Temperature is a crucial variable to evaluate and measure the energy consumption and efficiency of the Air Conditioning & Mechanical Ventilation System (ACMVS) in the buildings. To ensure the performance and efficiency of the ACMVS, an experimental study of long-term reliability for temperature monitoring to ensure the accuracy of temperature sensor measurement is carried out in this paper. Four experiments were conducted for the influence of temperature measurement under different operating conditions in the ACMVS, which are insertion depth, humidity, tube vibration, and water mixing. Finally, the drift of two types of temperature sensors (RTD and Thermistor) commonly used in industry are compared under the condition of long-term reliability measurement. The results showed that the non-waterproof thermistor installed on the open thermocouple sleeve at least 2/3 of the pipe where the temperature sensor is inserted at 10D away from the water mixing point could provide the best temperature reading.