A heat transfer unit for use in a location where heat will be transferred by the unit from a heat source in an enclosure to ambient hazardous air, the heat transfer unit comprising: a housing comprising a first compartment, configured for coupling to and removing heat from the enclosure, the first compartment having therein an evaporator, a motor, and all other potential ignition source within the heat transfer unit; a second compartment comprising an ambient hazardous air inlet, an ambient hazardous air outlet, a condenser and an ambient air movement element located between the ambient hazardous air inlet and ambient hazardous air outlet, the ambient air movement element being located such that, during operation of the heat transfer unit, the ambient air movement element will move and cause ambient hazardous air to pass into the ambient hazardous air inlet, past coils of the condenser, and out the ambient hazardous air outlet; wherein the evaporator and condenser are coupled to each other via piping so as to collectively comprise a cooling circuit for passage of a cooling medium therebetween; wherein the motor is coupled to the ambient air movement element so as to cause the ambient air movement element to move during operation of the heat transfer unit; characterised in that the first compartment is sealed off from the second compartment such that, during operation of the heat transfer unit, hazardous ambient air is prevented from entering the first compartment from the second compartment; and wherein the heat transfer unit is configured such that, during operation, an interior of the first compartment is maintained at an internal pressure that is higher than an interior pressure in the second compartment. a housing comprising a first compartment, configured for coupling to and removing heat from the enclosure, the first compartment having therein an evaporator, a motor, and all other potential ignition source within the heat transfer unit; a first compartment, configured for coupling to and removing heat from the enclosure, the first compartment having therein an evaporator, a motor, and all other potential ignition source within the heat transfer unit; a second compartment comprising an ambient hazardous air inlet, an ambient hazardous air outlet, a condenser and an ambient air movement element located between the ambient hazardous air inlet and ambient hazardous air outlet, the ambient air movement element being located such that, during operation of the heat transfer unit, the ambient air movement element will move and cause ambient hazardous air to pass into the ambient hazardous air inlet, past coils of the condenser, and out the ambient hazardous air outlet; wherein the evaporator and condenser are coupled to each other via piping so as to collectively comprise a cooling circuit for passage of a cooling medium therebetween; wherein the motor is coupled to the ambient air movement element so as to cause the ambient air movement element to move during operation of the heat transfer unit; an ambient hazardous air inlet, an ambient hazardous air outlet, a condenser and an ambient air movement element located between the ambient hazardous air inlet and ambient hazardous air outlet, the ambient air movement element being located such that, during operation of the heat transfer unit, the ambient air movement element will move and cause ambient hazardous air to pass into the ambient hazardous air inlet, past coils of the condenser, and out the ambient hazardous air outlet; wherein the evaporator and condenser are coupled to each other via piping so as to collectively comprise a cooling circuit for passage of a cooling medium therebetween; wherein the motor is coupled to the ambient air movement element so as to cause the ambient air movement element to move during operation of the heat transfer unit; characterised in that the first compartment is sealed off from the second compartment such that, during operation of the heat transfer unit, hazardous ambient air is prevented from entering the first compartment from the second compartment; and wherein the heat transfer unit is configured such that, during operation, an interior of the first compartment is maintained at an internal pressure that is higher than an interior pressure in the second compartment.