FCU stands for Fan Coil Unit, and AHU stands for Air Handling Unit. Both are important components of an HVAC system. FCUs are typically located in individual rooms and are responsible for controlling the temperature and air flow in that space. They are connected to the main HVAC system via ducts and are powered by electricity. AHUs are larger, centralized units that are responsible for controlling the air flow and temperature of the entire building. They are powered by either electricity or natural gas, and they are connected to the main HVAC system via ducts.