kiln wall in a 

storage room.
Heat Treatment of Firewood—Meeting the Phytosanitary Requirements

33

D9. The data logger was initiated 

using a laptop before heat-treatment operation started.

D7. A thermocouple probe was inserted into a firewood sample 4 in. 

deep from the end and the gap was 

sealed using silicon sealant.

D8. For a full load, firewood pallets 

are arranged in the kiln four rows 

by five pallets long and stacked two 

high for a total of 40 pallets.
General Technical Report FPL–GTR–200

34

Appendix E—Temperature conversion table (°F–°C)a

°F °C °F °C °F °C °F °C °F °C

31 –1 71 22 111 44 151 66 191 88

32 0 72 22 112 44 152 67 192 89

33 1 73 23 113 45 153 67 193 89

34 1 74 23 114 46 154 68 194 90

35 2 75 24 115 46 155 68 195 91

36 2 76 24 116 47 156 69 196 91

37 3 77 25 117 47 157 69 197 92

38 3 78 26 118 48 158 70 198 92

39 4 79 26 119 48 159 71 199 93

40 4 80 27 120 49 160 71 200 93

41 5 81 27 121 49 161 72 201 94

42 6 82 28 122 50 162 72 202 94

43 6 83 28 123 51 163 73 203 95

44 7 84 29 124 51 164 73 204 96

45 7 85 29 125 52 165 74 205 96

46 8 86 30 126 52 166 74 206 97

47 8 87 31 127 53 167 75 207 97

48 9 88 31 128 53 168 76 208 98

49 9 89 32 129 54 169 76 209 98

50 10 90 32 130 54 170 77 210 99

51 11 91 33 131 55 171 77 211 99

52 11 92 33 132 56 172 78 212 100

53 12 93 34 133 56 173 78 213 101

54 12 94 34 134 57 174 79 214 101

55 13 95 35 135 57 175 79 215 102

56 13 96 36 136 58 176 80 216 102

57 14 97 36 137 58 177 81 217 103

58 14 98 37 138 59 178 81 218 103

59 15 99 37 139 59 179 82 219 104

60 16 100 38 140 60 180 82 220 104

61 16 101 38 141 61 181 83 221 105

62 17 102 39 142 61 182 83 222 106

63 17 103 39 143 62 183 84 223 106

64 18 104 40 144 62 184 84 224 107

65 18 105 41 145 63 185 85 225 107

66 19 106 41 146 63 186 86 226 108

67 19 107 42 147 64 187 86 227 108

68 20 108 42 148 64 188 87 228 109

69 21 109 43 149 65 189 87 229 109

70 21 110 43 150 66 190 88 230 110

a

Conversion formula: T°C = (T°F – 32)/1.8