30 November 1998
(c) An output contact configured as PV and acting as a switch.
(d) An output contact configured as DEV and acting as a switch.
(e) There are exceptions to the above listed connections (such as when an economizer is used
in a multizone system.
(15) As the outside air temperature changes, the potential for using outside air for cooling is
affected. When the outside air temperature is lower by a specified number of degrees than the return air
temperature, outside air can be used for cooling; when the reverse is true, outside air must be at minimum
quantity. The PV contact setpoint prevents the use of outside air beyond minimum quantity until the return
air temperature rises to its setpoint. When the return air temperature rises to the PV contact setpoint, the
HVAC system is no longer experiencing a heating load. The DEV contact setpoint allows the use of outside
air until the outside air temperature approaches the return air temperature. The DEV contact then puts the
dampers under control of a minimum-position switch. A minimum-position switch is a device whose control
output is manually modulated to an actuator that then remains fixed until reset manually. Both conditions
must be met (PV and DEV contacts closed) for the use of outside air beyond minimum quantity.
p. Function Modules.
(1) There are control functions required with less frequency in HVAC control applications than
those included in the prescribed version of the single-loop controller. Control devices to perform specific
control functions not available in the single-loop digital controller are called function modules in this
manual. Function modules will be located in the HVAC control panel, except as noted. Function modules
accept contact, analog, or gradual manual adjustment input signals to provide:
(a) Signal selection,
(b) Signal inverting,
(c) Contact transfer output from analog signal input,
(d) Contact transfer output from the comparison of two analog input signals, and
(e) Generation of constant analog output signals.
(2) A minimum-position switch is a manually adjustable modulating output device used to hold an
outside air damper open to admit minimum ventilation air. This same device is used as a temperature
setpoint selector. In this application, it allows manual adjustment of the CPA input signal to a single-loop
digital controller. In the case of certain single-zone HVAC systems, this device is wall-mounted and
accessible to the occupant for adjustment of the space temperature controller setpoint.
(3) A signal invertor is a modulating input and modulating output device used to reverse the
direction of its input. The signal reversal is required when spring return position of a valve or damper
actuator has been chosen to operate with a direct acting signal, and when an actuator with which it must be
sequenced has been chosen to operate with a reverse acting signal. For example, a chilled water coil is
used for dehumidification and cooling. The chilled water valve is chosen as NC because it is to be
sequenced with a NO heating coil valve, and is to close when the control signal is removed during fan
shutdown and during the unoccupied mode. The direct acting temperature control signal is correct for this
combination. The humidifier control valve is also chosen as NC because it also is to close when the control
signal is removed on fan shutdown and during the unoccupied mode. The humidity control signal must be
reverse acting when it operates the humidifier valve, and must be direct acting when it operates the cooling
coil valve. A signal invertor is used to reverse the signal direction of the humidity controller signal to the
cooling coil valve.