30 November 1998
(1) Some HVAC equipment can be turned on and off as a method of temperature control. This
type of HVAC equipment is not applied where temperature control between close limits is required.
(2) When a thermostat or other control device cycles equipment to maintain its setpoint the control
mode is called two-position control. A thermostat used for two-position control opens and closes contacts
for control rather than providing a modulating output signal. The contacts either open or close when the
temperature is at the thermostat setpoint. The state of the contact reverses when the temperature changes
in the proper direction. Such thermostat contacts usually either open on a temperature rise (in a heating
application), or close on a temperature rise (in a cooling application). The temperature at which this
happens depends on the switch temperature differential (hysteresis).
(3) An example of two-position control is unit heater control, in which a space thermostat turns on a
unit heater when the space temperature drops to 18 degrees C (65 degrees F) and turns it off when the
space temperature rises to 19 degrees C (67 degrees F). The thermostat is said to have a differential of 1
degree C (2 degrees F) and a setpoint of 18 degrees C (65 degrees F). This type of control can result in a
slight undershoot below the lower end of the differential, and a slight overshoot above the higher end of the
(4) The thermostat may turn off the unit at 19 degrees C (67 degrees F), but the heating load may
decrease due to increasing outside air temperatures. In this event, water circulating through the unit coil,
which will then be acting as a radiator with the fan off, may raise the temperature in the space slightly above
19 degrees C (67 degrees F) as an apparent overshoot. Even though the unit turns on when the space
temperature drops to 18 degrees C (65 degrees F), the space temperature may fall slightly below 18
degrees C (65 degrees F) after the unit starts. This depends on the heating load at the time and on the
heating capacity of the unit. If the heating load decreases, the temperature may subsequently rise to 18.5
degrees C (66 degrees F) and stay at that temperature for a considerable time, while the fan continues to
run. A similar situation can happen after the unit heater shuts off at 19 degrees C (67 degrees F) and the
space temperature drops to 18.5 degrees C (66 degrees F). Consequently, the space temperature may be
18.5 degrees C (66 degrees F) with the unit heater fan either running or stopped. A graphic representation
of two-position control is shown in figure 2-11.
Figure 2-11. Two-position control.
b. Modulating Control. A simple control loop is shown in figure 2-12 as it would be applied to heat
outside air for ventilation using a pneumatic valve actuator rather than an electric or electronic valve
actuator. The controller operates an IP in response to the signal of the temperature sensing element in the
air duct, downstream of the coil, via a transmitter. The IP pneumatic output signal modulates the positioner
on the pneumatic valve actuator. The positive-positioner output throttles main air to the actuator, which
moves the valve stem. This example is used to explain two modes of modulating control that are applicable
to the control of valves, dampers, inlet vanes, and other devices. The modes applicable to most HVAC
control applications are:
(1) Proportional mode (P).
(2) Proportional plus integral mode (PI).
Figure 2-12. Simple control loop applied to outside air heating.
c. Proportional mode (P). The most common control mode in HVAC control is proportional mode.
(1) Proportional mode is used for the following applications: