Mammoth 800 to 5500 CFM: Dedicated Outdoor Air System with Enthalpy Wheels (VHC) - Manual (Page 13)

MAMM-VHC-IOM-1A (JANUARY 2013) 13
During cold temperatures, defrost and frost prevention are
controlled by the unit’s integrated controls as follows.
Preheat will prevent frost formation on the enthalpy wheel
to maintain proper operation and provide continuous venti-
lation and make-up air. This prevention occurs when a pre-
heater is energized to maintain an outdoor air temperature
higher or when the wheel speed is reduced to maintain an
exhaust air temperature higher than minimum required for
the enthalpy wheel to operate frost free.
The requirement for frost control is based upon the outdoor
air temperature and humidity content of the return air. In
areas where the winter outdoor air condition falls below 5°F
[−15°C] or the return air relative humidity is above 30%,
frost control is probably required.
Preheat Frost Prevention
Preheat frost prevention is an outdoor air temperature
controlled function that allows for continuous ventilation by
ensuring a minimum enthalpy wheel entering air tempera-
ture of 5° F [−15°C]. The temperature sensor is located be-
tween the open wire electric heating coil and the enthalpy
wheel. The electric heating coil is selectable in 1 kW incre-
ments and available in two-stage, four-stage or SCR con-
trol. With staged control, one stage cycles to maintain 5°
F [−15°C] temperature to the enthalpy wheel as the other
stages are continuously on as the outdoor air temperature
drops. If the selected kW is insufficient, based upon the
entered design conditions, selection software automatically
adjusts the minimum kW required to maintain t he minimum
wheel entering air temperature.
Non-defrost
No frost control is required in areas where the winter out-
door air condition stays above 5°F [−15°C] and the return
air humidity level is below 30%.
Frost Control
levels rise and fall the BMS must modulate the signal to
the supply and exhaust fans to change the ventilation rate
proportionally. See Appendix G . The minimum VFD speed
is factory default set to 40 Hertz.
Smoke Detector
VHCs can be equipped with a duct mount smoke detector
which will monitor the air when passing through the duct
system into the unit. When sufficient smoke is detected, an
alarm condition is activated. By connecting the occupied
terminals to the NC alarm auxiliary contacts on the duct
sensor, an alarm condition will open the auxiliary contact
and stop operation of the VHC. Locate in a normally oc-
cupied area of premises. Recommended 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 dehu-
midistat. 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
Appen-
dix G .
IMPORTANT
Removal of dry contacts that close on high speed termi-
nals is required for VFDs to modulate.