Optional Controls and Accessory Sequence and Interlocks
Free Cooling
Power connected, unit ventilating, free cooling call. Wheel
stops rotating.
Variable Speed Setpoint Free Cooling
Power connected, unit ventilating. Wheel modulates to
keep discharge temperature air at setpoint.
Coil Low Limit (Coil Units Only)
Low limit temperature protection is not provided. Coils
must be protected with glycol or field installed low limit
temperature protection if the coil entering air temperature
is expected to go below 35°F [1.7°C].
DDC units do not use low limit temperature protection. In-
stead, if discharge air temperature falls below the low limit
setpoint (adjustable), for five minutes (adjustable), the unit
will shut down and the HW valve will stroke to 100% heat.
Power to the DDC unit must be disconnected and recon-
nected to reset the ‘LS’ (low supply air) alarm.
Unit Fault
An external unit fault can be initiated by removing the dry
contact jumper to the field wire terminal in the control
panel. This contact is normally closed and requires an
open contact to initiate a fault and shut down the unit.
Dirty Filter Sensor
The VHCs can be equipped with dirty filter sensors which
monitor the pressure across the filters and close the con-
tacts when the filters become restricted with dirt. Dirty filter
sensor is factory set to close contacts at 0.8” w.c. [200
Pa]. Field wired connections can be made to the terminal
interface for both the supply and exhaust filter sensors
(electro-mechanical controls units only).
Remote Fan Control
Remote fan control can be achieved by connecting dry
contacts to the terminal interface (during occupied or un-
occupied recirculation). These controls could also be the
following: SPDT switch, dehumidistat, CO
2
sensor, light
sensor, heat sensor, timer, building management system,
etc.
CO
2
Ventilation Control
VHC units’ air volume with the VFD motor control option
can be directly controlled by a CO
2
ventilation control (ac-
cessory or field supplied) by connecting normally open
high speed contacts to the supply and exhaust fan high
speed input terminals for two-speed (High–Low) ventila-
tion control (see Appendix H,
Alternatively, connect the CO
2
ventilation control 0 to 10
VDC output signal to the supply and exhaust fan modulat-
ing (0 to 10 VDC) input terminals for proportional ventila-
tion control (see
2
levels exceed
acceptable limits, dry contacts close or VDC increase rais-
ing high speed fan ventilation. VFD motor control is factory
limited to a minimum motor speed of 40 Hz (double width
double inlet fans only) or 2/3 air volume.
Smoke Detector
VHCs can be equipped with a duct mount smoke detec-
tor which will monitor the air when passing through the
duct system into the unit. When sufficient smoke is de-
tected, an alarm condition is activated. By connecting the
occupied terminals to the NC (normally closed) alarm aux-
iliary contacts on the duct sensor, an alarm condition will
open the auxiliary contact and stop operation of the VHC.
Locate in a normally occupied area of premises. Recom-
mended for compliance to NFPA 90A and IMC code 606.
Cooling Override (DDC Controls Units Only)
These terminals are available for a room or return air sum-
mer thermostat. All thermostats are field installed and
wired.
Dehumidification (DDC Controls Units Only)
These terminals are available for a room or return air de-
humidistat. All dehumidistats are field installed and wired.
Heating Override (DDC Controls Units Only)
These terminals are available for a room or return air winter
thermostat. All thermostats are field installed and wired.
For standard field wiring terminals diagram, refer to
IMPORTANT
Removal of dry contacts that close on high speed termi-
nals is required for VFDs to modulate.
IMPORTANT
A separate low limit dry contact for DDC units is available
on the FW terminal strip 363–364. This contact signal is
provided by others. When the dry contact is made, it will
turn off the unit and stroke the HW valve to 100%.
VCES-VHC-IOM-1F – VHC-36, -42 & -50
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