An air handling unit is comprised of a filter, humidifier, blowers (fans) and heat exchanger (fins). Some air handling units are large enough for a person to enter, so engineers sometimes enter air handling units for maintenance.
An Air Handling Unit (AHU) is used to re-condition and circulate air as part of a heating, ventilating and air-conditioning system. The basic function of the AHU is to take in outside air, re-condition it and supply it as fresh air to a building.
AHU, which is Air Handling Unit is an appliance used to circulate air. HVAC is Heating, Ventilating and Air Conditioning system. HVAC is the central unit to which AHU is connected. AHU is only a part of HVAC and as such there is hardly any difference between the two.
There are basically two types of air handling unit that are in used and they are the "Draw-Through" or "Blow-Through". In the Draw-Through type, the fan pulls the air through the mixing box, filters and cooling coil before discharging it from the fan outlet to the space to be conditioned or to the ducting network.
The air handling unit, or AHU is part of the HVAC system and is a machine that is designed to handle and condition the air that is processed in the HVAC system.
Inside an AHU we will typically find two sets of filters. The first set is known as the primary filters and we also then have the secondary filters. These both have different functions in an air handling unit.