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77RT-PRC010-EN
Controls
CV Units Only
Sequence Of Operation
1
Occupied Zone Temperature Control
Cooling/Economizer
During Occupied cooling mode, the
economizer (if provided) and mechanical
cooling are used to control zone
temperature. If the enthalpy of outside
air is appropriate to use “free cooling”,
the economizer will be used first to
attempt to satisfy the cooling zone
temperature setpoint; then the
compressors will be staged up as
necessary. Minimum on/off timing of
compressors prevents rapid cycling.
On units with economizer, a call for
cooling will modulate the fresh air
dampers open. The rate of economizer
modulation is based on deviation of the
zone temperature from setpoint, i.e., the
further away from setpoint, the faster the
fresh air damper will open. First stage of
cooling will be allowed to start after the
economizer reaches full open.
Note that the economizer is only allowed
to function freely if one of the following
conditions is met: For dry bulb
economizer control, the ambient
temperature must be below the dry bulb
temperature control setting. For
reference enthalpy economizer control,
outdoor air enthalpy must be below the
enthalpy control setting. For comparative
enthalpy economizer control, outdoor air
enthalpy must be below the enthalpy of
the return air.
At outdoor air temperatures above the
enthalpy control setting, mechanical
cooling only is used and the outdoor air
dampers remain at minimum position.
If the unit does not include an
economizer, mechanical cooling only is
used to satisfy cooling requirements.
Outdoor air dampers may be set
manually for a maximum of 25 percent
outdoor air, if rooftop is equipped with 0
to 25 percent manual fresh air damper.
A
Heating
Gas Heating - Two-Stage
Upon a call for heating, the UCM closes
the first stage heating contacts
beginning the firing sequence. First, the
heat exchanger combustion blower
begins operation. Upon positive proving
of combustion airflow, a pre-purge cycle
is executed. Then the ignition sequence
takes place.
If ignition is not proven, the ignition
transformer is de-energized. After a time
delay another pre-purge cycle takes
place followed by another attempt to
ignite. If ignition fails a second time, the
cycle repeats on 235 and 350 MBh
modules. 500, 850 and 1000 MBh
modules, the heating section will be
shut down and locked out until manually
reset at the unit mounted Human
Interface Panel.
As additional heat is required, the UCM
will close the second stage heating
contacts and depending on heat module
size, will open either the second stage of
the gas valve, or a second stage gas
valve.
During heating operation, an electronic
flame safety control provides continuous
flame supervision. If combustion should
become unstable for any reason, heating
will automatically shut down. After one
minute, another 60 second pre-purge
and ignition cycle begins.
As the heating requirement is satisfied,
the UCM will open the second stage
heating relay, de-energizing the second
stage of heat. When the requirement is
fully satisfied, the first stage contacts are
opened, de-energizing the first stage of
heat. The specific sequence of operation
of the gas heat will depend on the size of
the heat exchanger.
(CV Units)