There are four main types of air handling units (AHUs) used in HVAC systems: single-zone, multi-zone, variable air volume (VAV), and fan-powered. Single-zone AHUs are designed to provide conditioned air to a single zone or space, while multi-zone AHUs are designed to provide conditioned air to multiple zones or spaces. VAV AHUs are designed to provide conditioned air to a single zone or space while varying the air volume based on the load requirements. Fan-powered AHUs are designed to provide conditioned air to multiple zones or spaces while varying the air volume based on the load requirements. Each type of AHU has its own advantages and disadvantages, so it is important to select the right type for the application.