Here is the regulation for using cargo heaters when transporting certain kinds of HazMat
Transportation includes loading, carrying, and unloading.
(l)(1) When transporting Class 1 (explosive) materials. A motor vehicle equipped with a cargo heater of any type may transport Class 1 (explosive) materials only if the cargo heater is rendered inoperable by: (i) Draining or removing the cargo heater fuel tank; and (ii) disconnecting the heater's power source.
(l)(2) When transporting certain flammable material-
(l)(2)(i) Use of combustion cargo heaters. A motor vehicle equipped with a combustion cargo heater may be used to transport Class 3 (flammable liquid) or Division 2.1 (flammable gas) materials only if each of the following requirements are met-
(l)(2)(i)(A) It is a catalytic heater.
(l)(2)(i)(B) The heater's surface temperature cannot exceed 54°C (130°F)-either on a thermostatically controlled heater or on a heater without thermostatic control when the outside or ambient temperature is 16°C (61°F) or less.
(l)(2)(i)(C) The heater is not ignited in a loaded vehicle.
(l)(2)(i)(D) There is no flame, either on the catalyst or anywhere in the heater.
(l)(2)(i)(E) The manufacturer has certified that the heater meets the requirements under paragraph (1)(2)(i) of this section by permanently marking the heater "MEETS DOT REQUIREMENTS FOR CATALYTIC HEATERS USED WITH FLAMMABLE LIQUID AND GAS."
(l)(2)(i)(F) The heater is also marked "DO NOT LOAD INTO OR USE IN CARGO COMPARTMENTS CONTAINING FLAMMABLE LIQUID OR GAS IF FLAME IS VISIBLE ON CATALYST OR IN HEATER."
(l)(2)(i)(G) Heater requirements under §393.77 of this title are complied with.
(l)(2)(ii) Effective date for combustion heater requirements. The requirements under paragraph (1)(2)(i) of this section govern as follows-
(l)(2)(ii)(A) Use of a heater manufactured after November 14, 1975, is governed by every requirement under (1)(2)(i) of this section;
(l)(2)(ii)(B) Use of a heater manufactured before November 15, 1975, is governed by every requirement under (1)(2)(i) (A), (C), (D), (F) and (G) of this section until October 1, 1976; and
(l)(2)(ii)(C) Use of any heater after September 30, 1976, is governed by every requirement under (1)(2)(i) of this section.
(l)(2)(iii) Restrictions on automatic cargo-space-heating temperature control devices. Restrictions on these devices have two dimensions: restrictions upon use and restrictions which apply when the device must not be used.
(l)(2)(iii)(A) Use restrictions. An automatic cargo-space-heating temperature control device may be used when transporting Class 3 (flammable liquid) or Division 2.1 (flammable gas) materials only if each of the following requirements is met-
(1) Electrical apparatus in the cargo compartment is non-sparking or explosion proof.
(2) There is no combustion apparatus in the cargo compartment.
(3) There is no connection for return of air from the cargo compartment to the combustion apparatus.
(4) The heating system will not heat any part of the cargo to more than 54°C (129°F).
(5) Heater requirements under §393.77 of this title are complied with.
(l)(2)(iii)(B) Protection against use. Class 3 (flammable liquid) or Division 2.1 (flammable gas) materials may be transported by a vehicle, which is equipped with an automatic cargo-space-heating temperature control device that does not meet each requirement of paragraph (1)(2)(iii) (A) of this section, only if the device is first rendered inoperable, as follows-
(1) Each cargo heater fuel tank, if other than LPG, must be emptied or removed.
(2) Each LPG fuel tank for automatic temperature control equipment must have its discharge valve closed and its fuel feed line disconnected.