Smart Buildings Systems for Architects, Owners and Builders

Air-Handling Units

Air-handling units (AHUs) provide warm or cool air to different parts of a
building, using chilled water to cool the air or steam or hot water to heat the
air (Fig. 3.7). An air handler is usually a metal box containing a blower, heat-
ing and/or cooling elements, filter racks or chambers, sound attenuators, and
dampers. Large AHUs for commercial use contain coils that use heated water
provided by a central boiler and chilled water from a central chiller. Small
AHUs usually contain a fuel-burning or electrical-resistance heater and an
evaporative chiller that are integrated in the unit itself.
The AHU draws air in, passes the air over heating and cooling coils, and
then forces it through air ducts. The AHUs have many of the networked
points of the HVAC control system to manage air flow, heating, cooling and
filtering. They can serve a building, a single floor on a building, or multiple
floors of a building. If the AHU is serving multiple zones, each zone typically
gets local control by having its own air premixed at the AHU. Some AHUs use
no ductwork at all, recirculating the air in the space served.
Smaller AHUs, sometimes called blower coil or fan coil units (FCUs), can
consist of only a coil, fan, and air filter, and operate using no outside air.
Fan coil units are typically found in places where cooling requirements are
small or on a room-by-room basis, such as hotel rooms and apartments.
Larger AHUs, known as makeup air units (MAUs), operate using only outside
air. One of the most common types of AHU is the roof-top unit (RTU), also
known as a unitary air conditioner. The condensing unit for an RTU is on the
roof of the building, with the cooling coils inside the AHU contained in the
building. RTUs are most commonly used for one-story commercial buildings.

Exhaust

Air

Exhaust

Air

Damper

Return Fan

with Variable-

Speed Drive

Return Air

Damper

Outside

Air

Outside

Air

Damper

Filter Heat and CoolingCoils with Variable-Supply Fan

Speed Drive

Supply

Air to

Te r m i n a l

Units

Return

T Air

T T

TTemperature Sensor HHumidity Sensor FFlow Sensor

T T T H F

H F

+− +−

Figure 3.7 Typical air-handling unit.

38 Smart Building Systems for Architects, Owners, and Builders