Frost Control
During cold temperatures, defrost and frost prevention are
controlled by the unit’s integrated controls as follows.
Recirculation and exhaust only defrost remove the frost
from the enthalpy wheel to maintain proper operation but
do not provide continuous ventilation or make-up air. This
removal of frost occurs when a damper closes the outside
airstream or closes both outside and exhaust airstreams
and opens a recirculation damper to allow warm room air
to circulate through the enthalpy wheel.
Pre-heat and Variable Speed Drive (VSD) will prevent frost
formation on the enthalpy wheel to maintain proper opera-
tion and provide continuous ventilation 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 tem-
perature higher than minimum required for the enthalpy
wheel to operate frost free.
The requirement for frost control is based upon the out-
door 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.
Recirculation Defrost
The exhaust fan is de-energized. There is no outdoor
air ventilation for the duration of defrost. Enthalpy wheel
stops rotation. Return air is recirculated through the wheel
and back into the building space.
Exhaust Only Defrost
Supply fan is de-energized. There is no outdoor air ventila-
tion for the duration of defrost. Enthalpy wheel maintains
rotation. Return air is exhausted outdoors.
Pre-heat Frost Prevention
Pre-heat 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, Venmar Select™ Web Edition
software automatically adjusts the minimum kW required
to maintain the minimum wheel entering air temperature.
Variable Speed Drive Frost Prevention
This variable speed frost control option is an exhaust air
temperature controlled function that allows for continu-
ous ventilation by reducing the enthalpy wheel rotational
speed. The rotational speed and therefore effectiveness of
the enthalpy wheel, is modulated to maintain an exhaust
air temperature of 33°F [1°C]. This modulation maintains
the wheel operating temperature at conditions that pre-
vent frost formation. Special consideration must be given
to applications where supply air is being heated, as the
heating capacity maximum condition will be during the
frost prevention cycle.
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 Options with Recirculation
In certain design applications, it may be required for the
energy recovery unit to have a frost control option and the
option for recirculation ventilation. For units requiring both
these options, it is important to recognize that these op-
tions do not operate at the same time. Recirculation op-
eration allows the unit to recirculate return air to the supply
airstream. Frost control options operate as described in
the previous paragraphs.
IMPORTANT
VSD frost control may cause supply air temperatures to
be below freezing. Adequate freeze protection, such as
glycol or low limit temperature protection for downstream
coils or re-heat to protect building systems, must be field
provided.
VCES-VHC-IOM-1F – VHC-36, -42 & -50
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