AHU (Air Handling Unit)

An AHU pulls in a mix of outdoor air and return air from the building, cleans it with filters, heats or cools it with coils, and pushes it into the supply ductwork with a fan. Dampers at the mixing section control how much outdoor air comes in. When outdoor air is cool and dry enough, an economizer opens the outdoor air damper wider and uses outside air for free cooling.

AHUs do not usually make their own heating or cooling. They get chilled water from a chiller, hot water or steam from a boiler, or refrigerant from a condensing unit through a DX coil. The building automation system controls fan speed, valves and dampers to hold supply air temperature, duct static pressure and outdoor airflow at their setpoints.

Air handlers vary in size and layout. Indoor units usually sit in an AHU room (a mechanical room) with their piping, valves and controls. Most contain these parts:

• Outdoor air, return air and exhaust or relief dampers, often arranged as a mixing box
• Filters, rated by MERV (Minimum Efficiency Reporting Value) under ASHRAE Standard 52.2
• Cooling coil (chilled water or DX) with a drain pan for condensate
• Heating coil (hot water, steam or electric), or a preheat coil for freeze protection
• Supply fan and often a return or exhaust fan, commonly driven by VFDs
• On some units, a humidifier, an energy recovery wheel or plate, and sound attenuators
• Sensors for temperature, pressure, airflow and filter pressure drop

AHUs are grouped by how they control airflow and what air they handle:

• Constant air volume (CAV) units deliver a fixed airflow and vary the supply temperature.
• Variable air volume (VAV) units vary fan speed to match demand from VAV boxes serving each zone.
• Dedicated outdoor air systems (DOAS) condition 100 percent outdoor air for ventilation, often paired with fan coils, chilled beams or VRF.
• Makeup air units replace air removed by kitchen hoods or process exhaust.
• Packaged rooftop units (RTUs) combine an air handler with built-in refrigeration and often a gas or electric heating section.
• Custom and modular AHUs are built to order for hospitals, laboratories and large buildings.

An AHU serves a large area or several zones through ductwork and usually relies on a central plant for heating and cooling. A rooftop unit (RTU) is a self-contained packaged unit with its own compressor, usually mounted on the roof. A fan coil unit (FCU) is a small terminal unit that conditions a single room or zone, usually without bringing in outdoor air.

Many buildings use more than one. A common arrangement is a central AHU or DOAS that provides ventilation air, with fan coils or VAV boxes handling zone-level temperature control.

Routine AHU maintenance includes replacing filters, cleaning coils and drain pans, checking belts and bearings, testing damper and valve actuators, and calibrating sensors. Stuck dampers, leaking valves and dirty coils are common causes of high energy use and comfort complaints, and they often go unnoticed without trend data.

Useful data points include supply, return and mixed air temperatures, outdoor air damper position, filter pressure drop, fan status and speed, duct static pressure and valve positions. Outdoor airflow should meet the ventilation rates required by the local mechanical code, which in most places is based on ASHRAE 62.1.

In a digital twin such as Syyclops, each AHU sits in the building model with its BMS points, work orders and documents attached, so the people working on it see live readings and history in one place.