Central Air Handling Units

Central air-handling unit systems are found in large buildings with multiple zones of at least 450 square meters (5000 square ft) and in tall multistory buildings. Hospitals with stringent air quality controls use central air-handling systems exclusively. Large central air-handling unit systems require routine daily checking and regularly scheduled maintenance. They are built on site, and take longer to install than prefabricated units, but may be more energy efficient. Central air-handling systems have a 20- to 30-year life expectancy.

In air-handling unit systems for large buildings, air is passed over the air-handling unit's heat exchanger coil, which contains steam or hot water pipes. Heating is transferred to the air, then to ducts and into the spaces where it will be used.

The air distributed from the central air-handling unit ends up in the building's individual occupied spaces. Small terminal units may be mounted directly below a window or in openings in the exterior wall of each space served. Window-mounted units typically are used to retrofit existing buildings.

Heating or cooling distributed by air or water

Return air is processed and recirculated.

Heating or cooling distributed by air or water

Return air is processed and recirculated.

Exhaust air

Blower air

Fresh air

Air handling unit with fans, preheaters, and filters treat and distribute air.

Figure 26-2 HVAC system.

Exhaust air

Blower air

Fresh air

Air handling unit with fans, preheaters, and filters treat and distribute air.

Figure 26-2 HVAC system.

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