Installation, Operation and Maintenance Manual
Marvel Mini
25
Subject to change without notice.
120.9-IM (1218)
Modulating Heat by Supply Air Temperature
Modulating Heat by Supply Air Temperature is based on
the Heating Demand This Heating Demand is calculated
by the Supply Air Temperature; Supply Air Temperature
Heat Set point and Supply Air Temperature Band Set Point
If the Supply Air Temperature is less than or equal to the
Supply Air Temperature Set Point minus ½ of the Supply Air
Temperature Band Set Point, the Heating Demand shall be
100%
The amount of modulating heat applied parallels the Heating
Demand A total of 50% Heating Demand equals 50 vdc on
the analog output controlling the Modulated Heating device
Modulating Heater outputs are only 0 to 10 vdc type
Important:
Heating Start and End points also allow for a
buffer around the Set Point helping to prevent overshoot
These Start and Hysteresis Set points are adjustable but it
is not recommended unless consulting with United CoolAir’s
support team first.
Heat Pump – Option
For systems with the Heat Pump option, Heat Pump mode
is available for heating if the heat rejection type (air cooled
or water cooled) temperature is above the Low Temperature
Lockout Set Point (applies to both air and water cooled
systems) United CoolAir strongly recommends Auxiliary
Heaters for those systems located within geographical
regions with inclement climates that heat pump mode will
not always be available due to low ambient conditions
As a standard, the heat pump mode of operation shall
operate down to low ambient lockout temperature Once the
Outdoor Temperature or Water Temperature is below the
Low Temperature Lockout Set Point, the heat pump mode
will be locked out from operation until the temperature rises
back above the Low Temperature Lockout Temperature Set
Point plus the Low Temperature Lockout Temperature Band
Set Point
Heat Pump Option with Supplemental
Heating (option)
If the Heat Pump option was ordered with the Supplemental
Heating Option, if the heat pump mode of operation is
locked out due to the Temperature of the Outdoor Air or
Water being colder than the Low Temperature Lockout Set
Point, the Supplemental Heaters are Staged or Modulated
to maintain the demand for Heating As the Temperature of
the Outdoor Air or Water rises above the Low Temperature
Lockout Set Point plus the Low Temperature Lockout Band
Set Point, the Supplemental Heaters are staged off or
modulated down to allow the Heat Pump Mode to restart
Defrost Cycle
Heat Pump systems have a defrost cycle that performs
defrost mode based on a temperature switches connected
to the refrigerant lines of the outdoor coil of each
compressor circuit Should one of these switches close due
to the temperature of the refrigerant line falling below the
switches low temperature cutout set point, the compressors
which are currently in operation will continue to remain on
and the reversing valves will be energized to cycle cooling
long enough to bring the temperature of the refrigerant line
back above the Defrost Switches reset set point Once the
Defrost Switch(s) reset, the Reversing Valves are then de-
energized placing the system back into heat pump mode of
operation
Should the system go into the defrost cycle and supplemental
heating is available, the supplemental heaters are brought
on to maintain the heating requirement
Modulating Hot Gas Bypass for Lightly
Loaded Compressor Management (option)
Modulating Hot Gas Bypass for Lightly Loaded Compressor
Management is an option used to false load the evaporator
coil and compressor system during reduced loading
conditions to the evaporator coil Since the cooling demand
has not been completely satisfied the compressor will
continue to operate and the evaporator coil may start to
freeze The unit’s controller regulates the amount of hot gas
applied to the evaporator coil to prevent the evaporator coil
from freezing based on the cooling demand
As the Cooling Demand increases, compressors are
staged ON based on the percentage of demand for each
Compressor as well as the overall Cooling Demand As the
Cooling Demand decreases to a point where the demand
for cooling required is less than the hot gas bypass start set
point, Hot Gas shall be modulated through the electrically
modulated Hot Gas Bypass Valve into the Evaporator Coil
If the cooling demand should then rise above the hot gas
bypass start point, the Modulating Hot Gas Bypass will
modulate closed and if the demand becomes high enough,
Compressor(s) 2 and/or 3 will stage on
