In accordance with the present invention compositions are disclosed. The compositions consist of (A) a refrigerant component consisting essentially of (1) at least one refrigerant having an OEL less than 400; and (2) a combination of refrigerants, each having an OEL greater than 400, consisting essentially of (i) HFC-32; and (ii) trans-HFO-1234ze; and optionally (B) a non-refrigerant component; wherein component (A)(2) of the refrigerant component is present in an amount sufficient to provide an overall OEL for the refrigerant component of at least 400. The refrigerant mixtures of the refrigerant component are useful as components in compositions also containing non-refrigerant components (e.g. lubricants), in processes to produce refrigeration, in methods for replacing refrigerant R-410A, and in air conditioning and heat pump apparatus.

A heating system is provided that includes a heater, a temperature sensor module, an interlock arrangement, a semiconductor switch, a driver module, an interruption switch, and a concentration sensor module. The temperature sensor module changes from a first state to a second state when a temperature associated with the heater reaches or exceeds a temperature threshold. The electrical interlock arrangement prevents supply of electrical power from power terminals to the heater when the temperature sensor module is in the second state. The semiconductor switch modulates supply of electrical power from the power terminals to the heater and is controlled by the driver module based on a drive signal. The interruption switch prevents the driver module from receiving the drive signal during a non-conducting state. The concentration sensor module causes the interruption switch to be in the non-conducting state when a concentration of refrigerant reaches or exceeds a concentration threshold.