Failure to follow the calibration procedures can
result in improper resistance values at the desired
1. Drive the actuator fully closed (0°) to fully open (90°)
and back again to the fully closed position to receive
the correct resistance readings at the appropriate
degree of stroke.
2. Check the resistance values of the potentiometer with
an ohmmeter at intervals in the stroke while referring to
the table in Fig. 12 and feedback potentiometer ratings
in the Specifications section.
3. Replace the range stop pins and/or the minimum
position setscrews using the appropriate procedures.
AUXILIARY POTENTIOMETER LEADS
FIELD-ADDABLE AUXILIARY POTENTIOMETER
Fig. 12. ML6174A,C with field-addable potentiometer.
ML6174, ML7174 NON-SPRING RETURN DIRECT COUPLED ACTUATORS
C O M
C C W
VAV systems control the temperature within a space by
varying the volume of supply air temperature. The system
delivers air to the space at a fixed temperature. The space
thermostat controls the volume of supply air by modulating
the supply air damper. When full heating and cooling flexibility
is required in a zone, it is handled by the air temperature
system, or with reheat capability in the air terminal units. As
individual zones shut down, a central duct static pressure
controller regulates the total air flow in the system. The fan
system is sized to handle an average peak load, not the sum
of the individual peaks. As each zone peaks at a different
time of day, extra air is borrowed from the off-peak zones.
This transfer of air from low-load to high-load zones occurs
only in true VAV systems.
In pressure-independent systems, individual zone airflow
sensors maintain the zone air flow rate regardless of
fluctuation in the total system pressure. When used with
controllers such as the W7620, pressure-independent
systems can react faster to changes in air flow demand;
therefore, these systems can use the faster 90-second
Pressure-dependent systems do not incorporate an individual
zone air flow sensor and depend on a stable system pressure
to maintain flow. These systems require slower actuators
such as the seven-minute ML6174 models that are typically
controlled by spdt floating wall thermostats.
The T641 is a mercury bulb floating control type thermostat
designed for use with the seven-minute ML6174 on pressure-
dependent systems (see Fig. 13 and 14). The T6984 is an
electronic floating control thermostat designed for use with
the 90-second and seven-minute ML6174 models (see
Over time, multiple actuators with similar timing that
are driving in parallel may not be in sync with each
other. In normal operation, driving all actuators to the
fully open or fully closed position restarts them in
sync with each other.
Using Fig. 16, parallel the CW, COM and CCW terminals. Up
to five actuators can be wired in parallel. Make certain the
total connected load does not exceed the current capacity of
the controller or thermostat.