30 November 1998
11. CONTROL SYSTEM VARIATION FOR BUILDING PURGE / FLUSH CYCLE.
a. This variation provides the means to utilize a time-clock scheduled 100% outside air "building
purge" cycle in situations such as laboratories, etc. where it is desirable to flush airborne contaminants
on a regular basis. The "purge cycle" variation is applicable to all of the systems represented in figures
4-13x thru 4-22x. An example, as applied to the standard multizone system is shown in figures 5-12a
through 5-12f. For a DDC control system, this variation can be incorporated by software changes, which
should be reflected in the schedules and the sequence of operation.
b. The purge cycle utilizes a third set of contacts on the system timeclock for initiation. When this
set of contacts close, a relay is energized to start the fan and a pilot light is turned on in the control
panel. Once the fan is started, a solenoid valve (EP-XX-01) is energized, supplying main air to the
mixed air dampers. This will move the outside air and relief air dampers to the fully open position, and
the return air damper to the fully closed position; thereby supplying 100% outside air to the space. When
the timeclock "purge" contacts open, ending the purge mode of operation, the system's mode of
operation is determined by the remaining timeclock functions. Note that the purge mode of operation
overrides the other modes, so far as the control of the mixed air dampers is concerned.
Figure 5-12a. Control system schematic for multizone HVAC system with building flush mode of
Figure 5-12b. Control system ladder diagram for multizone HVAC system with building flush mode of
Figure 5-12c. Control system equipment for multizone HVAC system with building flush mode of
Figure 5-12d. Control panel interior door layout for multizone HVAC system with building flush mode of
Figure 5-12e. Control panel back panel layout for multizone HVAC system with building flush mode of
Figure 5-12f. Control panel terminal block layout for multizone HVAC system with building flush mode of
12. CONTROL SYSTEM VARIATIONS FOR EMCS INITIATED BUILDING PURGE AND
a. When EMCS requires control of HVAC system outside air, return air, and relief air dampers, the
devices used by EMCS to assume control of the dampers are external to the HVAC control panel. The
devices required depend on whether the actuators are pneumatic or electric/ electronic.
b. Figure 5-13 shows the schematic variation for pneumatic actuators. The EMCS devices are: EPs
labeled "EMCS", the purge/ auto and recirculating/auto contacts, and the associated control circuit.
When EMCS is not in control, the EPs pass the pneumatic control signal from IP-XX01 to the PPs of the
damper actuators. When the "PURGE/AUTO" contact is closed, main air passes through EP-1 to the
PPs of the dampers, which causes the outside air damper to fully open. When the