Gas leak detectors for HVAC appliances; Gas detectors for detecting the presence of gas; Gas detectors for detecting the presence of flammable gas; Leak detectors for HVAC systems, heat pump systems, heat recovery systems, variable refrigerant flow systems, and HVAC unitary systems; Refrigerant leak detection system comprised of leak detecting device for coolant-supplied HVAC appliances, consisting of coolant sensor, control unit, and communications unit comprised of computer hardware and recorded computer software for communicating to other devices