36VCCX2 Controller Technical Guide
SEQUENCE OF OPERATIONS
Modes of Operation
If the HVAC unit is equipped with separate actuators for the outdoor
air and return air dampers, the return air damper will proportionally
close more as the Return Air Bypass Damper opens. The rate at
which the return air damper closes is user-adjustable. The purpose
of closing the return air damper more as the Return Air Bypass
Damper opens is to allow more air to bypass the evaporator coil
through the Return Air Bypass Damper. If more air needs to pass
through the Return Air Bypass Damper, enter a larger number in
the Return Air Damper Factor Setpoint. If less air needs to pass
through the Return Air Bypass Damper, enter a smaller number in
the Return Air Damper Factor Setpoint.
When the “Modulate RAB to control Supply Air Temp” is checked
the unit controller will modulate the Return Air Bypass damper
to attempt to reheat the SAT back up to the SAT setpoint during
RH mode. If the SAT is above the SAT setpoint the damper will
be modulated closed and locked at 0% open. When the SAT is
below the SAT setpoint in RH mode the controller will modulate
the damper open using the Return Air Bypass (SAT) adjustable
setpoints. The RAB Proportional Window sets the error calculation
range of operations. The higher this setpoint is the further the SAT
must be below setpoint to cause the control output to change from
0% to 100%. The RAB Control Rate is how often the calculation
and modulation adjustments will be made. The Reheat Stage Delay
setpoint is the amount of time the RAB damper must be at 100%
while the SAT is below the SAT setpoint by at least the Staging
Window amount before MHGRV will be allowed to operate.
When the RAB is at 100% and the SAT is below the SAT setpoint
by at least the Staging Window amount a timer is started. If the
SAT raises in temperature and is no longer below the SAT by the
Staging Window then no adjustments will be made until the SAT
once again goes below the SAT setpoint by the Staging Window
amount and the Reheat Stage Delay timer has expired. When this
occurs the RAB damper will be locked at 100% and the MHGRV
will be enabled and modulated.
When the SAT rises above the SAT setpoint the MHGRV will
modulate towards closed. When the MHGRV has modulated fully
closed and the SAT is above the SAT setpoint by at least the Staging
Window amount a timer is started. When the timer exceeds the
Reheat Stage Delay amount the MHGRV is disabled and locked
at 0%. Reheat Flush cycles are still allowed but the MHGRV will
no longer modulate for SAT reheat. The RAB damper is unlocked
so it can once again start modulating towards closed until the SAT
compared to the SAT setpoint is satised. If the SAT once again
falls below the SAT setpoint the cycle repeats.
Purge Mode
This is described under Supply Fan Operation on page 30.
Heat Pump
Air to Air Heat Pump Operation
Cooling Mode will operate in the same manner as described in the
Cooling section.
A reversing valve relay output can be congured to activate with the
rst compressor stage in the Heating Mode or the Cooling Mode
of operation.
In the Heating Mode, compressor heat, auxiliary heat, and
emergency heat can be used to achieve the active Supply Air Heating
Setpoint. auxiliary heat can be either a modulating or staged form of
heat, or it can be a modulating form of heat followed by staged heat.
When auxiliary heat comes on in conjunction with a digital
compressor heat stage, the digital compressor will be locked at
100% until the supply air temperature rises above the Supply Air
Temperature Heating Setpoint plus the heat staging window. At that
point, the auxiliary heat will stage o (after a stage-down delay) and
the digital compressor heat will be allowed to modulate.
When the outdoor air temperature is below the heating lockout
but above the outdoor air temperature compressor heating lockout,
compressor heat will be used and can be supplemented by auxiliary
heat.
When the outdoor air temperature is below the outdoor air
temperature compressor heating lockout, compressor heat is
locked out. Auxiliary heat will then be the primary heat and can
be supplemented with stage(s) of emergency heat (if available).
Emergency heat is only available when the outdoor air temperature
is below the outdoor air temperature compressor lockout.
Heat Pump Standard Defrost Operation
If using the VCCX2 Controller with an installed defrost coil
temperature switch, a defrost cycle is available.
If the compressor(s) are operating in the Heating Mode and the
defrost coil temperature switch closes, the unit will enter the
Defrost Mode, provided the user-adjustable defrost interval timer
has elapsed since the last defrost cycle.
In the defrost cycle, the reversing valve signal is switched to the
opposite operation, and the compressors are brought to maximum
capacity. Auxiliary heat will be used to attempt to maintain the
Heating Supply Air Temperature Setpoint. Emergency heat cannot
be used because it is locked out because the compressors are on.
The unit will leave the Defrost Mode after 10 minutes have elapsed
or the defrost coil temperature switch opens. If the unit leaves the
Compressor Heating Mode, the defrost interval will restart once the
unit re-enters the Compressor Heating Mode.
To Next Page
Loading...
Need help?
General
