30 November 1998
"RECIRCULATING/AUTO" contact is closed, air is exhausted from the damper actuator PPs, causing the
outside air damper to close. The return air and relief air dampers normally work in concert with the
outside air damper. When both EMCS contacts are open, the EPs are de-energized and IP-XX01's
pneumatic control signal is connected to the damper actuator's PPs.
Figure 5-13. Control system variations for EMCS building purge and recirculation modes for pneumatic
c. Figure 5-14 shows the EMCS devices required when the damper actuators are electric or
electronic. The relays labelled "R-1 EMCS" and "R-2 EMCS" function in a manner similar to the EPs
required for pneumatic actuators. The device labelled "EMCS MPS" is a minimum position switch set to
hold the outside air damper open in the purge mode. The signal to the actuators is interrupted in the
recirculating mode to close the outside air damper.
Figure 5-14. Control system variation for EMCS building purge and recirculation modes for electric or
13. CONTROL SYSTEM VARIATIONS FOR SMOKE CONTROL AND FREEZE PROTECTION. In
addition to the required HVAC control panels already shown, an HVAC system may have a smoke
control and freeze protection panel, to house the equipment required to connect these systems to the
HVAC control system. These systems can interrupt the control signals to starters, valves, and dampers
for these special purposes. The smoke control and freeze protection panels, if required, will be custom
designed specifically for each project. The standard HVAC control panels have interface provisions at
terminal blocks for smoke control systems, freeze protection systems, or other external systems
designed either to shut down the HVAC system or to bypass the shutdown circuits of the HVAC control
system as required. The contract documents must show the interface requirements and locations of
each non-HVAC control panel in the project. When direct digital controls are used to control an HVAC
system, smoke control or freeze protection may be provided by a separate system as above, or these
features can be integrated into one system.
14. CONTROL SYSTEM VARIATIONS FOR NON-ECONOMIZER HVAC SYSTEMS.
a. The economizer mode of operation is not appropriate for every HVAC application. When an
HVAC system is designed for comfort applications without humidity control (i.e., no humidification or
dehumidification), the control system will have a minimum-position switch to allow some adjustment of
the mixed air flow. The steps involved in modifying a standard single-zone economizer mode control
system (such as shown in Chapter 4) to convert it to a non-economizer control system are:
(1) Delete all economizer loop devices (such as the economizer controller and its transmitter,
relay contacts, and signal selector) from the control system schematic.
(2) Delete the economizer's PV and DEV contacts and their associated relay and pilot light from
the ladder diagram.
(3) Delete the economizer controller and associated devices from the equipment schedule.