Roger W. Haines

Dynamic Response and Tuning

HVAC system control systems can be especially difficult to adjust or tune because the process gain of the control loop varies with the control point and with a range of other variables. For example, if the air flow rate over a hot-water heating coil is cut in half (as in a multizone application), the gain of the coil (the ratio of the change in outlet temperature to the change in valve position) will increase significantly.

Retrofit of Existing Control Systems

The “energy crisis” of the 1970s created an awareness of the inefficiency of many existing HVAC systems and their controls. Most systems installed in the fifties, sixties and early seventies were concerned only with control for comfort, and minimization of energy consumption was seldom considered. Also, many existing systems have failed to perform well with respect to comfort or economy.

Central Plant Pumping and Distribution Systems

The term “central” plant refers to a grouping of one or more chillers and/or boilers supplying chilled and hot water (or steam) to HVAC units located at various points in a building or a complex of buildings. The plant is not necessarily centrally located with respect to the HVAC units. Some plants may have only one or two elements, whereas others may contain a dozen or more chillers and boilers. Some large plants, which have grown over a period of years as a campus developed, have been spread out in two or three locations as the originally assigned spaces have become too small.

Digital and Supervisory Control Systems

Central supervisory control systems allow one person, at a central location, to monitor and control the operation of up to several thousand elements of the heating, ventilating, and air-conditioning systems in the building or the complex. The addition of alarm circuits and audio or audiovisual communications improves security as well as simplifying and improving the degree of control that can be maintained. The additional first cost of these systems sometimes can be amortized in a few years by more efficient maintenance and operation. Where many tenants are involved, the increased tenant satisfaction may be sufficient justification for the added cost.

Complete Control Systems

In the previous chapter we discussed elementary control systems. These are, in general, the pieces and parts which can be fitted together to produce specific applications solutions.

Electric and Electronic Control Devices

Electrical controls are available in a wide variety of configurations, for every conceivable purpose in HVAC applications. All of these are based on one of four operating principles: the switch, the electromagnetic coil and solenoid, the two-position motor and the modulating motor.