An HVAC monitoring system that tests for an abnormal environmental condition, wherein the abnormal condition results in effectuating a selected response from an HVAC building system, the HVAC monitoring system utilizing an existing sensor for detecting the gas abnormal condition, wherein a first event marker signal is generated from the existing sensor detecting the abnormal condition. Further included is an audio sensor, a wireless transmitter, a wireless receiver, programmable control circuitry, a switching transistor, a relay, and a power supply for all the previous elements, wherein these components utilize the first event marker signal and through a series of subsequent signals to result in the relay being operative to be in an activated operational state upon being energized by the switching transistor to operationally effectuate the selected response from the HVAC building system.