Trane RT-PRC010-EN Application Considerations, Exhaust Air Options, Application Recommendations 1

Application Considerations

EXHAUST AIR OPTIONS

When is it necessary to provide building exhaust? Whenever an outdoor air economizer is used, a building generally requires an exhaust system.The purpose of the exhaust system is to exhaust the proper amount of air to prevent over or underpressurization of the building.The goal is to exhaust approximately 10 percent less air than the amount of outside air going into the building.This maintains a slightly positive building pressure.

A building may have all or part of its exhaust system in the rooftop unit. Often, a building provides exhaust external to the air conditioning equipment.This external exhaust must be considered when selecting the rooftop exhaust system.

IntelliPak® Rooftop units offer four types of exhaust systems:

1

100 percent modulating exhaust with Statitrac™ direct space sensing building pressurization control (with or without variable frequency drives).

2

100 percent modulating exhaust without Statitrac.

3

50 percent power exhaust.

4

Barometric relief dampers.

Application Recommendations

1

100 percent modulating exhaust with Statitrac™ control

For both CV and VAV rooftops, the 100 percent modulating exhaust discharge dampers (or VFD) are modulated in response to building pressure. A differential pressure control system, called Statitrac™, uses a differential pressure transducer to compare indoor building pressure to atmospheric pressure.The FC exhaust fan is turned on when required to lower building static pressure to setpoint.The Statitrac control system then modulates the discharge dampers (or VFD) to control the building pressure to within the adjustable, specified dead band that is set at the Human Interface Panel.

Advantages of the Statitrac™ 100 percent modulating exhaust system are:

a

The exhaust fan runs only when needed to lower building static pressure.

b

Statitrac compensates for pressure variations within the building from remote exhaust fans and makeup air units.

c

The exhaust fan discharges in a single direction resulting in more efficient fan operation compared to return fan systems.

d

Because discharge dampers modulate the airflow, the exhaust fan may be running unloaded whenever the economizer dampers are less than 100 percent open.

With an exhaust fan system, the supply fan must be sized to pull the return air back to the unit through the return system during non-economizer operation. However, a supply fan can typically overcome return duct losses more efficiently than a return air fan system. Essentially, one large fan by itself is normally more efficient than two fans in series because of only one drive loss not two as with return air systems.

The reason for either a return air fan or an exhaust fan is to control building pressure.TheTrane 100 percent modulating exhaust system with Statitrac does a better job controlling building pressure than return fans simply because 100 percent modulating exhaust discharge dampers (or VFD) are controlled directly from building pressure, rather than from an indirect indicator of building pressure such as outdoor air damper position.

The 100 percent modulating exhaust system with Statitrac may be used on any rooftop application that has an outdoor air economizer. However, when most exhaust is handled external to the rooftop or when building pressure is not critical, one of the other less expensive methods of exhaust may be used.

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