Chapter hvac engineering Fundamentals: Part 1 Introduction

8.2.3
Control modes
Control systems can operate in several different modes. The simplest
is the two-position mode, in which the controller output is either on
or off. When applied to a valve or damper, this translates to
open
or
closed.
Figure 8.2 illustrates two-position control. To avoid too rapid
cycling, a
control differential
must be used. Because of the inherent
time and thermal lags in the HVAC system, the
operating differential
is always greater than the control differential.
Design Procedures: Part 6
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