HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New Millenniumintegrated with space pressurization to provide indoor air quality sequencesand enhanced comfort.After Hours ControlAn override function allows access to the building by a tenant withconditioning provided during these times. It can be initiated by a pushbutton, switch or dial-up with the central operator interface or touchtone telephone interface to the controller.There is normally a fixed duration of time that the override willlast, but the amount of time that a controller is in override may also bea variable. It could be based on the time needed to log the status ofvarious points in the system.Modern leases may include access to the space on a 24-hour basis,but only provide space conditioning during normal working hours.Occupants who use the space after hours may be required to pay a usecharge for the lighting and space conditioning. This is known as afterhours tenant billing, and has become a common practice in many officebuildings.Space PressureA control loop that is used more often with VAV air handlers involvesmonitoring and controlling the space pressure. This type of controlis used in environments where a negative pressure is sustained toavoid the flow of contaminated air outside the controlled space.Maintaining a slightly negative pressure induces the flow of freshair into the building. Maintaining neutral space pressure prevents theloss of expensive conditioned air to the outside from positive pressure.Positive pressure can result in noise around diffusers that may botherthe occupants.AlarmingOne of the benefits to intelligent control is the ability to monitor avariety of alarm conditions. Most of these are simple control loops that©2001 by The Fairmont Press, Inc. All rights reserved.