This thesis presents design and implementation of a PLC-HVAC controlled system for a small commercial office. The project was built using Eaton easy-E4-DC-12TC1 PLC as a main controller. Using IEC 6131-3 programming Languages such as LD - ST, I could able to implement 5 different operation modes based on air quality, occupation percent, and user preferences ( Auto - Manual - Eco - Boost - Night) through three Structured Text User Function Blocks that compute mode-based fan speed, air quality demand from CO₂ and humidity sensors, and select the maximum of both to drive an Eaton DC1 Variable Frequency Drive with a 0–10 V analogue output. A supervisory layer was developed across an HMI touchscreen, a SCADA system, and a web-based visualization dashboard, with all 16 functional test cases passing successfully in the physics laboratory environment.