Sequence of Operation
RT-SVX34F-EN
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During cooling operation, the cooling valve (CLV) will be
open 100% and the reheat valve (RHV) will be closed which
will allow refrigerant to flow through the condenser coil
and not the reheat coil. During cooling mode the reheat
pump-out solenoid (RHP) will also be energized to allow
refrigerant to be removed from the reheat coil.
During cooling or dehumidification mode, to ensure
proper oil distribution throughout the reheat and cooling
condenser circuits, a purge is initiated by a hard coded
purge interval timer. After the purge interval timer reaches
60 minutes, the unit performs a purge for a fixed 3-minute
time period. During this state the reheat and cooling valve
will be driven 50% and the reheat pump-out solenoid is
energized.
See Dehumidification Low Pressure Control section for the
reheat low pressure control (RLP) function during active
dehumidification.
See Dehumidification Frost Protection section for the
control scheme during active dehumidification.
See the Condenser Fan / Compressor sequence section for
Condenser fan staging during active dehumidification.
Sensible cooling or heating control overrides
dehumidification control. For both multi-circuit and single
circuit units, any sensible heating request will terminate
dehumidification control. If heating is active at the time a
call for dehumidification control is received the heating
operation must complete and an additional 5 minutes
from the time heat is terminated must elapse before
dehumidification will be allowed.
Note:
Occupied VAV operation in cooling mode will
consider a critical zone temperature and when the
sensible cooling requirements of this zone are not
being met, the unit will terminate dehumidification
control.
Note:
Occupied CV and all unoccupied operation will
terminate dehumidification if the sensible zone
cooling requirements exceeds one-half the
available cooling capacity of the unit.
Gas Heat Sequence Of Operation
When heating is required, the RTRM initiates the heating
cycle through the ignition control module (IGN). The IGN
normally open contacts close to start the combustion
blower motor (CBM) on high speed. Next, the IGN control
energizes the hot surface igniter (IP) for 45 seconds. After
a preheat period, the gas valve (GV) is energized for
approximately 7 seconds. If the burner lights, the gas valve
remains energized. If the burner fails to ignite, the ignition
module will attempt two retries and then lock out if flame
is not proven. The unit will attempt to ignite at 60 minute
intervals until the heating call is removed.
An IGN lockout due to flame loss can be reset by:
1.
Open and close the main power disconnect switch.
2. Switch the MODE switch on the zone sensor to “Off”
and then to the desired position (VAV units – remove
and reapply the mode input).
3. Allow the IGN to reset automatically after one hour.
When ignition takes place, the hot surface igniter (IP) is de-
energized and functions as the flame sensor.
Two Stage—If, after 60 seconds, the unit requires 1st stage
heating only, the IGN will change the combustion blower
from high speed to low speed. If additional heating is
required and first stage heat has been operating for a
minimum of 10 seconds, the IGN inducer relay will change
the combustion blower motor (CBM) to high speed,
delivering second stage heat capacity.
Modulating—Units with modulating heat will fire the
modulating bank first at high fire for 60 seconds. The unit
will then modulate the heater to the necessary rate. If the
modulating heat bank cannot satisfy the zone needs alone,
the second bank will come on and the modulating will find
the appropriate operating point.
Constant Volume (CV) Unit Fan Operation
If the Fan selection switch is in the “AUTO” position for
constant volume units, the RTRM will delay starting the
supply fan for 60 seconds to allow the heat exchanger to
warm up. When the zone temperature rises above the
heating setpoint, the IGN control module will terminate
the heat cycle. The supply fan remains energized for an
additional 90 seconds.
Variable Air Volume (VAV) Unit Fan
Operation (2 Stage and Modulating Gas
Heat)
During Unoccupied heating, Morning Warm up, and
Daytime Warm up mode, the VFD must be at 100%.
Therefore, before the unit can heat, the VHR relay must
have been energized for at least 6 minutes to ensure that
the VAV boxes have driven to maximum. For example, 6
minutes after a Daytime Warm up mode is initiated, the
VFD output will go to 100% and then the heat cycle will
begin. The VHR relay is energized during Unoccupied
mode, Morning Warm up mode, and Daytime Warm up
mode.
