Home > Newsletter Archives > Vol: 09/14/2017
  • Heat Chamber Requirements

  • As facilities add heat chambers to their operations, the importance of understanding the requirements for ISPM 15 conformance increases.  The American Lumber Standard Committee, Inc. (ALSC) Wood Packaging Material Enforcement Regulations (November 7, 2014) Section 8 explains these requirements.  Below is a practical explanation of this section.



    For heat chambers using schedules:

    The required heat chamber chart used is based on the largest dimension of wood in the chamber.  If the size of the largest dimension is not identified on one of the schedules, the chart related to the next largest size up from the chamber's largest piece should be used.  There are three factors that determine the temperature the chamber should maintain for ISPM 15 conformance; the thickest piece of wood in the chamber, the coldest outside temperature during the treatment process and the temperature inside the chamber.  The heat chamber is raised to a minimum 160°F and kept there for the related period of time (not including ramping temperatures) based on the lowest outside temperature during the treatment.  Temperatures are monitored based on the type of heat used.

    When a heat chamber uses only dry heat, the heat chamber operates at a temperature based on the dry bulb reading.

    When a heat chamber also uses steam in their treatment process, the temperature is based on the wet bulb reading.


    For heat chambers with probes:

    The heat chamber probes must be sealed in the core of the thickest piece of wood in the heat chamber and maintain a temperature of 140°F or greater for a minimum of 30 continuous minutes.  Three probes are used and placed in the wood located in the coldest zones of the chamber.  Any probe dropping below 140°F at any point in the 30 minute treatment cycle will re-start the treatment process from the beginning.  Most facilities use heat chambers that utilize this method


  • Probe Placement:

    Proper probe placement is critical to correctly heat treat WPM.  Probe placement meets ALSC conformance when:

    • They are placed in the thickest wood in the chamber.
    • They are located in the coldest zones of the chamber.
    • They are the proper depth in the thickest wood.
    • They are sealed from heat conduction.

    When determining the thickest piece of wood in the chamber, boards paired together must have their thicknesses combined to determine the thickest wood in the chamber.  Two examples are:

    • Pallets made with ¾ x 4 deck boards and 5/4” stringers.  When the pallets are stacked the combined thickness of the ¾” deck boards are thicker than the stringer.
    • Recycled pallets with stringers repaired with companion boards.  The stringer and companion board thicknesses must be combined to determine the thickest wood in the chamber.

    When these situations occur placing a probe between the two pieces of wood is not allowed so solid wood surrogate blocks of the same thickness or thicker may be used for probe placement.  For a situation where two ¾ x 4 deck boards combined are the thickest piece of wood in the chamber, a 2x4 could be used for a surrogate block.  For a situation where the thickest wood is two 2x4’s paired in a stringer repair, a 4x4 could be used for the surrogate block.  Surrogate blocks must be allowed to cool down between uses and must be replaced when cracking or other factors could skew the probe temperature readings. 

    It is also important that the coldest zones are identified in the heat chamber to ensure all wood is properly treated.  Professionally manufactured heat chambers will have manufacturer recommendations for these zones.  Do-it-yourself heat chambers require a verification study by TP to determine the coldest zones in the chamber.  Any heat chamber that has a verification study successfully completed can run at 133°F as long as their probes are calibrated annually.

    Probes must also be placed at the proper depth in the core of the thickest piece of wood in the chamber.  This depth is determined based on the manufacturer recommendations for the probes used.

    Probes need to be properly sealed to prevent heat conduction from the opening the probe was inserted into.  Common seals include rubber stoppers, plumbers putty and ear plugs which are applied in a way that prevents the conduction of heat from the probe hole opening.  Seals need to be routinely checked and replaced if wear is found that compromises the effectiveness of the seal.

    One other area of note is the probe wiring.  Worn/frayed or exposed wires/sheathing can lead to inaccurate temperature readings so these must be inspected routinely as well and replaced if damage is discovered.

  • Heat Chamber Reports:


    Heat chamber reports are mechanically created documents that record the progress of the heat treatment.  These reports should show the ramping up process as the wood reaches the required temperature and the related continuous 30 minutes maintained before temperatures start decreasing as the treatment process concludes.  Once the treatment ends these reports should be reviewed for conformance by the facility prior to removing the WPM from the chamber, printed and kept for review by your inspector.  Any report found non-conforming by the facility or the TP inspector will require the WPM in that charge to be retreated to meet conformance to ALSC regulations.