30 November 1998
5. OPEN CONTROL LOOPS. When a control loop senses a variable, makes a control decision, and
sends an output signal to a control device without receiving input information related to the results of its
control action, the control loop is said to be an open loop. There are some open-loop control applications
used in HVAC control, such as:
(1) Operation of pumps above or below a certain outside temperature.
(2) Automatic stopping of HVAC systems based on outside air temperature.
(3) Scheduling of hydronic heating supply temperatures based on outside temperature.
(4) Timing and time-delay operations.
Figure 2-8 illustrates an open control loop.
Figure 2-8. Open control loop.
6. CLOSED CONTROL LOOPS. When the controller changes its output decision based on updated input
information, the control loop is said to be a closed loop. Most of the control loops used in HVAC control are
closed loops. Control of coil air discharge temperatures is an example. The transmitter, connected to a
temperature sensing element in the air stream passing through the coil, signals the temperature controller;
the controller makes a decision as to whether to open or close the valve that allows water to flow through
the coil; and an actuator operates the valve. The feedback in this example is the continuous input to the
controller of a changing temperature signal from the coil air discharge temperature sensor and transmitter.
The transmitter continuously updates the controller on temperature information from the sensor, and the
controller modifies its output to control the valve. See figure 2-9 for an example of a closed control loop.
Figure 2-9. Closed control loop.
7. APPLICATION OF OPEN-LOOP CONTROL AND CLOSED-LOOP CONTROL TO HVAC SYSTEMS.
a. Open loops and closed loops in combination. Open loops and closed loops are used in
combinations in some HVAC control-system applications. A perimeter hydronic heating system may have
open-loop components to start and stop the pump and to schedule the supply water temperature based on
outside air temperature. At the same time, it may have a closed loop for the control of the supply water
b. Closed loops in combination. There are some HVAC control applications that use two
simultaneously acting, closed control loops to actuate the same device. An example, as shown in figure 2-
10, is control of a duct humidifier. A space relative humidity transmitter is the primary input to a relative
humidity controller for the humidifier valve in a closed loop (loop 1). A duct-mounted humidity transmitter
downstream of the humidifier signals a high limit relative humidity controller, which provides a high limit
closed-loop control function (loop 2), by overriding the primary controller loop 1 to shut off humidification by
closing the valve if the relative humidity of the air stream rises to the loop-2 relative humidity high-limit
Figure 2-10. Two loops controlling one device (humidity control with high limit).
8. TYPICAL CONTROL MODES.
a. Two-position control.